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I've been thinking for some time about magnet mount mobile antennas and wondering how they establish their ground plane. Consider that many mag mount antennas have some kind of protective coating on the magnet. Consider also that most cars have a coat of paint between the mag mount and the sheet metal. Both of these should result in some level of electrical isolation between the antenna feed shield and the sheet metal.

I guess what I'm simply not clear on is how a mag mount antenna's ground plane is connected electrically. Or is this accomplished through inductance?

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    $\begingroup$ Hm... Somehow I internalized the concept as a capacitive coupling between the antenna and the sheet of metal. At the frequencies of interest, the impedance between the sheet metal and the base of the antenna could be sufficiently small, I think. $\endgroup$
    – AndrejaKo
    Nov 18, 2013 at 23:57

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There's always a ground. Whether it's what you intend it to be or not is another issue...

A mag-mount antenna is grounded through capacitive coupling between that magnet and the metal it's stuck to. At VHF/UHF frequencies, this effect is adequate for good results which explains the popularity of these mounts. Some folks advocate adding a wire instead of relying on the coupling effect, but in most cases, this has little or no effect.

At HF frequencies, it's a different story. The capacitive effect is not enough and thorough grounding of the vehicle is usually indicated. There are several good guides out on the internet, I suggest starting with K0BG's excellent site on the ins and outs of mobile amateur radio oeprating.

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    $\begingroup$ Some example calculations of a typical mag mount antenna for vhf. $\endgroup$
    – JSH
    Jun 21, 2016 at 21:17
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    $\begingroup$ @JSH That's a nice article and a good addition to the answer. $\endgroup$
    – WPrecht
    Jun 22, 2016 at 14:12
  • $\begingroup$ Most mag mount antennas I've seen have rubber on the bottom. I'm confused because everyone tells me that because a roof rack mounted antenna isn't directly connected to the metal of the car there is no ground plane, but then there wouldn't be for mag mounts either. So does it have to actually be connected? $\endgroup$
    – Mark Deven
    Aug 20 at 15:11
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MAG MOUNTS,GROUND PLANES, WORK AGAINST A REFERENCE GROUND. SOME BROADCAST ANTENNAS SIT ON A GLASS BALL. US OLD GOATS USED A PC OF PIPE SETTING ON A COKE BOTTLE. YOUR MAG MOUNT IS CAPACITIVELY COUPLED TO THE CAR CHASSIS WHICH IN TURN IS CAPACITIVELY COUPLED TO THE EARTH. CONSIDER A SATELLITE, THERE IS NO GROUND / EARTHING AVAILABLE. THERE WE US THE B- CONNECTION ON THE CKT BOARD FOR A GROUND. AGAIN IT IS A REFERENCE. ALL VEHICLES ARE ONE PLATE OF A CAPACITOR. THE EARTH GROUND IS THE OTHER PLATE. THINK ABOUT UR HT. THAT RUBBER DUCK USES THE CASE BODY FOR A GROUND. I AM ON MY IPAD. OTHERWISE I WOULD SEND A PIC./ DWG. HOPE THIS HELPS. REMEMBER - REFERENCE GROUND. ‘73 KF9F.

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I was wondering the same, and read this, but a friend has pointed out what could only be described as 'the bleeding obvious'.

The shield of the cable is connected to the chassis/ground of the rig, which is also connected to the chassis of the car.

Ergo, it makes no difference whether the shield of the cable is connected specifically where the antenna is sited, as it is electrically connected to the entire car.

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    $\begingroup$ If only it was this easy... $\endgroup$
    – user21417
    May 12, 2022 at 11:32
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    $\begingroup$ A ground-plane is not, strictly speaking, the same as RF ground. The ground-plane is about deliberately giving a monopole (which wants to become a sort of dipole regardless of what else you do) a place for its image antenna. This is why it is sometimes necessary to provide a more explicit ground-plane/counterpoise/etc. Especially if the antenna is some height from true ground (which isn't even the surface of the earth in most places; this is even more complicated on a metal body above the earth.) $\endgroup$
    – user21789
    May 12, 2022 at 17:22
  • $\begingroup$ Which leaves the question of how to any car roof mounted HF antenna works. Whether they be magnetic bases, or gutter mounts which of course do not break through the paint to the metal underneath, there is no local connection with the chassis of the car. Yet, one of my friends from years back did indeed use a mag base for his HF amateur band radio, and gutter mounts are/were common enough. What are your thoughts on that? $\endgroup$
    – Alexander Stollznow
    May 13, 2022 at 12:31
  • $\begingroup$ Because all antennas are compromise antennas. More to the point, at VHF and higher those compromises become different, and capacitive coupling to the ground for the purpose of the image antenna becomes most viable. As mentioned in the accepted answer, there is always a ground reference (especially when talking about monopoles), whether you provide one or not. It's often better to provide one for practical reasons. The RF ground lead is not that. Radio-electronics is famous for using the word "ground" to mean 3-4 different things. $\endgroup$
    – user21417
    May 17, 2022 at 21:13
  • $\begingroup$ The history and refinement of the so-called Marconi antenna is worth reviewing. In its simplest form you get a dipole almost for "free". For shorter antennas far from the actual earth or without any actual earth connection you have to start providing a better opportunity for that image antenna. It's not like the antenna won't work. It'll just be a poor compromise in terms of take-off angle, radiation pattern, and overall matching. $\endgroup$
    – user21417
    May 17, 2022 at 21:22

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