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.