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help...Multiple radios, one antenna?

Discussion in 'General Ham Radio Discussion' started by Moleculo, Jul 30, 2008.

  1. Moleculo Administrator Staff Member

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    Does anyone know of a way (on vhf/uhf or even hf) to have more than one radio on one transmission line / antenna and not use an antenna switch? I don't need to transmit at the same time, but i would like to be able to have one rig that is able to monitor on an antenna while another radio is actively using it on the same band. Is this even possible?


     
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  2. Beetle Well-Known Member

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    You want to use one antenna to transmit on, and to receive on at the same time?

    Unless you're talking about REALLY high frequencies (radar comes to mind), I'd guess not.

    With radar, the transmitted signal is pulsed briefly (microseconds of time) and then the transmitter shuts off while the receiver listens for the returned signal on the same antenna. While the transmitter is on, the receiver is muted.

    Don't use an expensive receiver if you're experimenting with this arrangement.
     
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  3. W5LZ Crotchety Old Bastard

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    That's basically how VHF/UHF repeaters work, which use a duplexer for simultaneous receive and transmit on the same band but not the same frequency. There are several "buts" when doing that, and not exactly cheap/simple to do. Same thing is possible with HF, but extremely 'not cheap' or 'simple'.
    - 'Doc
     
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  4. Captain Kilowatt Professional Amateur Staff Member

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    Three words come to mind. "Tunable bandpass filters." This approach is however neither simple nor cheap.

    Three more words come to mind. "I see smoke."

    Unless you have deep pockets and lots of know-how I do not recommend proceeding with this application.
     
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  5. Moleculo Administrator Staff Member

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    Well I can envision the smoke possibilities as I was thinking about this. I also understand how repeaters work, which is why I was trying to figure out how this might be possible. I really don't want to rx on the same frequency I'm transmitting on, just possibly within the same band. You guys understand the problem right? House in the city...limited antenna space, etc. Oh well, if it's too difficult or expensive to accomplish I guess I'll have to think up something else.
     
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  6. Captain Kilowatt Professional Amateur Staff Member

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    That's why I said tunable bandpass filters. They would allow what you want but they would have to be of extremely good quality (expensive) to have the filtering required to attenuate the TX freq enough to prevent desensing the RX.Differant bands would not be the problem but a TX and an RX on the same band gets dicey.
     
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  7. Escapade Member

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    1 antenna multiple radios

    Diamond sells a triplexer. HF/VHF/UHF or VHF/UHF/1.2 Ghz. this allows you to use 1 antenna with 1 feed line x 3 inputs. I use the 1.2Ghz version on my Ts 2000X, works very well with almost no loss. The cost depending on where you buy it is between $69.00-$89.00. I have seen them on E-Bay for 59 bucks. And yes you can receive on the other bands as you transmit on another, no switching involved. Did I just start a sentence with "and"?
     
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  8. Moleculo Administrator Staff Member

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    Ahh, but the question wasn't about TX/RX on different bands...it was about doing so on the SAME band. The triplexer/duplexer won't work for that.
     
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  9. Escapade Member

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    your right I did not realize he wanted to monitor the same band he was transmiting on. You may not be able to do that even with seperate antennas in close prox. with out all kinds RF interference let alone on the same antenna and feed line.
     
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  10. W5LZ Crotchety Old Bastard

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    There really isn't a simple answer. It's certainly possible, but not very practical, sort of. Even with different antennas, two radios on the same band in moderately close proximity will cause problems. Those problems might not rule out the possibility of doing it, but certainly makes for less than optimum receiving (lots of variations in that 'optimal' thingy). The problem arises because of the transmitter being active on some 'connected' way, not because of the receiving thingy. There really are ways to get around that energy being fed into the receiving antenna, but they are not going to be simple/cheap, or therefore very practical.
    Widely spaced transmit/receive frequencies don't interfere with each other all that much. Fairly closely spaced transmit/receive frequencies do (same bands). (I'm really telling you something new, right?) While your idea is possible, I'm too lazy to mess with it, rather try a different way of going about it (hide that @#$ antenna! ;)).
    - 'Doc
     
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  11. Happy_Hamer Administrator Staff Member

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    My Kenwood TM-D710A can rx on both sides of the radio on the same band but when I TX I cannot hear on the same band on the other side, which is good.

    is that what you mean?
     
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  12. Happy_Hamer Administrator Staff Member

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    OH and on my TS-2000 I can do the same, I talk on 144.260 USB on the A side of the radio and monitor 146.535 FM, our local simplex chat group, on the B side of the radio, you know, because I can (y)
     
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