RF radiation and grounding

Discussion in 'General Technical Questions and Answers' started by KN6NBG, Jul 1, 2021.

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  1. KN6NBG

    KN6NBG Ham Member QRZ Page

    Hi all, I just ordered an Icom 2730a transceiver for my car. I'm leaning towards going with the Larsen NMO 2/70b antenna. I have a Jeep grand cherokee and thinking about mounting it on a bracket at the rear hatch so that the top of the antenna will be probably 1' above the roof of the car.

    My question was that at 50w on VHF/UHF is the RF radiation too high given that my kids will be sitting within 5' of the antenna? Is there a better place to put the antenna to reduce radiation? When I input figures in the RF radiation calculator it says I need to be about 10' away, which isn't really possible in a car.

    My second question has to do with grounding the antenna. If it's mounted to plastic brake light housing would it be a good idea to run a separate ground cable from the metal nmo bracket to the negative battery terminal?

    Thanks for the help in advance!
  2. N3HGB

    N3HGB Ham Member QRZ Page

    If you want to go through the hassle, either DIY or get a radio shop to put an NMO mount in the center of the roof or close to it.
    You will have a large variety of NMO antennas you can use, your signal will be about as good as it can be, and the RF will all be outside the car.
  3. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Which calculator and what duty cycle did you input?

    Also, that antenna location will not work well at all. It would be much better to put it at the front fender seam, if you will not drill a hole. But as N3HGB said, the roof mount is the best for all purposes.

    Don’t use the plastic brake light housing, not only isn’t is conductive but it’s not meant to carry the force of having an antenna mounted to it at highway speeds and wobbling around. But if you did, you would want to run the shortest strap possible to the metal of the body.
    Last edited: Jul 1, 2021
  4. KN6NBG

    KN6NBG Ham Member QRZ Page

    what about through the roof glass if the car has the glass roof panels? Glass doesn't block the RF radiation does it?

    Thank you
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    The antenna needs to be mounted directly on metal. Long spindly "ground bonding wires act as random antenna elements which detune the antenna, increase SWR, and alter the antenna pattern. Poor practice.

    Re kids: Lower your power or just don't transmit while transporting kids.

    Everything you need to know about mobile installations:
    N0TZU likes this.
  6. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Those work by using the glass as a capacitor between the inner and outer surface. The trouble is almost all vehicles now use metal film or particles in the glass to provide tinting and some solar heat rejection, which increases losses of the capacitor or might cause high SWR.

    They also don’t work as well as a metal mounted antenna anyway because part of the coax is used as the counterpoise.
  7. KN6NBG

    KN6NBG Ham Member QRZ Page

    So would I be better off with 1/4 wave on the roof than the 1/2 wave at the rear hatch? If I'm not willing to drill the roof how bad is it to go with a magnet mount?
  8. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    A mag mount with a 1/4 wave would be much better than the low hatch mounted 1/2 wave in my opinion. Its low, and it’s very near all the metal of the hatch door, not efficient and not going to have a good pattern.

    But there are drawbacks, like having coax through the door seal, and possibly a low branch knocking the mag mount loose. If left on for a very long period there can be paint damage. They’re meant to be temporary not a permanent solution.

    Getting back to the RF exposure, FM has a 100% duty factor. But the duty cycle is the fraction of time spent transmitting in a 6 minute period. Few hams are long winded enough to to get that above 50%, and most of the time it would be far lower. So the average power in the calculator should be reduced by that duty cycle.

    Also it’s not necessary to run the highest power all the time, 10 watts is usually more than enough for hitting well situated repeaters - with a decent mobile antenna.
    Last edited: Jul 1, 2021
  9. KN6NBG

    KN6NBG Ham Member QRZ Page

    ok thats interesting, I thought that since the antenna would go to the same overall height as the 1/4 wave it would be as effective.

    Regarding the RF exposure, so do you mean that the radiation allowed exposure is measured in terms of % of time transmitting each 6 min period? The issue is that the radio i think only has 5/15/50 watt. nothing between 15 and 50. I was mostly interested in simplex range which I thought the 50w would be much more beneficial in comparison to a 5w HT.
  10. N3HGB

    N3HGB Ham Member QRZ Page

    The pattern of a 1/4 wave or 5/8 wave in the center of the roof is about as good as it gets for a mobile. Anything off to the side, in a corner, or below the roof is going to have a distorted pattern in various ways.

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