Re: tx/rx isolation and safe tx input power for K3

Posted by Jim Brown-10 on Mar 01, 2011; 5:25pm
URL: http://elecraft.85.s1.nabble.com/tx-rx-isolation-and-safe-tx-input-power-for-K3-tp6077343p6078008.html

On 3/1/2011 6:17 AM, pendulous wrote:
>   I recently
> installed the DX engineering bidirectional Beverage systems.

Are you also using the DX Engineering preamp?  It's got a lot of gain,
more than is needed to bring your Beverages up to match other antennas,
and that could be contributing to a high signal level from your TX
signal.   Consider putting a simple passive attenuator between the
preamp and the K3.  You can build the attenuator from a few resistors.  
Another name for such an attenuator is  "pad." I suspect that there are
some formulas in the ARRL Handbook to calculate the resistor values.

Just working off the top of my head, I'd start with a 68 ohm resistor in
parallel with the line at the input of the pad, a series resistor of
about 270 ohms, and a second 68 ohm resistor in parallel at the output
of the pad.   This will be good for something on the order of 14 dB, and
will be close enough to a decent match for either 50 ohm or 75 ohm coax.
In this application, that's plenty close enough. If you want more
attenuation, increase the 270 ohm to 560 or 680 ohms, which will get you
a bit more than 20 dB.

73, Jim K9YC.


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