Once I had upgraded the matching transformer for my EF-10/20/40 MKII "EndFedz" (see Part 1) my thoughts turned to the loading coil. The original coil is designed for low power portable applications. The slim coil is torpedo shaped to prevent it being snagged in trees when deployed in 'the great outdoors'. The coil uses 26 AWG (27 SWG) enamelled copper wire close wound on a machined plastic former. Although the original loading coil is very well engineered, I thought I could make a more efficient version given that I did not need to make it 'trail friendly'.
My design objectives were:
1. High Q, hence low loss. This could be achieved by: Using thicker wire; increasing the diameter/length ratio; using a low loss former.
2. High self-resonant frequency. This could be achieved by increasing the spacing of the windings, hence reducing the interwinding capacitance.
After some internet research, I settled on making the former from a paint roller. I got the idea for this from an old post by Mark Rotsch on the Antiques Radio forum (http://antiqueradios.com/). I used a brand new Harris 9" x 1.5" (229 x 38mm) roller sleeve. First of all I had to pluck, then shave the roller to remove the pile. I then gave it the microwave oven test* to see how lossy it might be. The plastic did not warm-up, suggesting low losses at radio frequencies.
The original "EndFedz" loading coil is wound with 27 SWG wire. I have used 22 SWG wire to reduce the resistance and increase power rating. Due to skin effect, at radio frequencies the current tends to be concentrated in the outer skin of the wire. This is why RF coils are sometimes made with copper wire plated with silver. Although this can reduce the wire resistance, it will not make a huge difference, so I ended up using ordinary enamelled copper wire.
I decided to space out the windings a little in order to minimise the interwinding capacitance. There are nomographs and online calculators around that can give the inductance for given coil dimensions and number of turns. These seem to assume that the coils are close-wound and therefore I used trial and error to determine the number of turns. I wound the coil with two windings and then removed one of them to leave a coil with turns spaced one wire diameter apart. I used a little hot melt glue to hold the turns in place.
The loading coil inductance should be around 30uH, but I found it difficult to get accurate readings of both the original and replacement coils on the bench. After a lot of experimentation I ended up with 34 turns. The coil was terminated with M5 stainless steel hardware via suitable solder tags. I covered the whole thing with a single layer of self-amalgamating tape to give some weather protection.
The finished dimensions of the coil assembly are 40mm diameter by 90mm long (see photo).
See Part 1 for matching transformer upgrade and Part 3 for conductor upgrade.
* This involves putting the plastic in a microwave oven for a short time (no more than 30 seconds) to see if it heats up at all. If it gets warm it shows that the material is lossy at radio frequencies. It is a good idea to also have a cup of water in the oven, to provide a 'load', for the microwaves. Microwave ovens can be damaged by running them empty.
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