Switching radios on 1 antenna

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

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

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

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

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

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

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

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

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/el...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

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