[Elecraft] tx/rx isolation and safe tx input power for K3

[Jim Brown]

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.