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I am building the T/R switching circuit for an HF linear amplifier covering 80m band to 20m band. The current circuit takes a potential-free contact from the exciter and use this contact to turn on/off the relay. When PTT is pushed relay contacts connect the RF output from exciter to HF linear amp input and output of HF linear amp to antenna. When PTT is disengaged the linear amplifier is bypassed to connect the antenna directly to the exciter.

I like to avoid the PTT connection from the exciter to HF linear amp, by sensing HF carrier from exciter output and using this input to switch the relay. Problem with this approach is that when using SSB modulation, relay opens briefly as carrier is absent when audio is briefly at a low level. How to get around this problem?

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  • $\begingroup$ I'm not very good at amateur radio terminology: what's an exciter, in this context? $\endgroup$
    – Marcus Müller
    Commented Dec 30, 2019 at 13:17
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    $\begingroup$ @MarcusMüller As far as I know, “exciter” is more common as broadcast terminology than amateur; it includes the RF oscillator and modulator, which generate the initial RF signal, and excludes the power amplifier which drives the antenna. $\endgroup$
    – Kevin Reid AG6YO ♦
    Commented Dec 30, 2019 at 14:35
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    $\begingroup$ Kevin is exactly correct. "Exciter" is used in the amateur realm, but in this day of one-box, off-the-shelf rigs, it's becoming an anachronism. In years past I heard it more commonly in the VHF/UHF world, but applies anywhere. $\endgroup$
    – Duston
    Commented Dec 30, 2019 at 15:11
  • $\begingroup$ Then I'm a bit confused, because why would a T/R switch connect the antenna either to the HPA or directly to the transmitter? Shouldn't it disconnect from both and connect to the receiver? $\endgroup$
    – Marcus Müller
    Commented Dec 30, 2019 at 15:59
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    $\begingroup$ To me, "exciter" is a term in fairly common amateur radio use, in the context of HF, to mean the transmitter whose output goes to the external amplifier. $\endgroup$
    – rclocher3
    Commented Dec 30, 2019 at 19:03

3 Answers 3

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If you rely on detecting RF output to activate the T/R relay, you will be switching it while there is already RF output (obvious, I know...) this leads to two problems: the switching can cause a transient high SWR that may damage the exciter, and switching under power can cause arcing which damages the T/R relay itself. Even if it doesn't cause an immediate failure it can put more stress on the equipment leading to premature failure. This is why best practice is to use a separate keying line to engage the T/R relay slightly before the exciter starts making power.

But to answer your question directly, equipment that does T/R switching for SSB usually has a circuit which adds a short delay before un-keying, usually a hundred to a few hundred milliseconds, and usually user-adjustable. This avoids excessive switching during speech.

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  • $\begingroup$ As you said, I understand that adding a short delay before unkeying prevents the PA from going into standby/bypass mode during short pauses of SSB transmission. The other problem can't really be fixed as the control circuit of PA takes a short while to detect the carrier and operate the the relay. To use this circuit i may have to make sure that exciter is operating at low power levels (so that there is no damage due to reflected power and no arcing in the relay). $\endgroup$
    – SRK
    Commented Dec 31, 2019 at 4:44
  • $\begingroup$ Another option i see is injecting the DC PTT line signal into the coax connecting exciter to PA, and recovering that signal inside PA.Why i needed this circuit was to reduce wiring. $\endgroup$
    – SRK
    Commented Dec 31, 2019 at 4:46
  • $\begingroup$ @SRK yes, the other problem is basically impossible to fix :) $\endgroup$
    – hobbs - KC2G
    Commented Dec 31, 2019 at 6:40
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As I understand you, your amplifier looks for a certain level of RF power at the input. If it is there, then the amplifier switches to active mode; if the RF power isn't at the input, then the amplifier switches to standby/bypass mode.

The advantage of using that switching method must be that no secondary connection for a switching signal between the exciter and the amplifier is necessary. The disadvantage is that in SSB, when the audio input level is low, the RF power from the exciter is so low that the amplifier doesn't go into active mode. Also, when the audio input level rises, then there is a switching delay.

You could try adjusting the threshold level, so that the your amplifier kicks on a little sooner, or work on making the amplifier switch on faster to reduce the switching delay. (Vacuum relays, or PIN diode switching perhaps?) If that doesn't work, then it seems to me that you need to either accept things the way they are, or else design your amplifier to include a separate switching input.

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Add a delay capacitor to the base of the switching transistor.....

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    $\begingroup$ Hello Carmine, and welcome to ham.stackexchange.com! Would you please edit your answer and expand it so someone new to the subject can get a handle on what you're talking about? $\endgroup$
    – rclocher3
    Commented Jun 30, 2020 at 0:15

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