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Thread: Scanner & APRS on one antenna

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    Default Scanner & APRS on one antenna

    Hi all!

    Working on adding APRS in to the mobile rig. I've got everything figured out except the antenna, but I'm really trying to avoid the "just add another one" strategy that some people love and turns clean vehicles into porcupines.

    I've got two VHF quarter waves right now, one for my scanner and another for my NX5k. I'm thinking of tapping into the scanner antenna and adding some kind of DIY duplexer that will notch out the APRS freq from the scanner's RX, and only pass that same frequency on to the modem. Essentially a duplexer, except with one port (the scanner) only having a notch around 144.39.

    I was thinking about re-purposing one of those el-cheapo VHF mobile duplexer sets, but even those are a couple hundred bucks and I'd really like to keep costs down as much as possible.

    Wondering if anyone else has done something similar. All ideas welcome, and DIY suggestions are a big plus. Even some kind of coax-stub assembly would probably be relatively easy to rig up. Despite my undergrad focus in EE, I'm not super familiar with all the tricks that are out there and figured this could be an interesting brainstorming session for the site.

    Thanks in advance!


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    It wont work. Unfortunatly the scanner front end is basically a wideband reciever. Anytime you change freq you will pass outside the notch and kill the scanner. you may do it anyway if the notch is deep enough scanners are notorious for missing stuff like overload protection on strong nearby signals.

    In the past I have mounted a super thin 1/4 VHF antenna (painted black) in the rear window or front for that matter. Look into the early suction cup style antennas that used to come with uniden scanners.

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    Quote Originally Posted by Notarola View Post
    It wont work. Unfortunatly the scanner front end is basically a wideband reciever. Anytime you change freq you will pass outside the notch and kill the scanner. you may do it anyway if the notch is deep enough scanners are notorious for missing stuff like overload protection on strong nearby signals.

    In the past I have mounted a super thin 1/4 VHF antenna (painted black) in the rear window or front for that matter. Look into the early suction cup style antennas that used to come with uniden scanners.
    Well, I wouldn't be changing frequencies, I'd be camping on just one: 144.39 which is the main APRS freq for North America (you probably know that though). Still I get what you're saying, but my line of thinking was that if I could get ~80dB notched out of the scanner's receiver at around that frequency I could probably make it work. A little desense is fine, plus it's small packet bursts so we're not talking more than a few hundred milliseconds of TX. I'm mostly concerned with protecting the frontend of the scanner and I wasn't really expecting to be able to get away with full duplex operation.

    Adding another antenna in the window is the other fallback plan though. Sadly even the best antenna on the inside of the vehicle won't work too well. In the last truck I had a quarter wave in the passenger rear window and it could barely hear the nearest digipeater even after getting it nicely tuned up. There's just too much metal surrounding it from what I've found.

    Third alternative, although it's probably even worse, is swapping my UHF phantom for one of those sketchy "dual band" can NMO antennas that are all over eBay and running that with a V/U diplexer. I'm sure they're essentially a dummy load on 2m though so obviously that's a last resort option.

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    If you can get your notch deep enough the math and experience says you should be ok. But its a fine edge your on as mentioned scanners dont have the front ends commercial radios have. I would suggest you get a cheap scanner do a proper sensitivity check on it. Then connect everything up and do some TXing. Once your satisfied the TX is enough. Pull the scanner and retest the rx sensitity. If it survived your good to go with your main scanner.

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    I've thought about how to solve this same problem in the past with my vehicles too. I ended up going the "add another antenna" route and abandoned the project but I was heading down a path looking for an RF triggered switch that could switch out the scanner when I transmitted into the antenna, hopefully protecting the scanner. The only concern I had was whether or not the switch would be fast enough not to kill the RX before it switched out.

    Just another angle to consider.

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    The switch wouldnt be fast enough. In theory you could use pin diodes like the PA deck uses to ground out the scanner antenna. My impression is the op wants to be able to scan and TX at the same time. The 2 antenna approach is the way to go. A notch on the scanner antenna for the TX freq would do a lot to prevent front end overload.

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    Mechanical switches can be plenty fast enough. Shakespeare makes or used to make a switch for combining two 25w VHF marine radios to a single antenna. When you transmit on one it senses the RF and routs the antenna there with a latching relay. It stays on that antenna until you transmit on the other radio then it switches and stays again. I have one somewhere around here in a junk box.

    You could probably use a 12v coaxial relay that is triggered off the PTT of the APRS radio. When APRS does its brief transmission it would switch the antenna then it would go right back to the scanner. Since each radio is on a separate port of the switch there would never be enough RF energy to damage the scanner.

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    Something like that could work. There's definitely a few options here. Don't even necessarily need a coaxial relay; something like an Omron G2RL with a properly-laid out board has great S-parameters up into VHF and beyond.

    Right now, I've just got a power splitter feeding two RTL-SDRs and my scanner from the second VHF whip. One of the SDRs is connected via Direwolf to an APRS client on my mobile PC. I'd have to scheme up a way to allow the client to use a second APRS modem for TX only, but that likely wouldn't be too difficult.

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    I guess I got hung up on the wants to scan and TX at the same time.

    If just isolating the scanner while TXing is the only issue then everyone is correct any good antenna relay should do the trick.