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I have almost finished building my first shortwave transceiver kit (mcHF). Remarkably it's fired up and not gone up in a puff of smoke. The kit still needs some finishing off and I need to decide on an antenna solution, but in the mean time I'd like to start hearing some sounds on it.

Is it ok to part strip a short length of Coax and tape it to my window for some RX testing? Is the whole antenna tuning exercise just needed for transmitting ?

rcx935
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    Congratulations! I built my first transceiver from a kit, and I remember how wonderful it was to hear the first signals from it. I'd like to say that a piece of coax doesn't make a very good antenna because the braid will shield the inner conductor from picking up signals. You would do much better with a long piece of wire poked into the center of the coaxial connector of your receiver. – rclocher3 Dec 15 '21 at 15:59

1 Answers1

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You are generally correct.

Antenna tuning is primarily about maximizing radiation for the transmission frequency, which is important because power emitted by the final transmitter amplifier stage that isn't radiated or consumed by ohmic resistance will reflect back into the amplifier, causing (potentially catastrophic) heating of the components.

This only matters, however, when significant power is being emitted -- and when receiving only, the transceiver shouldn't emit any power (or only a tiny fraction of a milliwatt), at least for a well designed set.

Therefore, antenna tuning matters almost not at all in receive mode; all you care about is that the antenna produces an RF voltage at the receiver terminals for the receiver to amplify, tune, detect, decode, etc. to eventually produce sounds of either Morse or voice. While impedance matching the antenna (which is what you're really doing when you "tune" to a given frequency) will improve the received signal strength (at best, maybe double it), with most modern receiver designs it's unnecessary. It might be helpful for things like a crystal set, or extreme DX, but the presumption here is that you just want to be sure the receive section of your freshly built radio works as it should.

From my personal experience, my Heathkit SB-102 receives fine with an FM antenna from an old stereo, on bands from 80 m up to 10 m (providing there is signal present), just as my old Hallicrafters S-120 multi-band receiver does. It wouldn't be a good idea to key up 100 W output to that antenna on 80 m, however; I'd be likely to damage one of my 6146 tubes or other components connected to them.

Zeiss Ikon
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    Fine answer! I'd like to add that I used to tune my HF antennas with my manual transmatch (antenna tuner), and I could roughly tune it by adjusting the knobs to get the loudest band noise. My point is that even though modern receivers have plenty of gain, receivers with a coaxial input connector will hear better from a matched antenna system. But considering that @rcx935 likely just wants to hear a signal, any signal, to validate that the receiver works, then a random wire antenna should be fine for that purpose. – rclocher3 Dec 15 '21 at 15:54
  • Thanks, @rclocher3 -- I've edited to note that weak signals or insensitive receivers could cause tuning to make a difference. – Zeiss Ikon Dec 15 '21 at 16:36
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    I'd like to add that a bad mismatch can mean 3dB of loss – it's pretty astonishing what communications engineering would go through to gain 3 dB (I wouldn't say it'd include murder, but some pretty sketchy things, I'm sure)! So, for testing, sure, makes little difference, but it's still pretty depressing to think that people would have a 400 GBP receiver with probably an excellent noise figure and consequently a good sensitivity and then throw out 3dB SNR just due to lack of tuning of the antenna. – Marcus Müller Dec 15 '21 at 16:42
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    Given my (typical?) urban RF noise floor, I often need to add attenuation rather than improving my HF wire antenna gain by impedance matching. – hotpaw2 Dec 15 '21 at 19:22
  • @MarcusMüller I might suggest that people who buy a 400 GBP stereo receiver expect the antenna that comes with it to work well. Then again, I've never owned any kind of stereo equipment that expensive. If that's a ham rig, then yes, they should know better (but might not care if they're doing no DX). – Zeiss Ikon Dec 15 '21 at 19:26
  • @hotpaw2 no questions there – but having the freedom to add an attenuation (and how much of it), or adding a notch filter instead of losing power across the whole band would always prefer the well-matched antenna over the loss. Not everything that doesn't work is an analogy, but have this analogy: My Ferrari becomes uncontrollable at 320 km/h, so I always drive with a trailer attached. – Marcus Müller Dec 16 '21 at 09:16
  • @ZeissIkon the mcHF is the 400 GBP kit I referred to, lifted that price figure from the first Google research for "mcHF shop" I found. – Marcus Müller Dec 16 '21 at 09:17
  • 100W? 80m? 6146 finals? Reminds me of the good old DX-100 – JustJeff Dec 18 '21 at 12:34
  • Heathkit SB-102 (which, unfortunately, has a problem in the transmit circuits). Wish I could get it running, but I'm not the guy to fix it, I'm afraid. – Zeiss Ikon Dec 19 '21 at 01:24