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I'm having an issue with some handheld transmitters, and not sure if they're bad, or the newly-installed antenna is bad, or if this even makes sense.

I've been using a handheld Icom aviation radio (model A23) within a hangar (ie, a metal-skeleton building) without any specific issues. We installed a base station antenna (B-ANT) with the hopes of being able to get better range from the handheld. The handheld was plugged in via an adapter cable and suddenly started transmitting intermittently. By that, I mean that when depressing the PTT the received audio sounded as if the PTT was being rapidly pressed and let go, at a rate of about 5hz. Additionally the red LED "RX/TX indicator" flashed at the same rate. In fact, I thought that it might be a bad PTT switch.

What's weird is that this continued even when the external antenna was removed and the stubby antenna was replaced when the handheld was in proximity to the antenna cable. It happened consistently when within a foot of the antenna cable, but only began consistently working properly when > 20 feet from the antenna cable.

This happened from a second Icom handheld radio.

It did not happen with a Yaesu handheld aviation radio, even when that radio was connected directly to the antenna.

What's happening here?

Additional Info:

I can't find an impedance rating for the antenna, but it's specifically recommended for some of their base stations (e.g., the A110), which have 50ohm impedance. Their handhelds (e.g., the A24) also have a 50ohm impedance.

I did find a spec sheet for the antenna if that's relevant.

While this antenna is sold with the base station radios, I haven't seen it recommended for handhelds; I assumed that's mostly just a function of not a lot of demand for external antennas with handheld tranceivers.

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  • $\begingroup$ I don't fully understand the scenario... Are you saying that the radio starts transmitting all on its own...OR... is it that when the radio is being keyed deliberately (PTT pressed) the radio's transmissions are intermittent? $\endgroup$ – mike65535 Jul 17 '18 at 18:26
  • $\begingroup$ The latter. When keyed (via the PTT button) it acts as if the PTT button is loose and isn't making a consistent closed circuit. The "TX indicator" would normally be solid red, but flashes at about 5hz, and the transmitted audio reflects that (sounds as if PTT is being tapped at about 5hz). $\endgroup$ – James S Jul 18 '18 at 17:09
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A nice theory that unfortunately does not explain everything is that the antenna is mismatched. If the impedance of the antenna, as installed, at the transmission frequency, is not near the value the transmitter expects, then some or all of the transmitted power is reflected and the transmitter may be damaged, or shut itself down or reduce power to avoid overheating.

The cycling behavior you observed is plausibly the result of a protection circuit being tripped, or of damage.

However, this does not explain why it would now happen when only near the now disconnected antenna. The only thing I can think of is that the radio is now damaged in a way which makes it especially sensitive to reflected signals, and in this case the reflection is coming from the base antenna. But it's fairly implausible that the disconnected antenna would cause this and that just, say, carrying the radio around in general would not.


The antenna may have other than the presumed 50 ohm impedance if

  • it is damaged or defective, or
  • it was installed close to metal objects in an inappropriate way.

To check this theory, find a radio technician or a ham friend with an antenna analyzer that goes up to the relevant frequency, and have them check the antenna as installed.

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  • $\begingroup$ I should have mentioned that I checked this before purchase -- both are 50ohm. The B ANT is made for base station radios but not handhelds, though both the handhelds and the base stations are 50ohm. See additional details I've added. \n Protection circuit would explain the cycling. \n Permanent damage also makes sense. I guess we'll soon see if the Yaesu exhibits similar behavior. Is there anything about the nature of a handheld that could cause this damage and extreme sensitivity? E.g., being underpowered for the antenna? $\endgroup$ – James S Jul 17 '18 at 6:17
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    $\begingroup$ @JamesS i think Kevin's theory is correct. The Icom recovery after disconnecting from the antenna is likely a function of time, not proximity to the antenna cable. The Yaesu radio is probably less sensitive to a high SWR. Ask a ham to check your antenna system with an antenna analyzer. Something is not right with the BNC adapter, the coax cable or the antenna. $\endgroup$ – Glenn W9IQ Jul 17 '18 at 10:17
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    $\begingroup$ @JamesS A ham can also check the Icom radios with a 50 ohm dummy load to make sure they are operating as expected. $\endgroup$ – Glenn W9IQ Jul 17 '18 at 10:36
  • $\begingroup$ I'm not sure about this theory - a handheld radio must work safely into an infinite SWR, because that's what it will see if you press PTT with a) no antenna or b) antenna held in hand or c) radio standing against a metal surface like a corner of a cockpit. SWR protection circuits act much faster than that, and gracefully as you still want to try to transmit. What about another nearby radio, tuned to the same frequency, having its PTT triggered intermittently by the fields near the antenna? Perhaps it has a cheap set of headphones connected? $\endgroup$ – tomnexus Jul 17 '18 at 18:17
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    $\begingroup$ Thanks. I'm marking this as the solution and I'll also keep in mind Tom's suggestion as I try to debug this further and find someone with an antenna analyzer. $\endgroup$ – James S Jul 18 '18 at 17:07

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