Dan Koellen AI6XG
End Fed Half Wave Antennas: More About the Primary Capacitor
Updated: May 5, 2022
I recently posted a blog claiming that a capacitor in the primary of the matching transformer is not needed for end fed half wave (EFHW) antennas operating on 20 meters and below. But what about operation above 20 meters in single band and multi band EFHW antennas? Let's find out!
While discussing the results of that experiment with other hams, there were questions about what happens at higher frequencies than 20 meters. There were suggestions to also look at the bandwidth of 2:1 SWR points. I was also interested in looking at the effect of the capacitor on an antenna operating at its fundamental frequency as well as multiple half wave length harmonics. I also got input from a comment on this blog from Danny E73M to measure insertion loss, those results have been added to this blog. I appreciate Danny's help in setting up the measurement and his input.
For more background and explanation please take a look at my other blog about the EFHW antenna matching transformer and the need for a primary capacitor. For this experiment, the transformer, antennas and mounting represent what I commonly use SOTA activations.
Five half wave antennas were constructed for 40, 30, 20, 15 and 10 meters. They were used at their fundamental frequency (one half wavelength) and harmonics (multiple half wavelengths) that are 10 meters and below. Each EFHW antenna was tuned to be resonant in the CW portion of the band.
A transformer was wound on a FT50-43 toroid using one winding, no capacitor was used on the primary. The winding had 29 turns of #26 enameled copper wire with a tap at 3 turns for the primary. An antenna analyzer was connected to the tap, the antenna was connected to the top of the winding. The transformer has a common return for the antenna analyzer and the antenna. A second transformer was wound for S21 transmission measurements.
The far end of the antenna was attached to a tree about 3 meters above ground, providing good isolation from other conductors. The other end was about 1 meter above ground and connected through the transformer to the antenna analyzer. No wire counterpoise was used.
Using a RigExpert AA-30 antenna analyzer, the SWR and other characteristics was measured on each antenna from the fundamental band to 30 MHz .
Two transformers were used to measure S21 transmission characteristics. One transformer primary was connected to port1 of a VNA and the second transformer primary was connected to port2. The secondary windings were connected together. This measurement was made in the configuration shown.
The measurements were repeated after a 100 pfd SMD capacitor was soldered across the primary of the matching transformers.
SWR minimum and 2:1 SWR bandwidth (BW) at fundamental frequency and harmonics were compared with and without the 100 pfd capacitor.
The transformers were initially connected to the 30 meter EFHW antenna, turns were removed until the best match to 50 ohms was found. The final turns ratio was 3:27 for a 1:81 impedance transformation.
All five EFHW antennas had a nice resonance, harmonics were also measured on the 40, 30 and 20 meter antennas.
For antennas operating at one half wave the SWR increases with the addition of the capacitor. The 2:1 SWR BW also increases slightly except for a large increase at 10 meters.
The insertion loss decreases by a few dB above 17 meters when the capacitor was added.
The addition of the capacitor significantly reduces the minimum SWR for harmonic operation with the effect increasing with frequency and harmonic. The 2:1 SWR BW also increases significantly with frequency and harmonic.
The capacitor's effect increases with increasing inductive reactance of the antenna.
The 100 pfd capacitor causes flattening of the SWR curve above 17 meters and a large dip in the 10 meter band. This exaggerates the capacitor's effect on minimum SWR and 2:1 SWR BW at 10 meters.
The capacitor lowers the resonant frequency for each antenna
The third harmonic of the 30 meter EFHW antenna could not be measured due to the antenna analyzer's upper frequency limit of 30 MHz . Though it could not be quantified, minimum SWR reduction and 2:1 SWR BW increase was observed consistent with other measurements.
The primary winding capacitor is not needed for EFHW antennas that operate as a single half wave for 20 meters and below. For 15 meters and above a capacitor should be considered, insertion loss is decreased for transformers with a primary winding capacitor.
A capacitor across the primary is recommended for EFHW antennas operating at multiple half wave harmonics. The capacitor significantly improves minimum SWR and 2:1 SWR BW.
The flattening of the SWR curve above 20 meters is significant with largest effect at 10 meters with a 100 pfd capacitor. What happens if the value of the capacitor is changed?
Since the capacitor's effect increases as the antenna is more inductive the capacitor is likely compensating for the inductive reactance. At a single wave the antenna's reactance is capacitive or slightly inductive. Multi half wave operation is increasingly inductive, increasing the capacitor's effect.
This is just one set of measurements. Try this yourself and share the data.