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Pollenating The Antennas – More 80m Folded Dipole

Whilst toying with some new KazShack toys, I found out an interesting and useful fun fact. The 80m folded dipole does a great job at ignoring some of the AM harmonic mixes. Death to harmonics! W4SAT seems to have the best on-line description of a folded dipole.There is very little written on the internet about this great antenna option. My own original post has a link to a web reprint of the original ARRL design article.

The KazShack is less than a mile from the 50KW WPTF(680) transmitter, and only about two miles from the 10KW WRBZ(850) transmitter. This results in all manner of harmonics and harmonic mixes that I can hear and identify. Pretty much any combination of the numbers 680 and 850 added and subtracted together produce a frequency that have audible audio artifacts that can be identified as from either AM station. Some of the mixes are much worse than others. Some are barely audible. The mixes that fall within the ham bands are obviously the ones of the most pertinent concern.

While playing with some new home brew band pass filters filters and the SixPak, I flipped the radio to 80m. While connected to either 40m antenna or the 20m antenna, I could hear a loud garbled mix centered at about 3570. The WPTF audio was clear. The WBRZ audio was also identifiable, although very garbled. That works out to the 4th harmonic of WPTF mixed with WBRZ’s 850. (i.e., [4*680]+850=3570)

This seemed really curious, since this never seemed to be a problem before. On the 40m antenna broadside to WPTF, the mix was S9+, and at least S7 on the other 40m and 20m antennas. So, it will probably blow my socks off when I switch to the 80m folded dipole, right?

Nope. Switching to the 80m folded dipole, the mix disappeared completely. No more WPTF audio on 3570. No more WBRZ. Both were Gone. Zip, zero, nada. Hmmmm.

Then I rememberated reading that folded dipoles were useful on their primary frequencies and their odd harmonics. So an 80m folded dipole could possibly be pressed into service on 30m, but it is deaf as a dummy load on 40m. That was a fact I have proven experimentally, both on purpose and by accidently flipping to 40m and wondering where all of the signals have gone(duh-uh!). The 80m antenna was rejecting the WPTF fourth harmonic well enough to eliminate the two station mix.

It would appear that a folded dipole also helps to reject the even sub-harmonics as well. This would probably have been obvious, but the thought had never crystalized within my addled gray matter before that moment. That fact could prove very useful for several settings. SO2R. Field Day. Field DXpeditions. IOTA. Sweet.

So, one additional yet seldom documented method to reduce the n/2 sub harmonic is to use a folded dipole. I would expect the folded dipole will also attenuate an interference your transmitter is generating on it’s 2nd harmonic, so switching either antenna to a folded dipole will probably help. This won’t help a 40m/15m problem, but applies to the other common harmonic situations.

Every little bit helps.

2 comments to Pollenating The Antennas – More 80m Folded Dipole

  • Dave lohnes

    Hi… I live in a trailer park with very limited space. I used to run on 3.870 and I miss HF like you wouldn’t believe. I am a big iron lover and 2 m just doesn’t cut it. I was wondering if you could send me the details to convert my 80 m long wire into an 80 m folded dipole that I might be able to actually use. I’m only really interested in 80 and 20 m operation but if it only works on 80 I would be totally happy with that . Any information help or insight would be greatly appreciated. Thanks in advance… 73s…

    • Well, a folded dipole requires the same amount of space as a full sized dipole. The ‘folding’ of a folded dipole is actually just adding a second closely spaced parallel wire to the dipole. So I’m not sure if it will help in a limited space, as it is more like a really thick wire.

      Instead, you might want to consider the folded counterpoise(FCP) for your existing wire. The counterpoise for 80m would be 32 feet long in total, or 16 feet on each side of the feedpoint. The difficult part there is the matching network. That will be a trial and error experiment, but a quick fix is to use an antenna tuner in a wx-proof enclosure as the matching network. My own 160m inverted L with the FCP seems to work reasonably well on 15m and 10m.

      Starting on page five in this document are the cookbook dimensions and diagram for a folded dipole.

      For information on the folded counterpoise,

      The FCP docs reference using the FCP on 160m, but it works FB on 80m when scaled down from 64 to 32 feet in length.

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