Saturday, April 17, 2021

A Simple 4 band EFHW for 40,20,15, and 10 meters

I like the EFHW for portable use because it is quick to deploy - unlike a quarter wave vertical, it doesn't require radials and unlike a dipole it's fed at one end. I have been trying out variations of the EFHW (end fed half wave) antenna for several years.These have included a 3 band trapped version (see post), a two band linked version, a two band lumped inductance version, and a fixed station 80-10 commercial version from MyAntennas.com. The later antenna is interesting because obviously the antenna is only a half wave on one band (in this case 80m). However, the 130 foot antenna is multiple half -wave lengths on all the other HF bands; 2 (half-wave lengths) on 40, 3 on 30m, 4 on 20m, 5 on 17m, 6 on 15m, 7 on 12m, and 8 on 10m. Interestingly, aside from the radiation pattern, the antenna behaves just like an EFHW on these other bands. It is resonant on these higher frequency bands with essentially the same high feedpoint impedance as 80m presents, resulting in a low SWR across multiple bands when matched with the transformer. So, let's call it an EFHW and just make the W plural = waves.

After getting good results with the MyAntennas 80-10 model at V31DJ, I got the idea to make a lightweight QRP 40-10 version of that antenna for SOTA and POTA activations. This antenna was cut for 40m, which is multiple half-wave lengths on three other bands; 2  (half-waves) on 20m, 3 on 15m, and 4 on 10m. The WARC bands, 30m, 17m, and 12m, can still be used, but an antenna tuner is required since this antenna is not resonant on these bands - it behaves like an end fed random length wire and needs at least one radial. 

In testing, the resonant frequency  of my 40-10 was high on 15m and 10m, so I added a small coil (6 uH) at 78 inches from the transformer to bring the resonant frequencies on these bands down to line up more with the CW SOTA frequencies.

Below is a plot of the SWR. Note that the SWR at resonance is less than 1.4:1 SWR on all four bands, making it ideal for radios lacking a tuner like the IC-705 and QRP radios like the Mountain Toppers. 




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