2015年1月13日火曜日

Building a 7 MHz small transmitting loop antenna, part 4

Part 4 of this article series focuses on the PWM circuit for precise and slow control of the motor that drives the capacitor.

Attempt 1: Analog-to-analog motor control circuit 

Before discussing the final PWM circuit I used to control my motor, allow me to discuss another highly interesting circuit: the 7N3WVM "Remote Motor Controller" described here: http://www.qsl.net/7n3wvm/DC_mot_control.html. Please have a look at that page before proceeding.

The 7N3WVM circuit is attractive because it seems to offer an intuitive interface to control the remote motor: a locally-operated DC motor that is turned to generate an analog DC control voltage, which is then amplified by a power amplifier and sent to the geared motor. If clockwise rotation of the control motor generates a positive voltage, then counter-clockwise rotation generates a negative control voltage. Either polarity is properly amplified by the power amplifier and is sent along to the remote motor, allowing bidirectional control by turning the shaft of the local control motor left or right. What's more, faster rotation of the local control motor generates a higher voltage, leading to a higher voltage at the remote motor and faster turning of the remote motor.

The system sounds ideal, and initial tests were quite encouraging. I actually wired up a previous loop antenna using this motor control method. However, there is unfortunately a subtle but crippling flaw in this system: the motion of the remote motor is unreliable (more on that in a moment), so when turning the local control motor shaft indoors, you never know if your local knob rotation had no effect, some small effect, or a too-large effect at the remote motor. So you're operating essentially "blind", randomly turning the local control knob left and right, fast and slow, but never really knowing if the remote capacitor is moving at all, moving slowly, or moving too fast.

So what causes this unreliability in the motion? The problem is that rotating the local control motor's shaft generates a varying analog DC voltage depending on how quickly and how smoothly you rotate the shaft. For very slow rotations of the control knob, the generated voltage, after amplification, is too small to turn the remote motor. So nothing happens, and worse yet, you have no way of knowing that nothing happened (unless you implement some means of sensing the remote motor's current angular setting, via e.g. a potentiometer as mentioned in Part 3).

Then, if your steady hand rotates the control knob at just the right speed, then the remote motor will rotate slowly and smoothly. However, if you rotate the control knob too quickly, the local voltage jumps very high, leading to a very high amplified voltage at the remote motor, leading to a quick and jerky rotation of the remote motor. And again, with no feedback at the local operating position, you have no way of knowing when this happens.

I experimented with the supply voltage of the power amplifier; 7N3WVM recommends 3 volts. But I found that 15 volts worked best (note that the NJM2073D chip is rated for a maximum of 15 volts supply voltage), using a 3V motor for the local control motor, and a 3V geared motor (with 661.2:1 reduction) for the remotely-controlled motor. With 15 volts supply voltage for the NJM2073D power amplifier chip, even relatively small voltages, caused by slow rotation of the local control knob, were amplified enough to cause rotation of the remote motor. But even in this best-case scenario, I still could not completely solve the issue of unreliable, jerky rotation. Sometimes, the remote motor would not rotate at all; sometimes, it would rotate too fast.

I think it is very important that any remote control scheme work reliably: if you take some action locally (push a button, flip a switch, turn a knob), then it is imperative to know that something well-defined definitely did happen at the remotely-controlled device. As elegant as 7N3WVM's scheme is, it unfortunately fails this test, at least in my setup with my choices of motors.

I think the 7N3WVM scheme might be made to work well if some of the following ideas are applied:

  • Use a higher-voltage motor for the local control motor, to ensure that even slow rotations of the local control knob generate relatively high DC voltages. This should help ensure that slow rotations more reliably generate enough voltage to drive the motor at the remote end.
  • Use a higher gear reduction ratio on the geared motor at the remote end. My motor used a 661.2:1 reduction ratio; a further reduction ratio would physically limit the maximum rotational speed of the remote motor's output shaft, which would make jerky motion more tolerable.
  • If possible, add a potentiometer to the remote motor shaft to sense the remote motor shaft's current rotation. Then, display the potentiometer value with an ohmmeter at the local control position. This will allow visualisation at the local control position of the effect (if any) that the control knob rotation had at the remote motor.
  • When operating the local control knob, make a series of short rotations, separated by a brief and full stop of the control knob, instead of one smooth slow rotation. The intermittent nature of the series of short rotations is probably generating a pulsed output voltage which seems more reliable in ensuring motion of the remote motor.

Attempt 2: Transistor PWM control circuit

The previous paragraph mentioned that in the 7N3WVM control scheme, a pulsing voltage (generated by a series of short control knob rotations) was more reliable in achieving remote motor motion, compared to a steady low voltage (generated by a continuous, slow rotation of the control knob).

Indeed, a well-known technique for achieving reliable slow rotation of a motor, without stalling, is to use a pulse-width modulated (PWM) voltage. The technique and its motivation are described in many places, so I won't repeat the explanation here; see for example this page for a good introduction: http://www.electronics-tutorials.ws/blog/pulse-width-modulation.html.

The above page presents a PWM circuit using a 555 timer IC. I wanted a discrete transistor solution, and found a simple PWM circuit here: http://www.techpowerup.com/forums/threads/make-your-own-fan-speed-control-under-1-usd.124633/#post-1932968.

After building the above transistor PWM circuit and hooking it up to my geared DC motor, I was very pleased to see that it did allow reliable and very low-speed rotation of the motor. Unfortunately, after a few minutes, the smell of burning electronics drifted through the air, and I realised that the last driver transistor was smoking hot and certainly on the verge of failure. I used 2N3904 transistors throughout, and the motor was obviously pulling too much current through the driver transistor.

An obvious solution would be to use a power transistor capable of handling the motor current, but I had none on hand. And anyway, it seemed rather pointless to think about power transistors, when I already knew that I had a component on-hand capable of handling the motor current: the NJM2073D power amplifier chip from my previous experiments.

Final circuit: BJT PWM + power amplifier to drive motor

So, the logical next step was to use the transistor PWM circuit to generate a pulsed voltage, that would serve as the input to the NJM2073D power amplifier, which would then drive the motor. The final circuit as as follows.


Q1 and Q2 form an astable multivibrator. The values of the base bias resistors, R3 (for Q2) and R4/R10 (for Q1) were determined experimentally to yield a low-frequency pulse train. (An easy way to verify the pulse train frequency is to connect a piezoelectric earphone between ground and the Q2 collector and listen for the clicking or buzzing sound.) Switch S1 allows shorting out one of the Q1 base bias resistors, R4, which reduces the total Q1 base bias resistance from 570k to 100k. This has the effect of increasing the frequency of the pulse train, which leads to faster rotation of the output motor. With S1 open, very slow motion of the output motor is possible; with S1 closed, faster motion is possible.

The Q1/Q2 circuit was designed for operation off of 1.2 volts and was initially powered off of a separate 1.2 volt battery. Later, when reworking the circuit to use my main 12 volt power supply, I originally used a few silicon diodes to form a simple voltage regulator to supply 1.2 volts to the Q1/Q2 circuit. But during testing I empirically found a simpler solution: simply inserting R12 (10k) between the 12 volt positive rail and the Q1/Q2 circuit yielded about 1 volt available for Q1/Q2, and the Q1/Q2 multivibrator functioned properly in this simpler configuration. This is a rather ad-hoc solution, but it works.

Q3 is the driver transistor. Originally, the motor was connected directly between the Q3 collector and the positive rail. Now, instead, the Q3 collector does not drive the motor, but instead only provides input voltage (from the multivibrator pulses) to supply to the NJM2073D power amplifier. In 7N3WVM's original NJM2073D circuit, driven by the voltage generated by a DC motor, the input pins 5 and 8 are floating and connected to the DC motor terminals. 7N3WVM's "floating input" configuration allows both positive and negative polarities to appear at the output pins 1 and 3. However, in my circuit, instead of a floating input at pins 5 and 8, I have permanently tied pin 5 to the positive rail (through R13) and pin 8 to the negative rail (through Q3). This means that the output voltage at pins 1 and 3 will have only one fixed polarity and that the output motor can only be driven in one direction. Therefore, I added a DPDT switch at the output (S2-S5) to allow reversing the motor direction by flipping the DPDT switch to reverse the motor connections to the output pins 5 and 8. Physically, the DPDT switch also has a "neutral" position in the center where it is not connected to any output, which allows stopping the motor once the appropriate capacitor setting has been reached. For operating convenience, I chose a spring-loaded DPDT switch that automatically returns to the central neutral position when pressure is removed (an "auto-return" DPDT switch). This allows momentary pressure on the switch in either direction to rotate the motor in either direction, and automatic stopping of the motor when one's finger is removed from the switch.

Resistor R13, between input pin 5 and the positive rail, was determined empirically to limit the input voltage available at pins 5 and 8 such that the geared motor was just barely able to rotate. If R13 is too small, the input pulses at pins 5 and 8 are so large that they result in a strong, jerky motion of the output motor for each pulse. If R13 is sized appropriately, the input pulses (and output pulses) are reduced in strength such that the motor smoothly and slowly rotates. If R13 is too large, then no rotation of the motor is possible because the input pulses and output pulses are too small.

Due to the number of empirical adjustments made to the circuit, it is likely that the circuit will need to be adjusted if used with a different motor and/or a different capacitor (whose shaft stiffness will determine how much power the motor needs in order to rotate it).

Video of PWM-controlled motor in action

The following video shows the operation of the PWM-controlled motor. Notice the slow rotation of the motor gears and the extremely slow movement of the capacitor vanes, which are important to allow fine adjustments to the capacitor setting.

The video starts out with the slowest PWM pulse train. Later in the video, I close switch S1 which leads to faster motion. I also switch the DPDT switch to show reversal of the motor motion.

There is some backlash when reversing direction; it takes several seconds for the capacitor shaft to start rotating in the opposite direction. This was expected and is tolerable.

Most importantly, the circuit (after the empirical adjustments described above) is very reliable. When voltage is applied, the motor always rotates, at a slow and reliable speed. This makes remote operation easy; in particular, it is easier than the 7N3WVM analog-motor-to-analog-motor circuit, which was unreliable and prone to both under-responding (motor stalling) and over-responding (jerky rotation). But with the PWM circuit, it is possible to know with certainty that the remote motor will rotate in response to the local switch positions: with the DPDT switch in the upper position, the motor will definitely rotate clockwise with a well-defined maximum speed and no jerkiness (due to the speed limit imposed by the multivibrator pulse train); with the DPDT switch in the lower position, the motor will definitely rotate counter-clockwise, again with no jerkiness; with the DPDT switch in neutral, the motor will stop.

The system of course is not perfect: as with any mechanical system, there will be some unpredictabilities in the operation like slightly varying rotational speed, temperature-dependent expansion of parts or heating of grease, slow startup and overshoot on stopping due to friction and inertia, and gear backlash when reversing directions. Nevertheless, I expect that the system will be sufficiently convenient, accurate, and reliable for tuning the loop antenna.


2015年1月10日土曜日

Building a 7 MHz small transmitting loop antenna, part 3

Part 3 of this article series focuses on the mechanics of the motorised variable capacitor.

Mounting the capacitor and motor

I had originally planned to use hot melt glue to mount the butterfly capacitor and the motor directly inside a small plastic box. However, working within the cramped confines of the box interior would make precise placement and alignment difficult. I therefore decided it would be easier to mount the capacitor and motor on a small flat board, which would allow me easy access to all the parts from all angles. Once the parts placement is finalised, I can simply mount the finished board inside the plastic box.

The board I used for mounting the capacitor and motor is a 1mm-thick polyethylene cutting board. I specifically chose polyethylene because of its low RF loss. The electric field in a small transmitting loop antenna is concentrated around the capacitor area, so it is important that any materials around the capacitor have low dielectric loss. As a counter-example, it would probably be a bad idea to mount the capacitor on a wooden board, because wood is a poor RF dielectric. Even though the capacitor losses, caused by poor dielectrics around the capacitor, will be small and likely in the milliohm range, in a small transmitting loop the radiation resistance is also small, making it important to minimise even seemingly trivial losses like dielectric loss. For more reading on the effects of dielectrics on the capacitor losses, here are a few links. This link describes an improvement in the Q of a variable capacitor by replacing the phenolic insulators with HDPE (high-density polyethylene): http://theradioboard.com/rb/viewtopic.php?t=1525#p12346. And, this link describes in detail various loss mechanisms of air variable capacitors, including dielectric losses: http://g3rbj.co.uk/wp-content/uploads/2013/10/Measurements_of_Loss_in_Variable_Capacitors_issue_2.pdf.

The capacitor is mounted on the polyethylene board as follows.


When mounted vertically, the height of the capacitor shaft was lower than the height of the motor shaft, so I mounted the capacitor on top of a stack of three scrap pieces of the cutting board, raising the height by about 3 mm. The scraps were affixed to the main board with hot melt glue, and the capacitor was also held in place by hot melt glue. A shaft coupler was attached to the variable capacitor shaft to enable interfacing with the motor shaft. The variable capacitor's shaft was slightly too thin for a snug fit into the coupler, so I wrapped some paper a few times around the capacitor shaft to ensure a snug fit. A snug fit is important to ensure that the center of the capacitor shaft's rotation is the same as the center of the shaft coupler's rotation (simply tightening the coupler's screws against a too-thin capacitor shaft would lead to eccentric rotation of the shaft coupler).

The motor looks as follows.


The motor shaft is very thin, much thinner than the capacitor shaft, and does not fit snugly into the shaft coupler. Unlike the solution for the thin capacitor shaft, where paper was wrapped a few times around the capacitor shaft to fatten it, with the motor shaft the thinness is so extreme that wrapping paper is not a viable solution for fattening the motor shaft; the paper would simply slip as the shaft turned. Therefore, to expand the small radius of the very thin motor shaft, I applied hot melt glue liberally to the shaft and allowed it to harden into a bulb. Then I carefully snipped away the hardened glue until the diameter of the glue bulb would just fit into the shaft coupler. The hardened glue, being a form fit for the motor shaft and being adhesive by design, grips the motor shaft well enough that it should not slip and can deliver the rotational power from the motor shaft to the load.

The glue-encased motor shaft was then inserted into the shaft coupler, the coupler's screws were tightened around the hardened glue, and the motor was glued in place to the polyethylene board.




Use of hot melt glue

I found hot melt glue especially suitable for this kind of hardware layout because it allows quick and easy initial positioning of elements, without requiring any drilling of screw holes or the like. Repositioning of elements, if needed, is also easy.

When applying the glue, I carefully apply a neat blob or seam to the key connection points required for strength, taking care not to apply too much glue that would be difficult to remove later if needed.

If repositioning is needed, I can pull away the glue with either my fingers or a pair of pliers.

Possibility of remote sensing of capacitor rotation

The current motor setup allows remote control of the capacitor by applying voltage to the motor, but it is not currently possible to remotely determine the current setting of the variable capacitor.

Notice, however, that in the above image, the motor shaft extends not only to the left side towards the capacitor, but also extends to the right side, which is currently unused. In the future, it might be possible to couple the right-hand shaft to a potentiometer, which would then allow remote reading of the potentiometer value to sense the current angular setting of the variable capacitor.

This sensing capability might be exploited to implement an automatic tuner, where for example the transmitter's frequency is automatically read via some interface and a computer activates a control voltage to rotate the motor appropriately, monitoring the potentiometer value to gauge the capacitor's rotation and stopping when the rotation reaches some previously-saved value appropriate for the current frequency. Of course, implementing such a system requires consideration of real-world robotics issues like backlash and overshoot, and would likely require some sort of a PID controller implementation in software.

RF choking, or lack thereof

By choosing to mount the motor directly next to the capacitor, I am placing the motor and its control wires in the vicinity of the capacitor's strong electric field. A more conservative approach would place the motor at or near the loop's zero-voltage point (diametrically opposite the capacitor), using a very long shaft to couple the bottom-mounted motor to the top-mounted capacitor. In a previous loop design I did exactly this, using a one-meter plastic coupling shaft. The problem is that a long coupling shaft will usually introduce extra backlash into the system, making the capacitor slower to respond to the motor rotation, due to time it takes for the long plastic shaft to twist until it has enough tension to drive the capacitor. And in the current loop design, a 2-meter shaft would be necessary, which would introduce even more backlash and would be mechanically challenging as well due to the long shaft length.

For the above reasons I chose to mount the motor directly next to the capacitor, to simplify the mechanics. Also, it's worth noting that the MFJ 1786 loop antenna (generally regarded as a well-performing, properly-constructed small transmitting loop) also places the motor directly at the capacitor.

However, the mounting of the motor and its control wires near the capacitor could conceivably cause RF currents to flow on the motor control cables, which could lead to problems both on transmit and receive: on transmit, the transmitted energy might make its way back into the motor control circuitry causing equipment damage or posing a shock hazard; on receive, the motor control wires might form part of the antenna system and allow induced noise on the control wires to couple into the capacitor and the loop antenna, where it then would be passed back into the receiver as unwanted noise.

The way to solve this problem is to use an RF choke or chokes on the motor control wires to prevent RF currents from flowing on the control wires. The MFJ 1786 loop antenna uses RF chokes for this purpose. Also, N4SPP's page on small transmitting loop construction (http://www.nonstopsystems.com/radio/frank_radio_antenna_magloop.htm) shows how he uses two ferrite chokes on his motor control cable.

I considered using ferrite chokes, but was concerned about how to decide on the specifics of the choke design: the proper core material, core size, number of turns, choke location, etc. Before diving too deeply into the choke design, I decided to run a 4nec2 antenna simulation to see if choking was even necessary or not.

I made a 4nec2 model of the loop and tried running a disconnected wire down from near the capacitor to ground, to represent the motor control wires (which are near, but not connected to, the capacitor). The separation between the capacitor segment and the motor wire segment was 1 cm. Also the motor wire deliberately ran away in an asymmetrical fashion (which makes it easier for unwanted currents to flow) and was running very close to the main loop conductor (1cm spacing). Even in this asymmetrical condition, the current induced on the unchoked wire, with 5 watts transmitting power, was only about 20 milliamps maximum, which I consider to be low enough not to be concerned about.


Although the simulation results lead me to believe that the unchoked motor cables will not carry significant RF current, it is possible however that for certain specific lengths, the control cable might be more prone to coupling into the capacitor and becoming an unwanted part of the antenna system. This will be determined during actual usage of the antenna. I plan to adjust the antenna for minimum SWR, then alter the routing of the motor control cables to see if the SWR changes. If it does, then choking of the motor wires may be needed.

Abrasion of the hardened glue

After testing the ability of the motor to drive the capacitor, some slippage started to occur after a few minutes. This was caused by the coupler's screws not being tightened enough around the hardened glue; the motor shaft and glue bulb were rotating, but the shaft coupler and the capacitor shaft were not rotating.

To fix this, I first tightened the screws even harder against the hardened glue. I cannot see inside the shaft coupler, but I believe the screws are now so tight that they are biting into the surface of the hardened glue. One concern is that the hardened glue is still quite soft compared to the usual metal shafts for which the shaft coupler is intended. Over time, as the motor is repeatedly rotated both clockwise and counterclockwise, I can imagine the shaft coupler's screws slowly grinding away at the hardened glue until the glue is completely stripped away from the motor shaft, leading to slippage and inability of the glue bulb to deliver power to the load. The expected failure mode will not be immediate; as the glue is slowly worn away around the contact points with the screws, I expect a widening hole in the glue to form, which will first lead to excessive backlash when the motor reverses direction, and may eventually lead to the wearing of a groove all around the glue bulb's circumference, at which point the motor-driven rotation of the glue bulb will no longer be able to push against the coupler's screws and will no longer be able to rotate the capacitor shaft.

In an attempt to delay this mechanical abrasion of the glue, I added more hot melt glue at the shaft coupler's opening where the motor shaft enters.


The hope is that the additional hot melt glue will allow the motor shaft to grip the outer surface of the shaft coupler, and that the required torque to turn the capacitor shaft will be partially provided by the grip of the glue on the coupler's surface. This might reduce the pressure, while the motor is running, of the coupler's screws on the inserted glue bulb, which in turn might lengthen the lifetime of the inserted glue bulb. Time will tell.

Addendum 2015-01-18: The glue seems to become harder after a 24 hours; it seems hard enough that moderate pressure from the screws will not significantly wear away the glue. The slippage mentioned above was caused by insufficient insertion depth of the hardened glue bulb into the shaft coupler, meaning that the shaft coupler's screws were only barely grazing the tip of the hardened glue bulb instead of firmly gripping the main body of the hardened glue bulb. With a sufficient insertion depth, 24-hour-hardened glue, and moderately-tightened screws on the shaft coupler, no further slippage has been observed.

Motorised capacitor in operation

The following video shows the operation of the motor when run from 4 AAA batteries.

https://www.youtube.com/watch?v=_CNhot_yUv8

The motor's rotational speed is still several RPM, which is too fast for precise tuning of a narrow-bandwidth small transmitting loop antenna.

The problem is compounded by the fact that a butterfly capacitor covers its complete capacitance swing in only 90 degrees, as opposed to the 180 degrees of a normal air variable capacitor.

The next post in this series will present a pulse-width-modulated circuit that can achieve a much slower rotational speed of 1 RPM or less.

Addendum: A note on motor lifetime

At the following page, some interesting data is presented on the lifetime of the type of motor I am using: https://www.pololu.com/docs/0J11/all#2. It can be seen that the lifetime for continuous operation is, in the best case, on the order of tens of hours, and in the worst case (when running at higher voltages) is less than ten hours.

This has some serious implications for the overall system design:
  1. The voltage used to run the motor should be kept within limits to lengthen the motor lifetime.
  2. In case of over-voltage, the motor will fail fairly soon. Assume for example that the motor is run at 6 volts. The expected lifetime will be only about 5 hours or 300 minutes. Then, assume the antenna is used such that the total motor on-time per day is 10 minutes (which is realistic if the antenna is frequently re-tuned, for example for short-wave listening purposes on various frequencies). Then, the motor will fail after only 30 days of operation.
  3. In the current loop design, it will be quite tedious to replace the motor because:
    • The loop dimensions (3m x 2m) are too large to allow transportation of the loop or laying the loop down flat (in the limited space on the balcony) for access to the capacitor box
    • The entire loop must be de-soldered in place, piece by piece (see part 1 of this article series), to access and dismantle the top-mounted capacitor box
    • After motor replacement, the entire loop must be re-soldered together
  4. To better accommodate the eventual necessity of replacing the motor, it may be better to use mechanical connections to connect the capacitor to the main loop. The mechanical connections could then be easily undone to completely disconnect (mechanically and electrically) the capacitor box from the loop, where it could then be taken inside and the motor replaced. Such mechanical connections could be wing nuts, hose clamps, or similar. These connections will introduce additional ohmic loss compared to a solder connection, but with a large contact area the resistance should be able to be kept within manageable bounds. Additionally, the radiation resistance of the loop is relatively high due to the quarter-wavelength circumference, making tolerable the additional ohmic loss through mechanical connections. 

2015年1月5日月曜日

Building a 7 MHz small transmitting loop antenna, part 2

I did some more exploratory work on the mechanics of constructing my 7 MHz small transmitting loop.

Fitting copper pipes together

The copper pipes I am using are 6mm in diameter with a wall thickness of 0.5 mm. This means that the inner diameter should be 5mm.

My plan on fitting the copper pipes together was to insert a short length of 5mm copper pipe into the 6mm pipe, as an inner segment to provide some structural stability. Then the adjoining 6mm copper pipe would also be slid snugly over the 5mm copper pipe, and butted against the other 6mm pipe. The butt joint, reinforced by the interior 5mm pipe, would be heated and soldered.


The problem in reality is that the 5mm pipe cannot quite be inserted into 6mm pipe due to manufacturing tolerances. The outer diameter of the 5mm pipe is slightly greater than 5mm and/or the inner diameter of the 6mm pipe is slightly less than 5mm.

Therefore, I needed to swage open the ends of the 6mm copper pipe, creating a flared opening at the pipe end, so that the 5mm pipe could fit inside.


My hand tools are limited, but I found that I had a screwdriver of just the right diameter to swage (somewhat crudely) the 6mm pipe opening wide enough to accommodate the 5mm pipe.



My swaging process is quite crude and requires me to wiggle the screwdriver in the 6mm pipe opening to flare it open. This process is not precisely repeatable, so each time I swage a pipe, the exact depth of the flare (i.e. the maximum depth into which the 5mm pipe can be inserted before it is stopped by friction against the unflared 6mm pipe wall) is different. This means that the required length of the inner 5mm connecting pipe segment will be different for each set of pipes to be connected due to the inexact and differing depths of each flared-open pipe end. This is illustrated in the above diagram by the different lengths of flared end A and flared end B.

To proceed, each end of each pipe to be connected must be individually flared open such that the 5mm pipe can be inserted to a depth sufficient to support the joint. Then, for each pair of pipe ends to be connected, a custom length of 5mm pipe must be cut specifically for that pair of pipe ends so that when inserted, the custom length of 5mm pipe allows the 6mm flared pipe ends to exactly butt against each other. 

Then, the pipes will be soldered together.

Copper soldering with a torch

I have no experience soldering copper with a torch. As an experiment, I grabbed some spare copper flashing I had lying around and tried soldering it with my small torch. The results, as you can see, were highly unsatisfactory, though I at least did get the solder to wet and bond in some places.


Clearly, I will need much more practice with the torch before I will have confidence soldering the pipes together.

The above experiments were done without flux. My next try at torch soldering will use flux applied to the copper surface.

Soldering flux to be used in future copper soldering experiments with the torch.

As an alternative, it may turn out to be easier to use a large-wattage soldering iron instead of a torch to solder these small copper pipes.

Attaching copper pipes to the capacitor box

The top-mounted and motorised capacitor will be mounted in a small plastic box for protection from the weather. Another mechanical problem to solve is how to attach the top-mounted capacitor box to the pipes at the top of the loop.

Due to the top-mounting of the capacitor box and the lack of a central spine in my loop design, the points at the top of the loop where the pipes connect to the mounting box may be subject to moderate stress if the loop flexes, as may happen in wind or during manual repositioning of the loop. And, indeed, we want the pipe-to-box connecting point to bear all of the stress caused by loop flexing, because otherwise that stress would be borne by the stator shafts of the delicate butterfly capacitor, which in my case is quite small and might easily be torn apart, cracked, or warped if the stator shafts are subject to too much stress.

Hot melt glue seems appropriate in this case to bond the copper pipes to the capacitor box. It seems strong enough to bear the expected stresses, yet it can be pulled apart for disassembly by a firm tug with a pair of pliers.

In the following image, I am applying hot melt glue to attach one pipe (that will eventually be at the top of the loop on one side) to the left half of the capacitor box. Later, another pipe will need to be connected to the right half of the capacitor box.

Applying hot melt glue to affix the one copper pipe to the plastic capacitor box. The glue forms a large blob completely covering the copper pipe and the surrounding plastic.

After the glue solidified, it formed a moderately strong bond that served to hold the copper pipe in place against the plastic box. In the following image, I am holding the capacitor box only. The full one-meter length of the pipe is unsupported and hanging freely in air, exerting maximum leverage on the hot glue joint. The joint held and showed no signs of breaking; I estimate the pipe would bend before the glue joint would break. 

The capacitor box is supported, and the one-meter length of copper pipe is hanging freely in air. The glue joint does not break.

Also, holding free the end of the pipe and allowing the capacitor box to hang freely, again the glue joint showed no sign of breaking. The joint still held even when the capacitor box was filled with the extra weight of the capacitor and motor.

The end of the pipe is supported, and the capacitor box hangs freely in air. The glue joint does not break.

These tests indicate to me that the hot glue joints at the capacitor box will be able to bear the stress of the loop flexing.

After these tests, I could break the glue joint by pulling it apart with a pair of pliers.

The above describes the solution for the mechanical attachment of the pipes to the capacitor box. For the electrical connection of the pipes to the capacitor, located inside the capacitor box, I will solder a flat strap to each of the two top-most copper pipes. Each flat strap will then be routed from outside the box to inside the box underneath the box lid, where each strap will be soldered to one end of the butterfly capacitor.

As with all aspects of this loop design, the mechanical and electrical connections should be capable of being reasonably easily disassembled for loop maintenance, transportation, or storage. To achieve this objective, the general approach for assembling the capacitor area will be:
  1. Solder copper straps to left and right copper pipes.
  2. Electrical connection: Solder each strap to one side of the butterfly capacitor.
  3. Insert capacitor in box, routing straps and the connected pipes outside the box.
  4. Mechanical connection: Glue (with hot melt glue) the pipes to the top of the capacitor box.
  5. Place lid on capacitor box.
In particular, note that this assembly sequence is easily reversible and does not require any tricky operations like torch soldering inside the plastic box.

2014年12月21日日曜日

Building a 7 MHz small transmitting loop antenna, part 1

This post will be the first in a series of posts describing the construction of a small transmitting loop antenna. The intended frequency of operation is the 40m amateur band (7 MHz), and the location of the antenna will be on the balcony of a concrete building.

The design parameters

I chose to use 6mm-diameter copper pipe for the loop conductor because it is lightweight and bendable by hand. It should also be fairly easily solderable without requiring a large torch. Though easy to work with, the pipe is solid enough to support itself even when formed into a large loop.

For the tuning capacitor, to be located at the top of the loop, I chose to use a 50 pF butterfly capacitor with welded vanes. The plate spacing is small, but it should be sufficient for my QRP power level of 5 watts.
The 50 pF butterfly capacitor (right) to be used in the small transmitting loop.
A geared motor will be connected directly to the capacitor (with a very short coupler) for remote control, with the motor wires dropping straight down from the motor to the bottom of the loop. 4nec2 simulations showed there should be minimal common-mode current on the motor wires, so I anticipate no choking of the motor wires will be necessary.
The Tamiya geared motor to be connected to the butterfly capacitor.

Loop excitation will be done at the bottom of the loop with a small and galvanically-isolated coupling loop. This should provide excellent suppression of common-mode currents on the feedline, requiring no additional balun.

I chose to make the loop as large as possible to maximise the radiation resistance. I decided the loop should be rectangular, 3 meters wide by 2 meters high, which would barely fit on the balcony where the loop will be mounted.

The challenges

There are two problems with my chosen loop dimensions:
  1. The loop is so large that it cannot fit through the balcony door. It must be constructed from shorter copper pipes, each transported separately to the balcony, with the final soldering of the pipes being done on the balcony.
  2. Once constructed on the balcony, the loop cannot be brought back inside again unless it is dismantled. Some thought needs to be given as to how to disassemble the loop in the event the loop needs to be moved or stored.

Solving the challenges

To allow easy assembly and disassembly of the loop, I considered using mechanical connections, such as hose clamps and bolts, to connect the pipe segments together. However, these would introduce ohmic losses and would require periodic maintenance due to weathering.  

Instead, I have decided to use a combination of pipe bending and pipe soldering to construct the loop, as follows:
  1. By hand, bend four copper pipes to 90 degrees, forming four corner elbows.
  2. Transport straight pipes and corner pipes to balcony.
  3. Solder pipes together in the following sequence.
Solder 2 corner pipes and 2 straight pipes together to form the bottom of the loop. After assembly, set this piece aside until the end.

Solder one straight pipe to each side of the butterfly capacitor.

Solder one corner piece to the left pipe.

Solder another corner piece to the right pipe.

Solder a vertical support pipe to the left corner pipe.

Solder a vertical support pipe to the right corner pipe.

Use two wooden poles to support the left and right sides of the loop. Fasten the loop to the support poles by using a hose clamp or similar.

Working alternately on each side of the loop, slowly slide the entire loop assembly upwards vertically along the support poles, to make room for the bottom-most pipes. 

Attach and solder the previously-assembled bottom pipe assembly.

Remove the support poles.

Disassembly of the loop must take place in the opposite order as assembly.

Soldering

The loop assembly or disassembly must be done piece-by-piece on the balcony. Due to the small 6mm diameter of the pipes, I do not expect much heat will be needed to solder the pipes together. While it might be possible to use a large 100-watt soldering iron, the iron's electrical cord would make it unwieldy to work with the iron outdoors on the narrow balcony.

A better option would be a torch. Again, since I don't think that a large amount of heat is needed, I plan to use the smallest torch possible: a tiny pocket torch fueled by a cigarette lighter. 

This pocket torch should be able to reach a temperature of 1,300 degrees Celsius and can be used continuously in a single burst for up to 60 seconds before it needs to cool down. I believe this should be sufficient for soldering of 6mm copper pipe.

Physically connecting the pipe segments together, prior to soldering, will be done by inserting a short segment of smaller-diameter (5mm) copper pipe to connect two adjoining 6mm pipe segments. The 6mm pipes will be butted together, heated with the torch, then soldered. The solder should flow into the butt joint with the 5mm inner piece providing mechanical stability.

Disassembly, when eventually needed, will be done by heating the butt joint and pulling it apart. 

Issues of weight

Note that there is no center spine in my mechanical design for the loop. Therefore, the pipes themselves must be strong enough to support the weight of the top-mounted capacitor and motor assembly. The butterfly capacitor and motor are both small and lightweight, and in my estimation the 6mm-diameter copper pipes should be able to support their weight.

In my particular loop design, the physical size (and weight) of the butterfly capacitor can be kept small for two reasons. First, the required resonating capacitance is small because of the large loop dimensions and correspondingly large inductance. The small size of the required resonating capacitance allows the physical capacitor dimensions to be small. Second, the loop is designed for QRP use, so the plate spacing can be kept small. This again allows small physical capacitor dimensions.

Physical construction

Construction is in the early stages. Future posts will show the progress of the construction.

The 6mm-diameter pipes used are shown below.

Here are the 90-degree corner pipes, bent by hand.

An idea about the capacitor mounting is shown below.


2014年10月5日日曜日

Fringe howling regenerative receiver

The following receiver is a remote-controlled regenerative receiver using hybrid Colpitts-Vackar feedback. A detailed write-up of this receiver will be posted in the future. Currently, at certain frequencies in the upper half of its tuning range, the receiver suffers from fringe howl, a crippling phenomenon that manifests itself as a loud audio oscillation just as the set enters RF oscillation. In a regenerative receiver, the point of just entering RF oscillation is where the detector is most sensitive, so fringe howl -- the unwanted AF oscillation at the most sensitive detector setting -- serves to make the detector almost useless, and is a problem that must be addressed.
Schematic diagram of fringe-howling regenerative receiver. 
The following video illustrates the fringe howl occurring in the receiver prototype. Although the audio and video quality are low, and the prototype circuit layout is not visually pleasing, I felt it important to create an audio-visual recording of the fringe howl phenomenon, because there is (as far as I know) no existing audio-visual documentation of the phenomenon, only written descriptions. 


Before watching the video you may want to review the following labeled screen captures from the video, which explain what you are seeing. 
Video from 00:00 to 00:20: close-up of the control board
Video from 00:20 to 00:27: close-up of the control cable connecting the control board 
Video from 00:27 to 00:50: close-up of the remotely-located receiver board.
Video from 00:50 to 01:14: smooth operation of regeneration control, with no fringe howl, when the antenna is disconnected. Tuning is set to slightly above the middle of the tuning range (4.5 MHz - 16 MHz). 
Video from 01:14 to 01:40: connecting a short antenna to the top of the tank through a 6 pF capacitor. 
Video from 01:40 to 01:48: close-up of the short antenna, a ~30cm piece of wire 
Video from 01:48 to 02:08: fringe howl occurring right at the RF oscillation threshold. Tuning is same as before. Video from 02:08 to 02:30: fringe howl disappears if regeneration is advanced further. 
Video from 02:30 to 02:40: tuning receiver downwards in frequency below the midpoint of the tuning range (4.5 MHz - 16 MHz). Video from 02:40 to 02:57: smooth regeneration control and no fringe howl at lower frequencies.
Video from 02:57 to 03:05: tuning receiver upwards in frequency above the midpoint of the tuning range. Fringe howl re-appears.

Addendum 2014-10-08

Regarding fringe howl: A number of readers have made suggestions as to how to fix the fringe howl, which I am investigating. Again, a full write up of this receiver will follow once the problem has been solved.

Some readers have asked what the V2 component is. For circuit analysis or construction, it can be ignored and considered to be a short circuit. In actuality, it is part of the LTspice circuit simulation that delivers a voltage pulse to the tank inductor as an aid in determining whether the oscillator has enough gain to oscillate with the given circuit parameters (Vcc, emitter resistance, inductor losses, base bias, feedback network). This in turn allows me to optimize the feedback network C1/C2/C5.

Also note that C1 and C2 are unnecessary. Further simulation has shown that the circuit still oscillates when C1 is removed and when C2 is replaced by a short-circuit. C1 and C2 were intended to provide some small amount of Colpitts-style feedback to boost the Vackar-style feedback formed by D1 and C5, but the Colpitts-style feedback is so small in this case that it is actually not needed, according to the simulation. In hardware, removing C1 and replacing C2 with a short circuit yields identical circuit operation.

Regarding the "Vackar-style" feedback network formed by D1 and C5 (with C1 and C2 removed), please see p. 16 of the following document for the derivation of this feedback network: http://www.kearman.com/vladn/hybrid_feedback.pdf. The following LTspice simulation of a simplified version of my circuit shows the open-loop gain behavior of the D1/C5 Vackar-style feedback network vs. the open-loop gain behavior of the C1/C2 Colpitts-style feedback network. As you can see the Vackar-style feedback has decreasing open-loop gain as the tuning capacitor is tuned higher in frequency, whereas the Colpitts-style feedback has increasing open-loop gain as the tuning capacitance is tuned higher in frequency.
Vackar-style vs. Colpitts-style feedback. Notice the difference in open loop gain as the tuning diode is swept across its capacitance range.

Addendum 2014-10-13: solved fringe howl

The fringe howl has been solved.

Measures tried

  1. Rewire DC control cable to physically move it far away from the hot end of the tank inductor. Result: Fringe howl still occurs.
  2. Remove C6 (RF decoupling for input of AF amp). Result: Fringe howl still occurs.
  3. Increase C20 (Vcc decoupling) to 120uF. Result: Fringe howl still occurs.
  4. Increase R14 (series resistor from emitter to AF amp) from 1k to 10k. Result: Reduced volume. Howling stops, but there is still a seashell sound indicating instability.
  5. Decrease R14 (series resistor from emitter to AF amp) from 10k to 0k. Result: Loud volume, but fringe howl observed at wider range of frequencies.
  6. Add emitter follower stage between detector and AF amp. Remove R14. New emitter follower base connects through 100 nF capacitor to Q1 emitter. Base bias set to Vcc/2 with 20k/20k divider. Emitter resistor set to 1k. Output taken from emitter resistor and fed into C3 (AF amp). Result: Fringe howl still occurs.
  7. Add 10k resistor between Q1 emitter and emitter follower base. Result: Reduced volume. Seashell sound still observed.
  8. Replace emitter follower with DC-coupled common-base buffer. New buffer emitter connects directly to Q1 emitter. Base goes through 100k to collector. Collector goes through 4.7k to decoupled Vcc at R15. AF output taken from collector and fed into AF amp at C3. Result: Fringe howl still occurs.
  9. Change DC-coupled common-base buffer to AC-coupled common-base buffer: Add new 1k emitter resistor to buffer, and connect buffer emitter through 100nF to Q1 detector emitter. Result: Fringe howl still occurs.
  10. Add 10k resistance between common-base buffer and Q1 detector emitter. Result: Fringe howl still occurs.
  11. Remove C7, C8, C9 (RF decoupling for intermediate stages of AF amp). Result: Fringe howl still occurs.
  12. Connect external LM386 amp to output of common-base buffer (at collector 4.7k load resistor). Result: Fringe howl still occurs.
  13. Connect external LM386 amp to Q1 detector emitter directly. Result: AF volume too low to determine whether or not fringe howl occurs.
  14. Remove common-base buffer, reconnect original AF amp (but still with C6, C7, C8, and C9 removed), allow fringe howl to occur, and monitor oscillator signal on nearby receiver. Result: fringe howl heard in radiated signal on monitoring receiver.
  15. Disconnect AF amp when fringe howl is occurring and observe radiated signal on nearby receiver. Result: fringe howl stops in radiated signal on monitoring receiver, indicating it is some interaction between the AF amp and the detector that is causing the howling.
  16. Reconnect AF amp and replace R13 (varactor DC bias at hot end) with 10 mH choke. Result: fringe howl still observed, but the generated AF oscillation has become lower in frequency.
  17. Add 10k in parallel with R12 (varactor DC bias at cold end) effectively reducing R12 to ~10k. Result: fringe howl still occurs in the same manner as #16.
  18. Change C5 (Vackar feedback capacitor) to 820 pF. Result: fringe howl still occurs in the same manner as #16.
  19. Change oscillator from Vackar-style to Hartley: remove C5, connect varactor anode directly to ground, connect Q1 emitter through 100 nF to tap on L3 (5 turns from cold end). With the Hartley feedback topology, the varactor anode directly grounded, and the varactor cathode biased through a 10 mH choke, most of the original RC networks in the detector itself have now been removed. Result: fringe howl still occurs, though AF frequency has changed. Fringe howl also now occurs with antenna disconnected.
  20. Remove C20 and attempt to take AF output off of collector resistor R15. Result: a constant high-pitched AF oscillation regardless of regeneration setting, indicating AF amp instability.
  21. Connect C3 (AF amp input capacitor) to ground. Result: quiet hissing from AF amp.
  22. Connect C3 (AF amp input capacitor) to decoupled Vcc at R15. Result: a high-pitched AF oscillation indicating insufficient AF amp power supply decoupling.
  23. Add additional 100 ohm resistor and 22uF capacitor (R16 and C23 in new schematic) to decouple Q3 power supply. Reconnect C6, C7, C8, C9. Result: high pitched AF oscillation of #22 stops. However, fringe howl is still observed.
  24. Add additional 100 ohm resistor and 22uF capacitor (R17 and C24 in new schematic) to decouple Q4 power supply. Result: fringe howl is still observed.
  25. Use decoupled AF amp with an older regenerative receiver using a common-collector oscillator, a 176 uH choke as the emitter RF load, and a 100 ohm resistor as the collector AF load. C3 from AF amp is connected to the detector's collector load resistor. Tank inductor is a ferrite rod antenna; no external antenna is connected. Result: no fringe howl observed, but a slight seashell sound is present at some frequencies.
  26. Sleep one night on the problem. In the morning, I think the problem may be the AF amplifier causing the Vcc voltage to vary, and the regeneration control voltage, being taken from the non-decoupled Vcc supply, may be varying in sync with the amplifier-caused Vcc voltage swings, which could cause motorboating: as RF oscillation starts, the detector noise comes up, causing the AF amp to draw more current, causing Vcc to drop, causing the regeneration voltage (taken from the non-decouled Vcc supply) to drop, causing a drop in detector noise, causing a drop in the AF amp current, causing a rise in Vcc, allowing the regeneration voltage to again rise.
  27. Rebuild new regenerative receiver to remove the temporary Hartley feedback of #19 and to again use the original Vackar-style feedback topology. Remove varactor bias choke and again use resistors to supply varactor bias. Reconnect AF amp back to the current regenerative receiver, but supply VR1 voltage (regeneration voltage applied to base) not directly from V4 but instead from the decoupled Vcc supply at R15. Result: fringe howl can no longer be observed in the new receiver when using either a whip antenna or a large antenna connected to C22. However, a slight seashell sound seems to be present around the middle of the tuning range. It is still possible to induce fringe howl by connecting a large antenna (a metal door frame) directly to the top of the tank L3, bypassing the 6 pF antenna coupling capacitor C22.
  28. Rewire older regenerative receiver of #25 to use decoupled Vcc supply for the regeneration control voltage. Result: no fringe howl observed in older receiver, but seashell sound is still present. Connecting whip antenna to the tank inductor (ferrite rod antenna) causes fringe howl at certain frequencies. Even with maximum decoupling the fringe howl could not be completely eliminated from the old circuit, so the decoupling solution -- i.e. decoupling of AF amplifier supply voltage, of detector supply voltage, and of detector regeneration voltage -- is not enough.
  29.  Reduce VR1 (regeneration control potentiometer) from 10k potentiometer to 1k potentiometer. Result: some reduction in volume, but no fringe howl observed in any case in either old receiver or new receiver, under all antenna cases (no antenna, short antenna, large antenna, connected directly to tank or through coupling capacitor C22). The reduction in volume was unexpected, but may be due to increased current through the detector transistor's base, leading to decreased AF detection efficiency.

Summary of solution to fringe howl

  • Ensure AF amp is stable by implementing Vcc decoupling. Test AF amp stability by connecting amp input capacitor to +Vcc rail; if it oscillates, more Vcc decoupling is needed.
  • Ensure the detector supply voltages, including the voltage used to control regeneration, come from the decoupled Vcc supply to avoid regeneration voltage varying with AF amplifier current draw.
  • The RC time constant of the regeneration control potentiometer and the bypass capacitors on the potentiometer wiper are important and may be a cause of fringe howl.

The current circuit


Revision history

  • 2014.10.13: added section on solving fringe howl and new circuit
  • 2014.10.08: added notes on Vackar vs. Colpitts feedback and open loop gain simulations
  • 2014.10.09: added video of fringe howl in operation. Verified in hardware that C1 and C2 are unnecessary.


2014年9月13日土曜日

The Wheatstone Bridge Regenerative (WBR) receiver: how does it really work?

A regenerative receiver design was published in 2001 by N1BYT called the Wheatstone Bridge Regenerative (WBR) receiver. The design was interesting because it achieves good reverse antenna isolation (little oscillator energy can escape to the antenna) while still allowing antenna energy to enter and be amplified by the regenerative detector. The WBR achieves the reverse antenna isolation passively; no additional isolating amplifier stage is necessary.

If you're not familiar with the WBR circuit, AA1TJ writes more about it here: http://aa1tj.blogspot.jp/2011/08/ancients-have-stolen-our-inventions.html. Please see that article to understand the basic idea.

I was intrigued by this idea of passive reverse isolation but couldn't completely grasp how the circuit worked. Until now. A 1928 patent clearly describes the WBR operation: https://www.google.com/patents/US1667513. I have seen no references in online discussions to this patent; I found it myself after a lengthy period of investigation, while attempting to understand the operation of and research the background of the WBR receiver.

The key point from the 1928 patent's circuit description is that, unlike a normal regenerative receiver, the antenna signal does not directly excite the oscillator's tank at all! Instead it flows through the inductor's center tap and simultaneously goes through both arms of the bridge, which leaves both ends of the capacitor, and both non-grounded ends of the inductor, at the same voltage -- thus not exciting the tank. By going through the bridge, the signal reaches the input of the amplifying active device. Only after amplification and regenerative feedback does the signal then first appear at the tank.

Another interesting point is that the antenna energy is not filtered by the oscillator tank before it reaches the active device. This means that the antenna signals (in N1BYT's design, where the antenna load Z1 is an untuned inductance) encounter the non-linear active device without the benefit of filtering and thus AM blanketing or IMD are more likely to occur than in a receiver where the antenna signals are first filtered by the oscillator's tank. The 1928 patent uses an additional tuned circuit in place of N1BYT's untuned Z1 for additional input signal filtering.

So to summarise: Forward coupling from the antenna into the detector is high (*NOTE: see addendum below), because the antenna signal flows symmetrically from the tap point through both arms of the bridge (without exciting the tank) whereupon the signal reaches the amplifier input, is amplified, and is regeneratively fed into the tank for the first time. Reverse coupling from the oscillator to the antenna is very low -- almost zero -- because the inductor voltages at opposite sides of the center tap cancel, leaving no oscillator voltage at the center tap to excite the antenna.

                                            
Addendum, 2014-12

After publishing the above explanation of the WBR circuit, I did some more research, both in the form of circuit simulations and in the form of reading 1920's radio publications.

My conclusion from my circuit simulations is that there is a mistake in my previous statement that "forward coupling from the antenna into the detector is high". The forward coupling is not high, because the antenna signal must first go through the amplifier before it reaches the tank. The problem is that this first trip through the amplifier leads to significant loss because the amplifier is configured as a regenerative amplifier, whose output is only a tiny, less-than-unity fraction of its input. That's the essence of a regenerative amplifier: only a tiny fraction of the input appears at the output, but this output is regeneratively fed back to the input, resulting in much larger final regenerative gain after several cycles. With the antenna connected as in the WBR circuit, the signal first gets attenuated by the amplifier stage before the signal is regeneratively amplified, leading to a serious loss of sensitivity. This is especially noticeable with a short whip antenna where signal levels at the antenna are very low to begin with.

My conclusion is supported by a 1924 article by Mr. Fitch, who also authored the above-linked 1928 patent on the WBR circuit. The 1924 Fitch article is here: http://www.americanradiohistory.com/Archive-Radio-News/20s/Radio-News-1924-10-R.pdf (see p. 496 of the magazine, i.e. p. 58 of the PDF). The important thing to notice is the difference in tone between the 1928 patent and the 1924 magazine article. The 1928 patent presents the WBR circuit in a positive light and fails to mention its weaknesses, which was probably necessary to convince the patent examiner that the new invention was worthy of a patent. On the other hand, the 1924 magazine article is much more honest about the circuit's shortcomings, and states that the receivers are theoretically interesting but suffer from very poor gain if perfectly balanced.

I wrote some more about this on the Yahoo Group "regenrx-simulations" which you can see here: https://groups.yahoo.com/neo/groups/regenrx-simulations/conversations/messages/111.

2014年8月29日金曜日

Simulation of regenerative receivers

At the Yahoo Group called regenrx, I got into a good discussion with another user about simulating regenerative receivers, a topic I am particularly interested in. There seems to be a small number of people doing these sorts of simulations and there are various tricks needed to gain insight into the behavior of the almost- or barely-oscillating detector. I've picked up a few tricks here and there, but I felt it would be helpful if all the relevant information could be gathered in one place.

Thus I created a new Yahoo Group called regenrx-simulations. There are already a few working examples posted showing AM detection and CW detection, including regenerative amplification and AF detection. Other examples include frequency determination via FFT and AC sweeps, determining the threshold regeneration level with parameter sweeps, and frequency shift with regeneration adjustment.

Feel free to stop by and have a look!

https://groups.yahoo.com/group/regenrx-simulations