[600MRG] TX RF protection for RX antenna

[Roger Graves]

Hi Paul,

If the termination resistor is 470 Ohm, then 1W dissipation would require 22 Vrms across it. You could put a back-to-back pair of circa 22V Zener diodes across the resistor to limit the dissipation to 1W. A low value resistor in series with the diodes would reduce some of the RFI generated by the Zeners. Capacitance of the Zeners would need to be evaluated.

Just a thought. 

73,
Roger VE7VV

> On Dec 10, 2024, at 9:03 AM, Paul N1BUG FN55mf <paul at n1bug.com> wrote:
> 
> OK, here is one I could use some help with.
> 
> I am using a large K9AY loop (twice the 160/80m size) for receiving on
> MF and LF. It's the best of any antenna tried so far.
> 
> The problem is that LF, MF and even HF RF can get into it in sufficient
> quantity to burn the 3 watt termination resistor and damage the
> transformer! How can I protect the loop components from damage?
> 
> Using a relay to disconnect or short something is not an option. I
> cannot run control cable to the loop. Coax is already used for the
> voltage to switch directions on the loop.
> 
> What do you suggest I should use to shunt the RF from both sides of the
> loop to ground so components won't get damaged? My first thought was
> diodes (I'm old school) but I'm not sure small ones eg. 1N4148 can
> handle the few watts of RF power and big ones appear to have too much
> capacitance.
> 
> Of course I also need to protect my delicate SDR receiver, that's
> another whole can of worms. At least for now I can try to remember to
> always disconnect it before transmitting.
> 
> 73,
> Paul N1BUG
> 
> 
> 
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