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September 21st 03, 12:25 PM

On 20 Sep 2003 19:15:23 -0700, (Jason Hsu)
wrote:

Jack Smith wrote in message . ..

Why don't you define your requirements a bit for us, such as:
(1) power to be switched
(2) speed for switch to operate/release
(3) power available to operate the switch
(4) cost target
(5) frequency range (assumed to be 3-30 MHz since you are working on a
noise canceller)
(6) Isolation between TX and RX port
(7) Maximum acceptable loss

What are these parameters for the T/R switches in the MFJ-1025/1026
noise canceller and the T/R switches in the receiver circuits of
transceivers? The advertising for the MFJ noise canceller explicitly
says that it uses an RF sense T/R switch. I'm sure that transceivers
need T/R switches to prevent the RF from entering the receiver. That
can't possibly be good, because receivers are designed to be very
sensitive, NOT handle large amounts of power.

Jason Hsu, AG4DG

Jason:

A T/R relay in a transceiver has an easier job of it, because the
designer can sequence the operation, along the following lines:

Key Down -- Start TR Relay in motion -- wait a few mSec -- turn TX on
and start putting out power.

By sequencing the operation, you have two advantages (a) the relay
doesn't have to hot switch RF and (b) the relay doesn't have to be
blindingly fast, a few mSec is fine.

Your case is different -- you will have to sense the RF and hot switch
it. Still, a relay should do what you want.

I have a JPS noise canceller and it uses a simple relay. I suspect
something similar would work for you. I can recommend an Omron
G2RL-24 relay. You will be able to obtain at least 40 dB isolation up
to 50 MHz, and closer to 75 dB below 10 MHz. That's more than enough
to protect the front end. And, we have put well over 1x10^6 operations
on one hot switching switching 100 watts RF without failure. In fact,
I've got close to 100,000 operations switching 400 watts without a
failure so far.

Jack K8ZOA