# How does a mag mount antenna achieve a ground plane?

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?

• 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. Nov 18, 2013 at 23:57

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.

• Some example calculations of a typical mag mount antenna for vhf.
– JSH
Jun 21, 2016 at 21:17
• @JSH That's a nice article and a good addition to the answer. Jun 22, 2016 at 14:12
• 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? Aug 20, 2023 at 15:11

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.

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.