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

5 comments:

  1. Hey Bob!

    Nice blog entry. I wouldn't change an apostrophe of what you said about the EFHW, and Yate's contribution; I love it. And I get 1.1:1 SWR too on my mickey moused EFHW.
    By that I mean, I don't have an antenna analyzer and don't really want to buy one for purist (snob) reasons; I just can't see building simple QRP rigs out of my junk and homemade parts for only a few bux but then having to use a $300 gadget to make it all work well.
    But I do want to make my antenna resonant somehow. That's what I'm up to now. C
    Bob, correct me if I'm wrong, but it's one thing to have an SWR of 1:1, but one can nonetheless have a non-resonant antenna which wastes power b/c it isn't optimally efficient. Do you know if I have that right? If I am, that's why I want a resonant skywire.

    Cheers!

    Les
    KE7SLX

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  2. Hey Bob I built one into a small hammond bp=ox with a key chain ring on it and it works great
    ve3ips

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  3. Hi John,
    Very nice! I was amazed at how small you can make one of those tuners. I was wondering about maybe making a one band end fed half wave with a fixed capacitor, well, maybe a small trimmer to fine tune. I have been very impressed with the performance of the EFHW, the only drawback is the single band aspect unless you change wires.
    Thanks for the comment
    72/73
    Bob

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  4. This is exactly what I was looking for. What type of capacitor would I use for a 40m tuner? Thanks

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    Replies
    1. Hi Daniel,
      Thanks for the comment and sorry about the delay in replying. Been a busy week. A standard or typical polyvaricon has two gangs of 335 pf. When I just connected to one set, it tunes 20m and up, but will not tune the 40 meter band. Once you connect the second set in parallel, you have enough capacitance to tune 40 meters. Probably the ideal method is to connect a switch to the second gang of 335 pf and switch it in when you want to go down to 40 meters. That way it will tune 40-10 meters. I have not even tried 80 meters, so you will have to experiment.
      73!
      Bob AD7BP

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