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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 26th Apr 2017, 6:06 pm   #1
3Phase615V
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Default Extending the range of RX Meters

Hi,

I have noticed that while aligning CB a few particular manufacturer's transceivers the expected S9 signal is much lower than 100uV and that in fact "+30dB" is set for around 100uV. Curiously, this is case even in one service manual where it advises setting S9 at 100uV but that's the end of the range, so no matter how much greater the signal strength the RF meter needle goes no higher. So, my question is how easy or difficult is it to modify the radio to allow up to what should be around an S+30 signal of about 3.16mV or more? I know these meters are not well calibrated and don't have anything like 6dB spacing but I would like for not just any old signal to near-ish station to hit +30 on the RF meter.

Thanks!
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Old 26th Apr 2017, 6:16 pm   #2
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Default Re: Extending the range of RX Meters

These meters are normally referred to as "S meters" meaning S for signal strength. If you look up RX meter, you'll come across the Boonton Rx meter, a classig piece of lab test gear for precisely measuring RF components (they're a bit of a devil to drive)

There is no standard whatsoever for readings versus applied signal voltage. Some people have preferences, manufacturers like to make them overly sensitive so it makes their sets look more sensitive. S meters on radios are often not very linear. One preference is for 6dB signal change per S-point. (6dB = doubling of voltage)

As most S meters run from the AGC control line, as seen in terms of the bias current in an AGC-controlled transistor (or valve) you can't do much to change the meter scaling without drastically changing the GC function and sensitivity. Playing about with an opamp adding a DC offset to the meter might be the only realistic option.

Usually you can expect more deflection means more signal, and that's about it, I'm afraid.

DAvid
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Old 26th Apr 2017, 8:51 pm   #3
3Phase615V
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Default Re: Extending the range of RX Meters

Thanks for the detailed explanation, David. Ah well, I guess I'll just have to make do with the way it works then.

Much appreciated. 73s!
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Old 26th Apr 2017, 10:21 pm   #4
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Default Re: Extending the range of RX Meters

Yeah, 'fraid so. And the ones on amateur radio gear are no better than those on CB radios.

I makes a right joke out of those people who read their meters to give signal reports.... that and the differences antennae make.

Cheers
David
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