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Thread: Sharing Antennas Between Radios

  1. #1

    Question Sharing Antennas Between Radios

    I have been thinking about installing a 2m/70cm ham radio into my jeep wrangler. While I was doing that, I was also considering also adding a GMRS, CB or HF radios.

    Unfortunately, that would be 4 antennas, and with the soft top, there isn't a whole lot of locations to be able to place antennas. I think I know how I would mount all 4 radios, but I am having trouble figuring out antenna locations that won't get knocked off by trees.

    It seems like you could use a diplexer or a duplexer to achieve this, but I haven't seen anything that would separate out 420-450 and 460-470 and I haven't looked into 10m/11m separation.

    Do you all have any suggestions for achieving this? Do you all have any ideas on other ways to achieve this?

  2. #2

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    I do not think you will need to worry about that for 70cm/GMRS as antenna bandwidth in MHz gets better the higher up in frequency you go. Ive seen many antennas that could easily cover 70cm and GMRS at the same time. 10m/11m could also run off a shared HF antenna with a simple tuner. The issue is that it is illegal to use a ham radio on 11m or GMRS. It is not illegal, however, to do it the other way around. If you can find a GMRS radio that you can modify for use on 70cm, your ham license allows you to do so. Similarly, a MURS radio can be used by a ham on ham bands if you possess the skills to convert or reprogram it. Same applies to putting an 11m radio on 10m. If you can do that while keeping to the emission standards, you'll be fine, but if you expect to use the ham gear in other radio services, you will be breaking the law. Unfortunately, I am unaware of any commercially sold GMRS radios (at least in the US) that are readily put on the ham bands.

    This leads to the inevitable problem of running 2 radios, say a 70cm and GMRS radio, into a common antenna. The isolation you would require to not cook the other radio when one keys up is a job for a duplexer. A diplexer is good enough if the radios are on different bands, but 70cm and GMRS are far too close in frequency to obtain the necessary isolation without a set of cans. Luckily, duplexer cans are small at that high of frequency, but now the issue is finding a set that covers both the ham band and GMRS band. They do exist though, and could be made at home if needed. The same goes for 10/11m. You cannot simply connect both radios to a common antenna. Ive tried that using an antenna switch that grounds the unused port thinking it would keep the power of one radio out of the other, but that little contact couples enough power, even when shorted, to cause issues. We are talking about receivers that detect microvolts here. As with 70cm and GMRS, a diplexer will not give enough isolation between 10m and 11m and duplexers are 8 feet long at 10m. This means for use on one antenna, you will need to run one radio only on that antenna. Luckily, CB radios are easy to put on 10m and a connex or similar radio with a tuner should get you going.

  3. #3

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    Do you have any suggestions about where I might find a duplexer that would separate out the GMRS and ham bands?

    Do you have any information about where I might learn about designing and building such a duplexer?

    That would be greatly appreciated.

  4. #4
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    Quote Originally Posted by brandon lind View Post
    I do not think you will need to worry about that for 70cm/GMRS as antenna bandwidth in MHz gets better the higher up in frequency you go. Ive seen many antennas that could easily cover 70cm and GMRS at the same time. 10m/11m could also run off a shared HF antenna with a simple tuner. The issue is that it is illegal to use a ham radio on 11m or GMRS. It is not illegal, however, to do it the other way around. If you can find a GMRS radio that you can modify for use on 70cm, your ham license allows you to do so. Similarly, a MURS radio can be used by a ham on ham bands if you possess the skills to convert or reprogram it. Same applies to putting an 11m radio on 10m. If you can do that while keeping to the emission standards, you'll be fine, but if you expect to use the ham gear in other radio services, you will be breaking the law. Unfortunately, I am unaware of any commercially sold GMRS radios (at least in the US) that are readily put on the ham bands.

    This leads to the inevitable problem of running 2 radios, say a 70cm and GMRS radio, into a common antenna. The isolation you would require to not cook the other radio when one keys up is a job for a duplexer. A diplexer is good enough if the radios are on different bands, but 70cm and GMRS are far too close in frequency to obtain the necessary isolation without a set of cans. Luckily, duplexer cans are small at that high of frequency, but now the issue is finding a set that covers both the ham band and GMRS band. They do exist though, and could be made at home if needed. The same goes for 10/11m. You cannot simply connect both radios to a common antenna. Ive tried that using an antenna switch that grounds the unused port thinking it would keep the power of one radio out of the other, but that little contact couples enough power, even when shorted, to cause issues. We are talking about receivers that detect microvolts here. As with 70cm and GMRS, a diplexer will not give enough isolation between 10m and 11m and duplexers are 8 feet long at 10m. This means for use on one antenna, you will need to run one radio only on that antenna. Luckily, CB radios are easy to put on 10m and a connex or similar radio with a tuner should get you going.
    the type of disconnect you need is one that disconnects both the center core and the shield conductor as well but there is another problem,
    common ground conduction through the chassis and ground wire from the power supply.
    if in fact you can isolate all these paths then the unused radio is protected from harmful feedback.
    to that effect I am going to design something that will do just that (complete isolation).
    too much blood in my caffeine system.

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