Switching radios on 1 antenna

[Nicholas J. Hirsch]

A friend of mine is trying to use one antenna with 2 radios in his RV-4

and wants to be able to and listen on both radios and transmit form

ether.... Is there a solution for this such as a combiner or RF

switch that can be used with 2 aircraft radios?

Thanks

Nick

[Jim Weir]

The military has developed a system whereby this is possible. It is complex,
heavy, expensive, and doesn't work all that well.

It is possible to mickey one up using relays and diode RF detectors, but it is
unreliable and at some time the failure mode will fire one transmitter directly
into the other receiver. It is a race to see which one burns out first.

Jim




"Nicholas J. Hirsch" >
shared these priceless pearls of wisdom:

->A friend of mine is trying to use one antenna with 2 radios in his RV-4
->
-> and wants to be able to and listen on both radios and transmit form
->
->ether.... Is there a solution for this such as a combiner or RF
->
->switch that can be used with 2 aircraft radios?
->
->Thanks
->
->Nick
->

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

[nauga]

Jim Weir wrote...

>....the failure mode will fire one transmitter directly
> into the other receiver.

Sudden, painful blinding flash of the obvious.
I'm glad I hadn't gotten around to trying what
I was planning with my handheld.

Dave 'smokey' Hyde

[Rob Cherney]

On Fri, 11 Jun 2004 07:02:03 -0400, "Nicholas J. Hirsch"
> wrote:

>Is there a solution for this such as a combiner or RF
>switch that can be used with 2 aircraft radios?

Yes, but it's not cheap (~$680). See:

http://www.comant.com/htmls/ci601.html

Rob-

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

[Jim Weir]

And let's do the math for the failure mode:

20 dB isolation in the receive mode, which is also the transmit failure mode.

20 dB is a voltage ratio of 10:1.

A 10 watt transmitter will put 22 volts RMS (32 volts peak) onto a 50 ohm line.

The receiver, 20 dB down, will see 3.2 volts of RF peak at its input IF the
input is purely resistive. All bets are off if there is a reactive component.

The little balancing resistor used in the Wilkinson hybrid in that device is a
quarter-watt resistor as I recall. Dumping 10 watts into it will frenchfry it
in less than a second, and THEN what is the isolation specification?

I don't know many base-emitter junctions that can take 3.2 volts and stay glued
together. Same for protection circuits at the input...100 milliwatts of RF is
pretty close to the dissipation limit of most devices used in light aircraft
radios.

$600 for two quarter-wave sections of coax, two reed relays, three BNC
connectors, and a metal box is a hell of a way to make a living.

Just some thoughts, mindya.

Jim





Rob Cherney >
shared these priceless pearls of wisdom:

->On Fri, 11 Jun 2004 07:02:03 -0400, "Nicholas J. Hirsch"
> wrote:
->
->>Is there a solution for this such as a combiner or RF
->>switch that can be used with 2 aircraft radios?
->
->Yes, but it's not cheap (~$680). See:
->
->http://www.comant.com/htmls/ci601.html
->
->Rob-
->
->------------------------------------------------------------------
->Robert Cherney e-mail: rcherney(at)comcast(dot)net

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

[Rob Cherney]

On Sat, 12 Jun 2004 09:04:51 -0700, Jim Weir > wrote:

<snip>

>Just some thoughts, mindya.

A reasonable implementation would receive PTT key signal, wait some
time to let the relays settle, verify that the relay contacts are all
in the correct positions, and then key the appropriate transmitter.
That guarantees that the race is won by the relays. This type of
circuitry would be relatively simple for a $700 product. Is that
what's done by Comant? I don't know for sure, but I would hope so.

I did, however, answer Nick's question. If he or his friend are
interested enough to pursue an informed answer, he can call Comant.


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

[Jim Weir]

And when the relay(s), an electromechanical device with a relatively low MTBF
goes south and dumps 10 watts into the other receiver, your analysis would
be...?

Jim


Rob Cherney >
shared these priceless pearls of wisdom:


->A reasonable implementation would receive PTT key signal, wait some
->time to let the relays settle, verify that the relay contacts are all
->in the correct positions, and then key the appropriate transmitter.
Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com

[Brian Whatcott]

On Sun, 13 Jun 2004 00:32:24 -0400, Rob Cherney >
wrote:

>On Sat, 12 Jun 2004 09:04:51 -0700, Jim Weir > wrote:
>
><snip>
>
>>Just some thoughts, mindya.
>
>A reasonable implementation would receive PTT key signal, wait some
>time to let the relays settle, verify that the relay contacts are all
>in the correct positions, and then key the appropriate transmitter.
///
>------------------------------------------------------------------
>Robert Cherney e-mail: rcherney(at)comcast(dot)net

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.)

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.

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.

Brian W

[Rob Cherney]

On Sat, 12 Jun 2004 22:22:46 -0700, Jim Weir > wrote:

>And when the relay(s), an electromechanical device with a relatively low MTBF
>goes south and dumps 10 watts into the other receiver, your analysis would
>be...?

I guess we're having a critical design review...

Since, for this exercise, I'm doing the design, I get to choose the
relay. I'm going to choose a Teledyne RF300-series relay. Here are
the specs:

http://www.teledynerelays.com/pdf/electromechanical/rf300rf303.pdf

So, what's the isolation at around 140 MHz? It's not 20 dB, it's more
like 40 dB. In addition, since there are two internal switches, they
can be wired in series. But, for the sake of argument, let's say that
we're limited to 40 dB of isolation. That's 0.32 volts instead of the
3.2 volts that you quoted. That's at or below the conduction voltage
of the protection diodes, assuming that they're placed at the input of
the receiver.

So, given that information, what are the failure modes of a relay that
we're talking about? It's not fair to say that all relay failures
would result in a fried receiver. The one that would cause damage is
where the relay contact (the one to the input of the receiver hybrid)
closes after it was commanded open. What are the chances of that?
Pretty low, I would say, for a relay that is rated for millions of
cycles. If you're really worried about this failure mode, put two
independent relays in series.

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

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

[Jim Weir]

Rob Cherney >
shared these priceless pearls of wisdom:

->
->I guess we're having a critical design review...

I guess so.


->So, what's the isolation at around 140 MHz? It's not 20 dB, it's more
->like 40 dB. In addition, since there are two internal switches, they
->can be wired in series. But, for the sake of argument, let's say that
->we're limited to 40 dB of isolation. That's 0.32 volts instead of the
->3.2 volts that you quoted. That's at or below the conduction voltage
->of the protection diodes, assuming that they're placed at the input of
->the receiver.

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.

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.


>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.

Jim


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

[Jim Weir]

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

[Rob Cherney]

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

[rip]

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

[Rob Cherney]

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

[[email protected]]

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$) *
************************************************** ***********************

[Rob Cherney]

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

[[email protected]]

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$) *
************************************************** ***********************