Switching radios on 1 antenna

Brian Whatcott
shared these priceless pearls of wisdom:

-If we are going to brain-storm it, let's dump the relays in favor of
-SSRs (that can be had in sizes less than a finger nail in real
-estate.)

Finding a SSR that will handle 10 watts is difficult. Finding a SSR that will
be relatively loss-free at 137 MHz. is more difficult. Finding a SSR that will
do both is difficult in the extreme.


-
-Let's by all means add fifty cents of security with back to back
-diodes to limit receiver input excursions to 0.6 volts or less.

At that sort of forward current, the excursions will be more like a volt. And,
those diodes have a burnout mechanism also.


-
-Come to think of it, how hard could a functional isolator be?
-They've been around for sixty years. But wait: its the bandwidth that
-makes those tuned things less practical - great isolation midband, but
-skanky at the band-edges.

You are kidding, no? A ferromagnetic isolator at 127 MHz. would be about the
size of a computer printer and weigh something on the order of a small child.

Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

On Sun, 13 Jun 2004 08:58:42 -0700, Jim Weir wrote:

But that's not the common failure mode. The common failure mode is for the
PTT/relay circuit to fail to respond to a "put me in transmit" mode. Thus, the
only isolation you have is from the splitter.

I've already said that you don't key the transmitter until all relay
positions are verified. You can do this at DC and still pass RF.

If you're really worried about this failure mode, put two
-independent relays in series.

Isn't that flawed logic like carrying a bomb with you on an airliner? The odds
of TWO people having a bomb are much smaller.

No. It's more like checking for a bomb twice. See above.

Lastly, I would not select a Wilkinson divider for the receiver. I'd
-use a transformer hybrid.

Your call. I note that under the common failure mode that your transformer
hybrid will have to handle the whole 10 watts. That's going to be one hell of a
transformer. The Wilkinson splitter will handle 50 watts without a whimper.

If the failure mode for the receiver is addressed, the failure mode
for the hybrid is addressed.

R-
------------------------------------------------------------------
Robert Cherney e-mail: rcherney(at)comcast(dot)net

You all seem to have lost sight of the fact that just about every
transceiver HAS a T/R switch in it already. If it's good enough
internally, it's good enough externally.


Jim Weir wrote:
Brian Whatcott
shared these priceless pearls of wisdom:

-If we are going to brain-storm it, let's dump the relays in favor of
-SSRs (that can be had in sizes less than a finger nail in real
-estate.)

Finding a SSR that will handle 10 watts is difficult. Finding a SSR that will
be relatively loss-free at 137 MHz. is more difficult. Finding a SSR that will
do both is difficult in the extreme.


-
-Let's by all means add fifty cents of security with back to back
-diodes to limit receiver input excursions to 0.6 volts or less.

At that sort of forward current, the excursions will be more like a volt. And,
those diodes have a burnout mechanism also.


-
-Come to think of it, how hard could a functional isolator be?
-They've been around for sixty years. But wait: its the bandwidth that
-makes those tuned things less practical - great isolation midband, but
-skanky at the band-edges.

You are kidding, no? A ferromagnetic isolator at 127 MHz. would be about the
size of a computer printer and weigh something on the order of a small child.

Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

On Sun, 13 Jun 2004 22:54:20 GMT, rip
r*nospam*quinby@snet.*nospam*net wrote:

You all seem to have lost sight of the fact that just about every
transceiver HAS a T/R switch in it already. If it's good enough
internally, it's good enough externally.

How do you tell Transceiver-A to isolate it's receiver yet not
transmit when Transceiver-B is transmitting? What connector pin on
the radio does this?

------------------------------------------------------------------
Robert Cherney e-mail: rcherney(at)comcast(dot)net

IIRC there's a "receive inhibit" on most radios. I had to add that wiring to
my KLX-135 and KY-197 setup so transmitting on one wouldn't open the squelch on the
other (probably 40 dB isolation between the separate antenna... enough for IMD).

-Cory

Rob Cherney wrote:
: On Sun, 13 Jun 2004 22:54:20 GMT, rip
: r*nospam*quinby@snet.*nospam*net wrote:

: You all seem to have lost sight of the fact that just about every
: transceiver HAS a T/R switch in it already. If it's good enough
: internally, it's good enough externally.

: How do you tell Transceiver-A to isolate it's receiver yet not
: transmit when Transceiver-B is transmitting? What connector pin on
: the radio does this?

: ------------------------------------------------------------------
: Robert Cherney e-mail: rcherney(at)comcast(dot)net

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

On Mon, 14 Jun 2004 11:10:23 +0000 (UTC),
wrote:

IIRC there's a "receive inhibit" on most radios. I had to add that wiring to
my KLX-135 and KY-197 setup so transmitting on one wouldn't open the squelch on the
other (probably 40 dB isolation between the separate antenna... enough for IMD).

I knew that such a thing existed, but it appears that it's not a
universal feature. My Apollo SL30 doesn't have it, even though it's a
pretty modern radio. I'll look again, though.

Are you sure it interrupts the RF input, rather than just squelching
the audio?

Rob-
------------------------------------------------------------------
Robert Cherney e-mail: rcherney(at)comcast(dot)net

Now that you mention it, I'm not sure... it'd be easier to do the latter.
The radio itself already has the hardware to switch the RF though (TX/RX), so it could
actually turn off the receive. Perhaps someone with service schematics could chime
in?

-Cory


Rob Cherney wrote:
: On Mon, 14 Jun 2004 11:10:23 +0000 (UTC),
: wrote:

: IIRC there's a "receive inhibit" on most radios. I had to add that wiring to
: my KLX-135 and KY-197 setup so transmitting on one wouldn't open the squelch on the
: other (probably 40 dB isolation between the separate antenna... enough for IMD).

: I knew that such a thing existed, but it appears that it's not a
: universal feature. My Apollo SL30 doesn't have it, even though it's a
: pretty modern radio. I'll look again, though.

: Are you sure it interrupts the RF input, rather than just squelching
: the audio?

: Rob-
: ------------------------------------------------------------------
: Robert Cherney e-mail: rcherney(at)comcast(dot)net

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************