Loud feedback problem, mobile HF

Discussion in 'Mobile Radio' started by N0JMP, Jun 2, 2016.

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  1. N0JMP

    N0JMP Ham Member QRZ Page


    I have been running an Icom 725 into an Icom AH-4 auto tuner into a screwdriver antenna on a magmount on the roof of a 2003 dodge caravan. The power goes straight to the battery through a cigarette lighter plug that I installed and everything is grounded to the roof of the car. The station seems to work exceptionally well, as I have made numerous DX contacts in the last month or so, except that when I have the rig running on the car battery I will usually get very loud feedback from the receiver when I transmit. When I use an external battery this problem does not occur. Earlier today I opened the hood and ran the radio straight to the battery and I still got the feedback, however it was not as loud. I also moved wires and tried many different setups. The only way I could get rid of the feedback completely was to either run the radio to the antenna barefoot or to use a power source other than the car battery. If anyone knows why this would occur or has ideas on how to fix it I would be very happy to hear what you have to say.

    Thanks and 73
  2. WR2E

    WR2E Ham Member QRZ Page

    You sure the lighter plug can handle the current?

    What do you mean by 'feedback'? Loud squealing? Hearing your voice? Please elaborate.

    You connect the radio to an external battery with it still installed in the vehicle, and using the same antenna?

    You mean that you also have an amplifier?

    My SWAG is that you have RF getting into the power wiring.

    Try some of those snap-on ferrite cores on the power cable to the radio, see if that helps.
  3. N0JMP

    N0JMP Ham Member QRZ Page

    The only reason I believe it can handle the current is because I used to run the same setup with an Icom 706MKIIG with no issues. As far as feedback goes, I hear my own voice, usually very loud and very distorted. By external battery I mean I use a car battery that is in a battery box. It is usually sold as a jump starting box, I've just adapted it. I have also tried a power supply to the radio from the 120 volt socket at my house with an extension cord just to test it. I didn't have the issue when I did that. Please excuse my misuse of terminology, I was tired and trying to post this quickly, it should've read that I did not have the problem when I ran the setup without a tuner. I will try your suggestion and report back.

  4. WR2E

    WR2E Ham Member QRZ Page

    No problem on the terminology, I just wanted to make sure I understood the problem!

    Why use a tuner with a screwdriver? Isn't the idea of the screwdriver that you tune the antenna with the up/down buttons and don't need another tuner?

    Is the AH-4 well grounded?

    It does sound like 'RF feedback' so the snap-on ferrites might work.
    Last edited: Jun 3, 2016
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Is the feedback coming from a external speaker ? If so, RF may be getting on the wire.

    When you transmit the receiver is muted in the radio, not sure what you mean.

    Does it do the same when only 1 VFO is on ?

    Good Luck.
  6. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Well, the most likely problems are
    -Pure RF overload
    -Common mode on coax, power line, control lines
    -Lack of antenna grounding, as a magmount

    What sort of chokes are you using on the coax and control lines?

    Please read Alan's excellent site: www.k0bg.com
    KB0TT likes this.
  7. N0JMP

    N0JMP Ham Member QRZ Page

    A few turns through a ferrite bead solved that issue. I used the radio for about 30 minutes before I encountered another problem. The audio would suddenly quit working. Only way to bring it back was to turn on the preamp or to key the tuner. The problem came back quickly though. Tried it without the tuner, problem still occurred.
  8. WR2E

    WR2E Ham Member QRZ Page

    I'm still curious about this though...

    You're talking about the NEW problem here, right? The ferrites fixed the feedback problem? You don't mean that the feedback problem returned, right?

    OK, so now you've got a problem with the radio.

    Are you still taking the power from the lighter plug? If so, you need to get rid of that. I don't believe I've ever seen one that was rated at a high enough current for running a radio, unless it's a QRP rig.
  9. WA4HMR

    WA4HMR Ham Member QRZ Page

    If you "must" use a connector for your power, get a trolling motor connector male and female combo. Most are rated in the 40-50 amp range.
  10. K4KWH

    K4KWH Ham Member QRZ Page

    I was going to say the same thing. Go DIRECT to the battery and try to go the opposite way from the car's engine computer. Keep wiring as short as possible (ya gots to get to the radio of course ;) ). Check all grounds; if you think it might need a ground, DO it. I would, first, be highly suspicious of that lighter plug. That's designed for a cig lighter OR low current draw electronics such as a GPS, etc--NOT for an HF radio that *may* draw as much as 15-20 amps @ 100 (?) watt transmitter.

    Every HF operator needs to know that HF is NOT the same as 2 meters and has entirely different characteristics and needs than VHF. So many times we assume that because we once were able to plug in a 2 meter rig, slap on a mag mount antenna and go, we can do it with HF. It would be nice (?), but it ain't so. There ain't no "shortcuts" for HF, and no compromises, either. That is, if you want truly successful HF operation, one must go "the extra mile". "It takes a big 'ugly(?)' antenna for true HF success". (Don Johnson, W6AAQ silent key), and that holds true for most every other aspect of HF mobile.

    I do suspect that, since the op has already tested this by bypassing this lighter plug, and it STILL fed back, there could be a ground problem. How is the antenna mounted? Where? Is the vehicle actually metal? Lots of things to consider. Its hard to "diagnose" a problem from afar. So these are just suggestions to help.


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