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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 |
#2
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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 |
#3
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#4
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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 |
#5
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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 |
#6
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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 |
#7
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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 |
#8
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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 |
#9
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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 |
#10
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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 |
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