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Radio being powered through Antenna this normal?

Discussion in 'Mobile Radio Systems' started by K6JYC, Oct 25, 2020.

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  1. K6JYC

    K6JYC Ham Member QRZ Page

    Hello All,

    I apologize in advance for the noob question, but I want to ask the group if what I am seeing is normal.

    I have a Kenwood Tm-V71 mobile radio, powered by a direct connection to the vehicle battery with both the negative and positive leads fused closed to the battery.

    The feedline runs from the radio, through the firewall and to a hood lip mount with a dual band antenna attached with a PL-259 connector. The antenna mount bracket is attached to the vehicle with an existing sheet metal bolt creating a solid electrical connection. I'm assuming this means the system is now grounded to the vehicle, and negative current is flowing thought the mount and thus through the external coax shielding back to the case on the radio.

    I have discovered that even after removing the fuse in the negative power lead, the radio turns on--which indicates that system is being grounded & powered by the negative return through the antenna and feedline.

    My question is: is this normal? Or should I insulate the antenna mount so no current flows through the system in that fashion?

    Thank you!

    AG5DB likes this.
  2. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    It is normal and inevitable. You just want to be sure your "official" negative power lead is conducting a lot more current than your coax shield.
    WA7ARK and KB0MNM like this.
  3. KC3PBI

    KC3PBI XML Subscriber QRZ Page

    In a mobile installation, the electric supply ground and the antenna ground often need to be the same thing, the car itself. Usually this is fine with regards to vehicle and radio operation.

    Only the fuse on the positive leg is serving as overcurrent safety for your radio. The fuse in the negative leg isn't really doing anything. I'd eliminate it to cut confusion.
    WA7ARK and N3EG like this.
  4. WB2LBV

    WB2LBV Ham Member QRZ Page

    Remove the negative battery connection and replace it with a good body or frame ground as close to the radio as possible (i.e. a seat bolt if the radio is under a seat). The cable from the radio to ground should be short and NOT fused.
    You do want the antenna to have a good ground for RF but you don't want your DC power flowing through that ground path.
    WA7ARK, N3EG, K3UJ and 5 others like this.
  5. AI7PM

    AI7PM Ham Member QRZ Page

    What WB2LBV said! I can't figure out why the ham eqiup manufacturers are still publishing this "direct to the battery" non-sense. Especially in this age of vehicles with battery monitoring/maintainng systems that specifically say do not connect directly to negative.
    N3EG and W9WQA like this.
  6. K7JEM

    K7JEM Ham Member QRZ Page

    Most properly installed mobile radios will have three grounding paths. The first is the negative lead, tied to ground near the radio or near the battery. The second is the coax shield, which ties to the radio chassis on one end, and vehicle body on the other. The third ground is the screws that mount the bracket to the vehicle body. Any one of these will provide a DC connection that will power the radio to some extent.
    N0TZU likes this.
  7. WA0CBW

    WA0CBW Ham Member QRZ Page

    Don't forget that most newer vehicles have a current sensing device in the vehicle-to-battery negative lead. Connecting external devices directly to the battery will bypass these devices. This can cause electrical system issues especially with high current devices such as radio transmitters.
    WA7ARK and N0TZU like this.
  8. WB2LBV

    WB2LBV Ham Member QRZ Page

    An example of the above-

    KB0MNM, KC3PBI, WA0CBW and 1 other person like this.
  9. W9WQA

    W9WQA Ham Member QRZ Page

    the important thing here for a ham to know is how his radio is wired. the case is ground for the entire radio. in reality the neg black is not needed IF the radio case is properly grounded.i agree with a previous post using a seat bolt is usually very good.
  10. N3EG

    N3EG Ham Member QRZ Page

    Also, using the antenna lead for ground puts the grounding in the absolute wrong place and causes increased receive noise.
    KB0MNM likes this.

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