|The 20-17-15m trapped EFHW|
Anyway, this had been bugging me for some time. I tried modelling a version that could use parallel-fed half-wave radiators, but it didn't work.
I had always dismissed traps as these are used on quarter-wave radiators so they wouldn't work on a half-wave design – or would they? Turns out they do. I started modelling an antenna in MMANA-GAL and found that you could make a 20-15-10m EFHW using traps for 10m and 15m. But you would need a variable capacitor to bring the EFHW tuning unit to resonance.
This actually makes sense as the end of an EFHW is at the same impedance as the end of a quarter-wave vertical, so a trap does the same thing on a half-wave design. All you have to control is the feedpoint impedance.
Doh! – why didn't I think of that earlier?
The result is my first working prototype of a 20-17-15m multiband trapped EFHW, which seems to work well. I have updated the PDF to show how this was done. Development work has continued and one can now be built with a single matching box that doesn't need a variable capacitor at all, just the original coax capacitor.
Download the PDF with the complete EFHW story.