Dan Koellen AI6XG
End Fed Half Wave Antennas: Is a Primary Capacitor Really Needed?
Updated: May 5, 2022
In my research on End Fed Half Wave (EFHW) antennas I found that nearly all have a capacitor in the primary (rig) side of the transformer. I am building an EFHW antenna for my new MTR3b, but do I need to include the capacitor for 20, 30 and 40 meters? Let's find out!
Note: More extensive experiments including operation at HF frequencies higher than 20 meters, harmonic versus fundamental operation and transformer efficiency is posted in this blog. Be sure to take a look for additional analysis and conclusions.
Looking at several builds and discussions, such as commercial antennas (e.g. MyAntennas), G0KYA, K1RF, K1TA, K6ARK, N4LQ, W8JI AND WA7ARK; they all have a primary capacitor of 100 pfd to 220 pfd.
An example is shown in this figure of a 49:1 transformer with a 100 pfd capacitor in the primary side of the transformer. I was curious why this capacitor was added to the transformer. The purpose of the transformer is to step down the high impedance of a resonant EFHW, not as a resonant circuit or tank itself. A few comments have been made about why the capacitor is needed:
'Improves higher frequency UNUN performance' and 'Compensates for UNUN primary leakage' - K1RF 'Capacitor flattens SWR at higher frequencies' - WA7ARK
My question is what is 'higher frequency' specifically? And since I am interested in building EFHW antennas for a single band or a few bands, how will that impact my design? Can I get away with no capacitor in my design?
As it turns out K1RF had similar questions about the capacitor and published data showing that the capacitor does improve SWR performance at frequencies higher than 20 meters.
The above data from K1RF shows that the SWR for 20 meters (my dark red markings - SWR scales differ in each graph) is about the same with or without a capacitor in the transformer primary. The effect of the capacitor is very apparent for 17 meters and above, flattening and lowering the SWR curve through 10 meters. Will there be the same performance for single band EFHWs? Let's find out by checking the performance of a transformer with no capacitor for single band EFHWs for 20, 30 and 40 meters.
Three EFHW single band antennas were prepared for 20, 30, and 40 meter bands. They will be adjusted for resonance at or close to the CW portion of the bands. The antennas are set up as a sloper as in my SOTA outings. One end is about 5 meters high on an extended fishing pole. The other end, attached to the transformer and antenna analyzer, is about 1 meter above the ground. No wire counterpoise is used to reproduce the SOTA setup.
A transformer was wound on a FT50-43 toroid with 3 bifilar turns followed by another 31 turns of #26 enameled magnetic wire. No capacitor was installed.
The SWR was measured for each of the three antennas across the CW portion of the bands
The transformer was attached to the 30 meter EFHW antenna and turns were removed for best match to 50 ohms. The final turns ratio turned out to be 3:29.
The SWR for 30 meters, 1.05:1, was the lowest of all three antennas since the transformer was tuned for impedance match at 30 meters.
The SWR for the 20 meter EFHW antenna was < 1.2:1
The SWR for the 40 meter EFHW was ~1.4:1 for 40 meters and 1.2:1 for 20 meters (full wavelength)
For my 20, 30 and 40 meter EFHW antennas I will not use a capacitor in the primary
Don't sweat the digits to the right of the decimal point too much, the SWR readings changed as the feed height and droop of the antenna changed. Nothing drastic but likely the SWR on the summit will vary as conditions and the setup change.
I hope to do more experiments with single and multi band EFHW antennas above 20 meters - stay tuned -- The experiments have been performed, check it out!
Do your own experiments with your own set up