Dealing with desensitization in mobile

Discussion in 'On the Road' started by W0TLP, Oct 23, 2013.

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  1. W0TLP

    W0TLP Ham Member QRZ Page

    I just installed a second VHF radio in my truck to run APRS. It's a 2-meter mobile with a Tracker 2 TNC interfaced to the GPS. This rig runs only APRS.

    It is causing desensitization on the second radio used for voice. Whenever it sends a packet, the APRS radio it silences the RX on the other. I've noticed only on VHF. I have not tried it on UHF. It's a minor annoyance in a QSO, but I'd like to try to address it.

    The dual-bander antenna is a Larsen 2/70 mounted over the left front quarter panel with a trunk lip mount. The APRS antenna temporarily is a mag-mount near the center of the pickup cab.

    I intend to mount a more permanent antenna for the APRS as far from the other antenna as I can, but I am limited in location: It's a crew-cab truck with a fiberglass topper shell. I frequently carry a canoe, so roof center is sketchy.

    Does anyone have suggestions for antenna, mount and placement for the APRS antenna that might address this? It just needs to be good enough to deal with packets, and I will usually transmit at 5 or 10 watts, but may occasionally give it the whole 55 watts when the nearest digipeater is a long distance.

    If I can't address it through antenna separation, do I have any other options?

    Thanks.

    Teak
    W0TLP
    St. Louis
     
  2. W0TLP

    W0TLP Ham Member QRZ Page

    I've realized this should be under "Mobile Radio Systems." Is a moderator able to move the thread?

    Thanks
     
  3. AA4HA

    AA4HA Ham Member QRZ Page

    It's really difficult to cure a desense problem on receivers in the same band when the antennas are so close to each other. The first choice would be to separate the antennas as far as possible. Could you mount the APRS antenna at the back of the truck by the tailgate? (I assume it is a pickup type truck). If you created a small ground plane you could even mount the whip on the fiberglass shell.
     
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Vertical separation is best.

    If you could lower one and raise the other, Or put one antenna at a slight tilt. Or build a very sharp filter.


    Good Luck.
     
  5. K2HAT

    K2HAT QRZ Volunteer Volunteer DX Helper QRZ Page

  6. KK4BQT

    KK4BQT Ham Member QRZ Page

    I have a 22' pontoon boat with a marine radio mounted at the stern. No matter where I go on the pontoon, laying down on the carpet or standing up on the seats with my arm stretched out, the HT I carry suffers from desense. Only one frequency though. The one I want to monitor.
     
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