[Boatanchors] Front End Protection

[Carl]

Any decent coaxial relay will have 70 dB isolation at 10M and more at 
lower frequencies. The better relays are in the 80 dB range,

Either one is fine for an AM KW as long as its adjusted properly and the 
contacts are clean. At -70 dB worse case thats 150 microwatts into the 
receiver at 1500W PEP. A signal generator can put out more power.

Dow Key went to the spring loaded center pin on the NC side in the late 
50's for better isolation. See what is in yours and that it is free and 
functional.

Ive been running a Dow Key transfer relay, which is like a pair of 
regular ones in a single frame, at 1500W on 2M CW/SSB for 20 years 
without a problem or preamp failure.

I can run a fair amount of power on AM with either a pair of 250TH's or 
304TL's and use a Dow Key on one and a mil surplus no name on another. 
Isolation measured on a HP VNA at 160-20M is 82 dB worse case. No 
receiver coils are in danger even when running full bore.

Bias up a respectable size neon to fire at a half volt for backup to the 
relay or electronic TR.

Many commercial ham amps that are capable of 1200-2200W out use an open 
frame relay, even they dont cause damage. Ive measured the SB-220 and 
others in the -60 dB isolation range at 10M.

It was always nice to work in a RF lab with the necessary test 
equipment. Now retired, Ive duplicated a lot of it here at home.

Carl
KM1H

----- Original Message ----- 
From: "BLIMPY" <blimpboy at sonic.net>
To: <boatanchors at mailman.qth.net>
Sent: Saturday, November 08, 2008 7:15 PM
Subject: [Boatanchors] Front End Protection


>
> Even if you DO have a working coaxial relay... there is significant 
> lack of isolation of RF to the RCVR when running high power.
>
> Bias or B+ interuption in the RCVR will mask the fact that the front 
> end is being hammered with bunches of RF.
>
> Go measure it..
>
> Now.. it isnt very hard to actually Ground the RCVR Antenna with a set 
> of relay contacts across the input during transmit.
>
> This is low tech cheap insurance, and it works.
> Good idea to continue with the B+ interuption or biasing off the RCVR 
> as well.
>
> --  Thanks... glad to know I am not the only one who has seen the 
> coils on Dow Key brand coax relays.. litteraly blow thier tops across 
> the room.
>
>     Compare the coils and mechanics of the Advance and Dow Key  Series 
> 60 coax relays.. examine the contacts inside -
> then tell me which looks robust, and which doesn't.
>
> Kenwood, Yaseu, and others DID incorporate fuses in Reciever Antenna 
> circuits.
>
> Neon bulbs will provide protection from "normal"  high power adjacent 
> transmitters ( including broadcast).
>
> Seems like.. when a T/R switch fails... the end result is the xmtr 
> transmitting into nothing at all.. rather than the reciever.
> Least the way I wire them up !
>
> 73
> _______________________________________________
>