Monday, June 11, 2012

End Fed Half Wave Antenna


OK, call me an antenna snob, but in my opinion the End Fed Half Wave antenna is about as good as it gets for an easy to put up, great performing antenna. I am a coffee snob too, but that is another story. I came across EFHW antennas a while back when I was just starting to figure out antennas. My best and favorite source for end fed information is Steve Yates, AA5TB's great site http://aa5tb.com/efha.html The tuner is “Stupid simple” to assemble, but an antenna analyzer is really helpful in tuning the antenna wires. The only downside to EFHW antennas is the need to cut a different wire for each band. The upside to EFHW antennas is no long counterpoise. The antenna tuner needs a 0.05 wavelength counterpoise to work properly. Three turn primary from the radio and 24 turn secondary with a capacitor across it. That is it.

Lets take 20 meters, 468/Frequency = Half wavelength: 468 / 14.0 = 33.42 feet for the half wave wire.
Move the decimal point over one to get the 0.05 wavelength counterpoise of 3.34 feet.

I thought it odd at the time that a 3.34 foot counterpoise was necessary to make the antenna work, but when I disconnected it, the antenna would not tune very well. When I connected it back up, I could tune to 1.0:1. Below is the small version I built for my MTR radio. I typically use T50-2 toroids for most of my QRP tuners, but I did not have any of the T50-2's handy, so I substituted a T30-2 from the junkbox. Since the bandpass filters on the MTR use T30-2's, I figured the antenna tuner would be fine with that small a toroid. It did work! I used the tuner on 20 meters for the first time, for the first QSO, and made a nice contact with Gary, K5ON in NM at just over 1,200 miles. Nice!
Tuning the wire is an interesting process. AA5TB has you connect a 4.7K resistor across the tuner antenna connections and tune the analyzer to the band portion to which you want to center the antenna. Tune the capacitor on the tuner to get the lowest SWR. Leave the antenna tuner alone and start pruning the antenna wire until the SWR drops to the value with the resistor connected. One quick comment about that. Even though I tune for the CW portion of the band, the antenna SWR up in the phone portion is still very low.

I tune the the wire a tad different than AA5TB. (For 20 meters) The length of 468/14.0 drops you to around 13.8 MHz. What I do is fiddle with the analyzer and antenna tuner to get 1.0:1. Once I find the sweet spot on the tuner, I nibble a quarter inch at a time off the antenna wire and slowly walk the wire resonant frequency up to where I want it to be. You typically need to tweak the tuner a little bit (less capacitance) as you walk the analyzer up. Again, I tune the 20 meter wire for 14.050 MHz, but up in the SSB portion of the band, the tuner still tunes 1.2:1. Nice bandwidth too. One last thought, you can tune the EFHW by ear for max band noise. I know some of the QRP tuners have a resistive bridge LED SWR indicator gizmo attached, but you really do not need it.

Give an EFHW antenna a try. You will become a believer too!

72, Bob