Connecting 2 radios to 1 antenna feed...

Discussion in 'General Technical Questions and Answers' started by KE5KZX, Apr 28, 2007.

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  1. KE5KZX

    KE5KZX Ham Member QRZ Page

    Is there a device available that will allow connection of two tranceivers to one antenna feedline and allow for simultaneous receive on both radios with some kind of RF relay to cut out the non-transmitting radio when transmitting on the other one?

    I currently have a jumper running from my antenna tuner to a coax switch, thence to the radios, but I cannot receive on both at the same time with this setup.

    Thanks and 73!

    Dennis - KE5KZX
  2. N3ATS

    N3ATS Ham Member QRZ Page

  3. KA9VQF

    KA9VQF Ham Member QRZ Page

    There is a thing called a duplexer.

    What kind of radios are you trying to listen to.

    I know you are smart enough not to try transmitting on either one. You probably would fry the front end of the one you had in receive mode.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    It can be done using relays that are "keyed" by the respective transceiver. Two relays are used, one going from the antenna to transceiver "A" and the other going from the antenna to transceiver "B". When transceiver "A" has the push-to-talk activated it opens the relay between the antenna and transceiver "B". When transceiver "B" has the push-to-talk activated it opens the relay between the antenna and transceiver "A".


    A duplexer would work for fixed frequencies on a VHF or UHF band. A diplexer would work if one radio was on one VHF or UHF band and the other radio was on another VHF or UHF band. However, to use on HF this is possible, but not practical because of the equipment required.

    Remember that a diplexer is the device that is used to couple two radios on different bands to the same antenna (or two antennas for different bands to the same radio). A duplexer is a device that is used primarily on repeater stations which allows the same antenna to be used for simultaneous transmitting and receiving. However, a duplexer can be used for coupling 2 transmitters or 2 receivers to the same antenna so long as the frequency is fixed.

    Now I know that several manufacturers use the terms duplexer and diplexer in their advertising where the correct term is diplexer. However, there is a BIG difference between a duplexer and a diplexer.

    Glen, K9STH
  5. VE1IDX

    VE1IDX Ham Member QRZ Page

    How do you figure that will work? It is simply a remote antenna switch meant to reduce the number of feedlines needed. It certainly won't do as KE5KZX wants.
  6. W5HTW

    W5HTW Ham Member QRZ Page

    It is certainly possible. But it is not easy to address. Of course, we really should know what kinds of radios (HF, VHF, UHF?) Are they both HF, both UHF? Is one UHF and one HF? Is one VHF and one UHF?

    However, it IS possible regardless. Receiving ONLY, (neither radio transmitting) is not a problem. Doesn't matter a whole lot whether HF, UHF, VHF, though it makes some difference as we have to have an antenna that is designed for the frequency in use. So let's pretend they are both VHF, and we have a VHF antenna in use.

    First, will we only be transmitting on Radio A? Or will we at times transmit using Radio B? If it is only Radio A, it makes it easier. If it is also Radio B that we will transmit on, it becomes markedly more complicated.

    For the sake of discussion, I will propose the following. Radio A is an amateur band VHF radio, and will be used in the amateur bands. Radio B is a volunteer fire radio, and is used exclusively in the VHF fire band. First problem of course, is the difference in frequency. 146 mhz versus 155 mhz. You need a broadbanded antenna. Such things exist, but are not really very common.

    When Radio A is keyed, it must disconnect the antenna from Radio B. A simple external relay accomplishes this. Since Radio B is far removed in frequency, overload is not a consideration, unless Radio A is running substantial power.

    However, when Radio B is keyed, it must disconnect the antenna from Radio A. We now have to have TWO antenna relays in the coaxial line. One of them is not operated when Radio A is keyed, and the other is not operated when Radio B is keyed.

    This can be done with the two external antenna changeover relays, and a few coax patch cords.

    But it gets worse if both radios are in the VHF band. For example, a fire dept radio and a police radio, both around 155 mhz. Now we have to not only change the antenna to the radio in use, and disconnect from the radio not in use, but also we have to desense the radio that is not being use for transmitting at the moment.

    There are ways to do that, too, and if the right changeover relays (like the Dow Key with extra contacts) are used, they can be wired to short the receiving radio's antenna directl to ground while the other radio transmits. We have done that in HF for decades, when using two receivers.

    So yes it can be done. But there may be an easier way. Think about it. I believe I would purchase a commercial radio (assuming this involves fire and/or police) that could also be programmed to operate in the ham band. Put the commercial radio in scan mode and you will be scanning both ham, and public safety frequencies.

    Again, there is a matter of antenna, but for low power and short range, you can operate with a considerable mismatch.

  7. N3ATS

    N3ATS Ham Member QRZ Page

    You're right! I inserted the wrong link. My bad! Sorry KZX!

    This will do it...
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    It will still require 2 relays to be able to transmit on either transceiver.

    Glen, K9STH
  9. KE5KZX

    KE5KZX Ham Member QRZ Page

    Thanks for all the ideas!

    The rigs involved are a Yaesu FT-897D and a Kenwood TS-2000 and my question is regarding the HF bands. I would like to be able to simultaneously listen to different frequencies and/or bands on each radio, and then transmit on the radio of choice. I understand that the antenna tuner is only set for one band at a time, but I can still hear, albeit not in an optimum fashion, on bands that the tuner is not set to.

    Hopefully this will answer some of the questions above.

    Thanks again and 73!

    Dennis - KE5KZX
  10. W5HTW

    W5HTW Ham Member QRZ Page

    Yes, two relays.

    OK, both are HF radios. Presumably you are using an external automatic tuner. If that is the case, the two relays, and a way of shorting the receive antenna to ground, on the radio not being used to transmit on, will work.

    Or, lacking an automatic tuner, a fan dipole or other multiband antenna that does not require tuning can handle the operation. That makes it fairly easy in the antenna department. Now you need the two antenna changeover relays.

    Not a difficult project at all, now that we know more about it.

    An external tuner such as the LDG that memorizes many settings and tune to them quickly would work. Resolve the antenna problem with such a tuner and/or with a good multiband antenna.

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