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.