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Inspired by this discussion, I wanted to check if this is something I can get away with.

I've got an old FM Broadcast antenna that came with a home stereo receiver -- it's 75 ohm flat cable, in a T shape (not sure if it's a loop or folded dipole, but I'll check -- let's assume, for this question, that it's a folded dipole). It's close to resonant length for the 3m approximate center of the FM Broadcast band.

I've read that antennae often work well if they're on a third harmonic of the transmit frequency. This antenna is close to a third harmonic of the upper end of the 10m ham band (a little short, but not much).

I currently have it connected to a Heathkit SB-102; I was using it to test the receive functions. It occurred to me that I might be able to transmit meaningfully (possibly at reduced power) on the highest 10m segment supported by this transceiver (29.5-29.8 MHz), or, if there's enough matching capability, on the rest of 10m band. I'm concerned, however, not to damage anything while trying to adjust the transceiver's matching network.

Is that a realistic concern?

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  • $\begingroup$ You bet it will. See the answers and comments explaining why. $\endgroup$ – Mike Waters♦ Jul 11 at 18:21
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It won't work very well.

antennas often work well if they're on a third harmonic of the transmit frequency

This is calculating the third harmonic the wrong way around. A better statement would be something like

dipole antennas often work well at the third harmonic of the design frequency

A simple dipole antenna like an inverted-V, designed for 10 MHz will probably work at 30 MHz. The radiation pattern will be broken up into three beams, not one, but that doesn't matter much at HF. It might even help, as the middle lobe has a higher gain than a dipole.

But it doesn't work the other way around. An antenna designed for 100 MHz will be extremely short for 30 MHz, and hardly radiate at all.

I'm not certain that a folded dipole will resonate nicely again at three times the design frequency, because of the interplay of the transmission line and antenna modes. This needs to be simulated to be sure.

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    $\begingroup$ Sigh. I knew it couldn't be that easy... $\endgroup$ – Zeiss Ikon Jul 9 at 17:28
  • $\begingroup$ @ZeissIkon Tom answered nicely. Sub-harmonics are what you are thinking of; unfortunately, they don't exist. $\endgroup$ – Mike Waters♦ Jul 9 at 17:41
  • $\begingroup$ Tom, there was an ARRL construction article years ago about a 40/15m folded dipole. It used twinlead and 4:1 balun. Feedline and balun made from RG-59 CATV coax. I made one for myself and another (at the time) Novice class licensee. Worked for us. I recalibrated my SWR meters for 75 ohms (still do to this day). $\endgroup$ – Mike Waters♦ Jul 9 at 23:33
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    $\begingroup$ It's also important to remember that the resonance of the third harmonic is likely to fall outside the band of interest and the feedpoint impedance is likely to be quite different from $50\Omega$. This tradition began when a tube-type transmitter's $\pi-$ or $\pi-L$ output network was able to drive a wider range of impedances. The tradition may be extended by the use of a matching network between a modern solid-state transmitter and the feedline. $\endgroup$ – Brian K1LI Jul 10 at 13:42
  • $\begingroup$ I did specify in the question that I'm using a genuine boat anchor, which has a built-in pi matcher -- but based on short experimentation, it's not capable of matching that little antenna. $\endgroup$ – Zeiss Ikon Jul 10 at 21:26
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Will transmitting on this antenna cause problems?

It certainly can! Forget that FM BCB antenna, my friend. You can damage your SB-102.

You are not only wasting your time with it, but you may very well damage your 6146B PA tubes or related components in the PA circuit.

Even if you had an external tuner that could match that antenna on 10m, you would have enormous losses. You need a dipole in the air that's fairly close to resonance, say less than 5:1 SWR.

But before you go that far, I strongly suggest that you buy a 100 watt (or more) non-resonant 50 ohm dummy load. Once you can load that, then you can build and connect a proper antenna.

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