This is part 3 of my series on building a cheap loop antenna. If you haven’t read part two, it’s here.

It has been a while since I did anything on this project, but I intend to finish it this year, so here goes! One of my other goals this year is to publish at least one article per month – so this is a short update.

The topic of loops and their capacitors came up at a recent meeting of the West Kent Amateur Radio Society, with some very constructive feedback on my capacitor build, which I’ll relate here. Thanks to Chris G6HTH, Robert, Richard, Barclay, Bob, and Alex.

The main problem Chris pointed out is that I’ll have an unacceptably high resistance by connecting the loop (RG213 braid, copper) to the stainless steel/aluminium vanes of the capacitor. Soldering such dissimilar metals will be ineffective (or impossible). Brazing is advised (I know nothing about this, so research required).

Also, why not construct the capacitor vanes/rods from copper? I had not considered this – all the designs I’d seen on t’Internet were aluminium, so naturally I chose that. Looking closely at proper transmitting variables, the vanes are brazed to their supports, not bolted, and both are the same material.

I was also considering – based on the project I was following: A Simple RG213 Loop by Mario G8ODE – on using RG213 (the outer braid) connected to the box housing the capacitor via PL259/SO239 connectors. Inside the box, these SO239 connectors would connect to the vanes of the capacitor with very thick wire, soldered to ring terminals bolted under the nuts.

The advantage of this scheme would be that I could switch the 1m loop for a longer version as needed. The disadvantage is that it introduces much resistive loss, and for loops, minimising this is critical to increase efficiency.

A loop of RG213 is also attractive since I also have some, and can work with it. Using a solid copper pipe loop would be better, but I don’t know how to bend copper effectively. I have seen schemes for bending it using rollers fixed to a Black & Decker Workbench. I could also fill pipe with sand, or get a spring and try bending a length… but the problem was solved very nicely when Chris gave me a 1m diameter loop of pipe, pre-bent at the next WKARS meeting. Thank you, Chris!

After some discussion, a method of attaching this to the capacitor has been suggested: two strips of copper plate, of similar thickness to the vanes (~1mm), bolted across the two terminals of each side of the capacitor. Bend the plate along its length into a right angle with the horizontal half bolted to the capacitor, and the vertical half connected to the loop. To connect the vertical half to the loop, drill a hole in the vertical half through which the loop end can pass, and braze the loop to the right angle.

So I’ll need a small sheet of copper with which to make the two loop-capacitor right-angle sections.

Item Cost each Total cost
Copper Sheet Plate 0.7mm Thick – Various Sizes, Copper, 100mm x 100mm £4.00 £4.00

We also discussed how I’d mount the capacitor and loop – I was thinking of putting the capacitor in a large plastic box, but the loop isn’t going outside in wet weather, only in the loft, or outside on pleasant warm days operating /portable. If I did encase the capacitor, I’d have to get the loop ends out of the box, through holes. Chris raised a valid point on this – if the loop touches the side of the insulating box, any moisture on the surface of the box between the ends would short the loop out. So, no need for the box – I’ll attach the capacitor to a board that’s fixed to the frame. I will place a “Danger! High Voltage!” sticker near the capacitor though!

The overall frame will be made of thin wood, in the shape of a cross. The loop will be attached to the top, left and right extremities of the cross by plastic clips – usually used for fixing radiator heating pipes to walls.

The coupling loop will be fixed to the upper section of the cross; the capacitor to the lower, mounted on a board, where I’ll possibly place a reduction drive and stepper motor.

I read the Feb 2019 issue of Practical Wireless with interest: there’s an article by Hamish Storie, MM0GW0, relating his experience building a variable capacitor – he built his with thin brass plates and fixings. It’s not a butterfly capacitor like mine, but he does point out some aspects of engineering that I’ve learned the hard way: use a metal nibbler to cut the vanes out, rather than shears, as shears will deform the metal.

I’ll continue next month, hopefully, with pictures!