Minimum Ground Plane Size For Magnet Mount Antenna

ByKen J. Last Updated
38 Comments

I have several mag-mount antennas that I use on my truck. I may use one or another during road trips – depending on what radio I’m listening on. But what if you wanted to use one of these antennas at home – as opposed to not mounted on your vehicle? Would it work well enough? Yes, provided you come up with the right size ground plane to plop it on…

Note: Some mobile antennas don’t require a ground plane. Check the specs of your particular antenna.

Though Ham operators may cringe at the following admission, and although this article is focused on my GMRS mobile antenna, I do throw on my Wilson-1000 CB-radio magnetic mount antenna with my Midland-75 for long road trips. Yes, CB radio, like “Breaker 1 – 9…” ;)

Why would I admit that? Well it has saved me numerous times around traffic jams. I do find out where the “bears” are… The trucker chatter provides some humor. And it’s the most listened-to band while on the freeways in case you get into trouble. Yes, I know, cell phones work too – but this is more fun…

I also use UHF radios around here quite a bit. More specifically the GMRS/FRS bands for short range communications around the homestead, local, or wherever. I have a variety of handheld VHF/UHF radios for this purpose.

I also have a Midland MXT400 mobile radio, designed to mount in-vehicle ( GMRS/FRS ). Presently I’m using that radio as a in-house base station for comms around the homestead and local area.

I also use it for monitoring local surroundings in general. Why? Well for one thing it’s interesting/fun. Most people who are out in the woods and trails around here will likely be using these frequencies for communications.

For example it’s deer hunting season right now. I let my radio scan the channels and will occasionally hear hunters communicating – especially during the weekends. Generally, it enables intel (for the lack of a better descriptor) on the most common & popular consumer communications bands. I live rural, so there’s not a whole lot. However I hear various activity out there at times. Plus Mrs.J and I sometimes use it for comms on our property.

So let me get to the point of this article. Though it may only interest a few of you here on the blog, maybe it will prove helpful over time for anyone who happens across this post…

What if I wanted to use my magnetic mount antenna as a sort of base station antenna?

Normally it would stick on to the metal roof of a vehicle. What would I need in order to simulate that if not mounted on a vehicle with its metal roof?

A Ground Plane With One-Quarter Wavelength Radials or Half-Wavelength Diameter

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.

How do I figure out the dimension size of the antenna’s ground plane?

Radials / radius:
Quarter-wavelength in feet: 234 / frequency in MHz

or Diameter of ground plane:
Half-wavelength in feet: 468 / frequency in MHz

For example, the GMRS band is right around 462 MHz.
468 / 462 = 1.01 (feet)
1.01 x 12 = 12.12 (inches)

So as long as I plop my magnetic mount antenna in the middle of a piece of conductive metal at least 1 foot in diameter, I’m good to go… (for GMRS).

Simply calculate for your own frequency. If you’re doing it for a UHF mag-mount antenna for say, 150 MHz, you’re looking at a ground plane diameter of 3.1 feet. I happen to have a few metal outdoor tables that I use on my deck during the warmer months. I could simply stick that particular mag-mount antenna right in the middle. Good to go…

Here’s my experiment for that GMRS mobile antenna:

I used an upside-down small galvanized pail. I then checked my SWR (Standing Wave Ratio) measured on my DAIWA to be 1.2 which is ideal (If your SWR is under 1.5, you’re in great shape).

Don’t laugh, but here it is:

I do have a better antenna for this band (designed as a base station antenna with some gain). But I just wanted to play…

Generally, the bigger the better as ground planes go. But 1/4 wavelength radials or 1/2 wavelength diameter will be fine.

What materials would be good for a ground plane?

If it something conductive, then it will probably work just fine.

Keep in mind that some materials won’t grab as well as others to a magnetic antenna base. If outdoors, you’ll want it secure enough so the wind won’t knock it over.

A note regarding aluminum: Since aluminum doesn’t have any ferrous metal in it, the magnet isn’t going to stick, but that’s OK. All it’s got to do is sit on top of the aluminum to achieve the capacitive coupling it needs to utilize the ground plane. True, the mount will be relying on gravity to hold it in place. If it’s up in your attic or inside, but that won’t be a problem.

Finding materials for a ground plane? You might just go to the Mrs. kitchen cabinets if you can get away with it. A pizza pan. A cookie sheet. Shhhh…don’t tell!

Or you might go to the hardware store and start wandering around and see what you find:

Garbage can
Aluminum window screen
Oil drip pan
Roof flashing
Chicken wire
Wire mesh
Corrugated roofing material


Probably 100 different things you’ll find by just wandering around.

How I Get Antenna Coax Through The Wall

My home is framed with 2×6 lumber. By the time you add layers of exterior sheathing, exterior finish, interior drywall… I needed 10-inch barrel connectors! Although the 10-inch length protrudes a bit, the 8-inch wasn’t quite enough to enable the PL-259 connectors to screw all the way on.

Here’s a photo:
SO-239 Bulkhead Coaxial Connector, 10 inch:

Here’s my current Ham stack. Always in a state of flux ;)

The radio on top is the Midland MXT400 that I use as a base station for GMRS comms.

Okay, hope some of you enjoyed…

[ Read: Best BaoFeng Antenna Upgrade for Ham Radio and GMRS, FRS, MURS Bands ]

38 Comments

    1. I laughed as well, after I saw the request not to laugh. I was laughing with him, not at him, I thought it was brilliant, I’m a telecommunications network engineer.

  2. Mine require a ground plane too, yet I hadn’t gotten around to doing the math for the type of application that you’re covering. Great info! It’s already given me some ideas to adapt to my needs.

    Thanks, Ken! It’s nice to have something fun to look forward to playing around with when time permits.

  4. Ken,
    Good article. There are many bands, as you laid out nicely. Nothing I see here is laughable. Whatever works. If you’re making your own (homebrew) you can incorporate a loop into the very top. Makes for an easy attachment for hoisting it up high. I encourage everyone to write down the formula(s). None of this stuff is hard. It’s just daunting when ya start. Next thing ya know, wow it works.

    Try this guys: You surely have some short pieces of copper wire around. You know, scraps from whatever electrical project. I’m talking single strand wire, like romex. Strip all the insulation off. Place two pieces of wire into the chuck of your drill. Put the other ends of the wire in a vice or whatever. Engage the drill. You’ll end up with a really strong twisted copper strand. Hey, it even looks good. If ya do one longer wire with a screw driver in the vice and the wire around it, both ends of the same wire in the drill, you’ll end up with a loop at the top. Play with it.

    Hopefully, this is all for fun and convenience. Radio is fun. It could be the only comms ya have. Figure out what ya need (distance and terrain) then figure out the band that will do what ya need. Oh, and don’t forget faraday.

  5. Know it’s apples and oranges….but, I’ve found that my little Baofengs are the best portable FM broadcast radio receivers I’ve found. I live in a fringe reception area with only one conservative talk station I can sometimes pick up. The Baofengs will pick it up pretty clearly if you hold the radio in your hand, but if you set the radio on a table the signal becomes real scratchy. By accident, a couple of days ago, I found that if I sat the radio on top of the popcorn tin I had beside my easy chair….instant clear reception, allowing both hands to be free.

    If others have this problem…might find this to be a solution.

    1. Dennis,
      Yep, ya just made it a ground plane. You’re a smart guy, so get off your butt and get the tech license so you can get “on-the-air.” I learned far far more by actually being on the air. I’m betting you will too. BTW that is NOT intended as an insult. Ya might check out RCA superadio III for long distance am/fm receive only. Has a really good internal am antenna and a telescoping external fm antenna. Also has external antenna hook-ups. The antenna is everything.

  6. On my HF I use a 130 ft inverted dipole and for UHF/VHF I use a vertical on a 35ft pole. My house is located on the north side of a 3500’ mountain with mountains on the northwest through the southwest so the only clear path (the valley) I have is from approx. 220 degrees thru 320 degrees. Luckily the the two repeaters I normally use are west so I have no problem hitting them. I have a portable J-Pole ladder line antenna with 50’ of coaxial and PARACORD I can use for my handhelds if I need to bug out.

    Communications will be essential in in a SHTF situation and the more bands and freqs you have available the better intel you will be able to gather.

    73’s everyone

    2. Romeo Charlie,
      You might look into a NVIS antenna for that HF rig, you’ll find you find you can hit everyone in a 600-800 mile radius regardless of terrain on 40-160m.

    3. Romeo Charlie,
      First off, listen to minerjim. He knows of which he speaks. You are a good example of my mantra (distance and terrain). I’m sure you know this, but others may not. Repeaters are fine when all is well. Repeaters will likely fail in any grid down. Seems like ya have a good plan. 73

      1. Plainsmedic,
        So true. Repeaters are not reliable as you are at the mercy of the repeater owner and the grid. I am hoping to improve my antenna setup soon and I’m going to look at the NVIS Minerjim recommended.

  8. I have no clue.
    I thought that the vehicle acted as a kind of rf reflector because there is no connection between the metal of the vehicle and the antenna base.

    I am looking for a magnet mount for a basic cb with weather channels.
    less than special, it’s a realistic brand.
    was going to ask the junk yard for cheap.

    When younger I loved to listen to the truckers.
    Much younger, like 35 years ago.

  9. Metal roof, stick it to the ridge cap,,,,
    had asked a different question, but then saw the reply and discussion in the comments 🙄

    1. Not sure of all the different kinds of ridge caps for a metal roof… but the ridge cap on my metal shop roof is not really flat on top. I don’t believe there’s enough horizontal ‘flatness’ on the top of the ridge cap for a mag-mount to properly orient (antenna pointed straight up). Instead the antenna would be crooked, and end up at the same angle as the roof’s slope on one edge or the other of the ridge cap (and mine is pretty steep)…

      If I were to go through that trouble, I would rather just buy a “real” base station antenna and mount it on pole with brackets fastened to the side of the structure up near the peak.

  10. Ken,

    I ordered a Browning BR450 450-470 mghz antenna last night, but ordered a NMO to PL239 2″ bulkhead adapter instead of a magnetic mount. My thinking was trying this antenna as both a mobile on my side x side, and or, as a base antenna.

    My question to you, minerjim, plainsmedic….or anyone who might know…when you build an antenna, internet experts stress that ground plane radials should match frequency same as the transmitting element (not too long, not too short). You (Ken) are saying, essentially, that too big is OK, too small is not. Does this only apply to antennas with a base load? Would transmit power/efficiency increase if the ground plane was more closely matched to frequency?

    Having fabricated my own GMRS ground plane antenna, I could fabricate a ground plane made of radials cut to frequency, but was thinking of using some flat steel sheets instead, if preciseness is unnecessary with these loaded antennas

  11. Hello , I work on the Rail road right of way as a safty man and have a VHF Icom base radio wired in my Jeep JL “aluminum body and fiberglass roof ” . I am no radio aficionado. I have a zero ground plane antenna now but I am not receiving signals from trian as my coworkers who have steal vehicles and magbase antennas. Do you know how large of a piece of sheet metal I can attach to my roof to achieve a good ground plane.

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