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Rf sense keying cir cap size


Yeah its turned off, touch it
May 9, 2011
Anyone tried or noticed a difference in keying activation wattage when using a different size cap to sample the rf?

Yes. In my experience, generally the higher the input wattage, the smaller the cap (pF) can be. If it is an AM only box, then it has to be large enough for the carrier wattage to keep the relay keyed. If using SSB, it will need to be a bit bigger to try to minimize relay chatter. Obviously, adding an electrolytic for SSB use has it's pros and cons.
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The current gain of the keying transistor determines how much diode current is needed to drive it. The series cap's reactance serves to limit that current. Typical practice is to add a resistor in series with the disc cap. The less diode current you need to activate it, the more headroom you have to withstand excess drive power without being damaged.

The keying circuit we sell on fleabay uses a darlington transistor. The high DC current gain reduces the diode current needed. This allows using a relatively high reactance plus resistance between the radio and the diodes.

Here's the one we sell.


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I'm using a 33uf at c1 right now. I see a around 1 watt drop for each keying cir I add. I dont have a resistor inline with mine though.
The resistor is a big deal. At least 1k ohms. Our setup uses higher values. This will eliminate the elevated SWR problem.

You did mean 33 picofarads (pf), right? A 33 microfarad (uf) cap is way beyond left field by a factor of 1000.


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