How close for mobile antennas on the same band?

Discussion in 'Mobile Radio Systems' started by KD4UPL, Nov 17, 2018.

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  1. KD4UPL

    KD4UPL Ham Member QRZ Page

    I want to have two 2 meter radios in my truck, one for voice, one for APRS. I was thinking of having one antenna on the left front fender and one on the right front fender. This would put them only about 6' apart. If transmitting 50 watts on one rig would the received signal on the other rig be too strong and damage the receiver?
     
  2. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    This question gets asked about 4 times a year. Other than desense to the other transceivers, no damage will be done unless the antennas touch each other when damage might occur.
     
  3. KC8QVO

    KC8QVO Ham Member QRZ Page

    If you are using both radios at the same time the radio that is transmitting will make the radio receiving unusable. IE - when your APRS radio is transmitting it will wipe out voice comms on your other radio. That is the desense that occurs from the close transmitters.

    There is no way around this that is practical in a Ham application.

    Several years ago I worked with commercial radio systems on the UHF band (above 70cm ham band). We had a transmit-only base station. Some of the locations we put them in had existing UHF repeaters. So we were basically doing the same thing you are asking about - 2 radios on the same band next to each other (mounted on the same tower).

    How I got around the desense issue is I put filters on the radios. The repeaters already had duplexers so I put a bandpass filter on mine. I started with dual can Telewave filters, then upgraded to triple can filters. At over $1000 a piece they weren't cheap, but that is what we had to do to make it work. The bandpass filters are tuned to a very specific frequency. That is why they work in a commercial application where you are licensed to operate on one frequency. That is also why they will not work in a ham application where you need to have frequency agility - there is no way to get the filter to track the VFO on the radio (although, an interesting tidbit is the U-tune units that Yaesu came up with many years ago do this for tighter filtering on the HF bands).
     
  4. KD4UPL

    KD4UPL Ham Member QRZ Page

    Thanks guys. I figured this had probably been done before and would work but I wanted to make sure before I tried it and burned out a receiver.
     
  5. WD6USA

    WD6USA Ham Member QRZ Page

    Why not go to a single radio like the Yaesu FT400XDR and talk on the A Band and APRS on the B Band automatically? It's a neat and clean system.

    I'm far from being an expert, but this Yaesu radio has worked great in my space constrained Jeep.
     
  6. KD4UPL

    KD4UPL Ham Member QRZ Page

    I already have the Yaesu 400. It's a great radio. I was wanting to add a 220 rig to the truck. I happened to find a deal on an old Icom dual band 2m /220 rig. So I'm going to put that in the truck too. Which got me to thinking: I can run APRS and 70cm on the Yaesu and have 2m and 220 on the Icom. That will cover just about everything around here but I didn't want the Yaesu's APRS transmissions to hurt the Icom's 2m receiver.
     
  7. AI7PM

    AI7PM Ham Member QRZ Page

    I did not have this issue. My APRS antenna was about 30 inches from the other antenna. Other's results may vary.
     
  8. M0GVZ

    M0GVZ Ham Member QRZ Page

    I'd be surprised if you didn't. A friend of mine and I were talking to a few people via a repeater 10 miles away. We were both parked on the opposite side of the street to each other, both using 25W. When I talked he couldn't hear me and vice versa. De-sense. You will be experiencing it but by how much will depend on how near you are to the frequency the APRS is broadcasting on. The further away you are the less the effect.
     

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