[Elecraft] K3 Sub Receiver and protecting the Sub RX input

[Edward R Cole]

Semi-related topic:

Recently, I was hosting a local 2m-SSB net and had my KX3-2M 
connected to a 7-element yagi pointed 90-degrees from the 
transmitting antenna array (K3+xvtr+150w amp).  Spacing is about 
40-feet horizontally and 30-feet vertically (Tx ant higher).  Its 
normal to run the Preamp with the KX3 on 2m and when I transmitted 
the signal was strong enough to trigger overload protection in the 
KX3 which disables the preamp.  Probably 25-dB front-back ratio on 
the transmitting antenna and maybe 15-dB rejection off the side of 
the Rx antenna (both yagis) so estimated power at the KX3 is +52-dBm 
- 40dB = +12 dBm (not including space loss between antennas).  +12 
dBm into the KX3-2M is pretty marginal for safety of the 
preamp.  Probably actual level is closer to 0 dBm (because I didn't 
include coax loss or space loss).  -33 dBm signal from my XG3 will 
drive the KX3 to S9+40 (-73 dBm=S9*).  *with preamp=ON.

Since antennas are within near-fields, ordinary space loss formula do 
not apply.  That is the typical situation with the KRX3 used on HF.

1-watt is way too high for safety for my eme preamps.  +5 dBm is my 
maximum tolerance.  >+15 dBm is certainty of blow out.  (I disconnect 
other antennas when running 1300w on the main array)

73, Ed - KL7UW
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