[Elecraft] Using K2 rant input

[Trevor Day]

Hi Vic,
Have you thought of just switching the rx antenna out of circuit whilst 
transmitting?  Unless you are using fast break-in, a small externally 
switched relay should solve your problem.  If you are feeling 
particularly paranoid:-), then use the same relay to earth the rx input 
via a 50 ohm resistor at the same time.

gl, Trev G3ZYY

In message <420C3EF6.2060509 at rakefet.com>, Vic Rosenthal 
<vic at rakefet.com> writes
>I am experimenting with using the K2 rant input (that's receiver 
>antenna, not verbal outburst).
>
>I want to use my 20-meter quad as a receiving antenna when I am 
>transmitting on 160 with an inverted L.  It is a lot quieter.  The L is 
>right next to the quad, so a lot of voltage is induced in the quad when 
>transmitting with high power.
>
>I tried putting a couple of back-to-back diodes and a 50-ohm resistor 
>across the quad feedline.  I put my scope across it and found that the 
>signals from BC stations, etc. were well below the clipping level of 
>the diodes, and received signals were plenty strong with the K2 preamp 
>on. So far so good.
>
>Transmitting resulted in the clipping level being reached with about 
>100 watts output (I have an external amplifier capable of 1500 watts).
>
>All of a sudden, I realized that I have built a huge harmonic QRM 
>generator!  If I use this setup,  as soon as the diodes clip they will 
>generate harmonics all the way up into VHF, which will be re-radiated 
>by the quad (it will do a real good job on the 4th harmonic, near 20 
>meters, too).
>
>Am I right?  If so, this isn't a good way to do what I want.  Does 
>anyone know a) how much RF voltage can be safely applied to the K2 rant 
>input when transmitting, or b) how to protect the input without making 
>harmonics?  I thought of using the diodes after a lowpass filter that 
>cuts off around 2.5 MHz,  but this is getting complicated.
>

-- 
Trevor Day
UKSMG #217
www.uksmg.org