The best mobile antenna money can buy isn’t any better than the ground plane it is mounted over. Remember the basic dipole—it needs two elements to be complete. Your vertical element is one, the body of your vehicle is the other, functionally similar to the radials used on a ground-mounted vertical or the user’s body when holding an HT.
In very simple terms, a ground plane is a mass of conductive material such as steel or aluminum right below your antennas—in this case, your vehicle.
This ground plane is required for certain types of antennas such as a 1/4 wave mobile antenna, and it reflects the RF energy into the sky to complete a portion of a radiated signal.
HF: More is Better
A vehicle will actually provide a good ground plane for permanent or VHF/ UHF magnetic-mount antennas with a typical three- to four-inch base. The antenna placement diagram (below) shows only 0.02 dB difference between a mag mount and permanent mount antenna.
But for HF antennas, it’s a different story.
With HF, using a vehicle roof for the ground plane is problematic simply because it’s too small. For an effective ground plane, ideally you want at least a quarter wavelength in the desired direction. With a big SUV you might get up to three feet of ground plane, which won’t quite make a quarter wavelength on 15m, 20m, or anything longer. And the three feet is stretching it, putting the antenna at one end of the roof. It’s not really an issue on 2m because it can easily fit within about a 19-inch radius on almost any vehicle roof.
If you want to use any of the HF bands when mobile, you’ll want to extend your ground plane as much as possible. Bonding trunk lids, hoods, tailgates, and other parts to the rest of the vehicle body will help improve the antenna match, along with other benefits. First is noise abatement—bonding minimizes the leakage of RFI into and out of the various bolted on parts of the vehicle. It is not uncommon to see a drop in noise levels once they’re properly grounded.
Depending on the vehicle, there can be several dozen other places where ground straps will provide a benefit. These include, but are not limited to, bumpers, suspension parts, rear axles, tailgates—virtually any bolted-on piece of hardware. Engines are also overlooked because most of them have visible ground straps. They’re for DC, of course, so adding wider ground straps can help minimize ignition noise.
The Hole Truth
Mounting an antenna on a car or truck requires planning. You need to see how the locations will affect radiation patterns and possible losses. Check the image above for data on dB losses based on antenna placement.
We know the best place for the antenna is the center of the roof, mounted directly to the metal. In locations other than top and center, the radiation pattern will be affected to some degree. For example, if you have an antenna on the right side of a vehicle, the pattern will skew to the left to some degree—across the car body. If it’s not mounted at the highest point on the vehicle, parts of the vehicle higher than the antenna can block part of the signal.
But some folks can’t bring themselves to drill a hole, especially in a brand-new car. Some are driving lease vehicles and believe they may be assessed for the damage. Others feel that it affects the car’s trade-in value. Then there’s the spouse/significant other’s opinion: “You’re not gonna put that on our car.”
That’s why there are such a variety of alternative mobile antenna mounts for the hatchback, trunk lip, luggage rack, mirror, and fender.
You can avoid poking holes in the roof, but there’s still a great likelihood you’ll need to scrape off some paint and tighten some set screws into bare metal. Fortunately, it will be hidden.
An Attractive Alternative
I’ve heard some amateurs and CBers bash the use of magnetic-mount antennas. Concerns include them flying off the vehicle, scratching paint, and not providing a true connection to the conductive body of the vehicle. Yet they can work well in many situations and provide the bonus of putting the antenna in the best position without the need for a hole in the roof.
How well a magnetic mount works depends on proper usage and understanding. These mounts require a steel surface, typically the roof of a vehicle, to function effectively. The vehicle’s roof acts as a ground plane, allowing for better signal reception and transmission. One advantage of using a magnetic mount is ease of installation and removal, making it ideal for rental cars or vehicles where permanent mounts are not wanted.
Mag mounts operate by utilizing the metal surface beneath them to create capacitance. This capacitance acts as an electrical connection, which is important for antenna performance. The mount itself forms one side of a capacitor while the vehicle’s roof constitutes the other side. Experts suggest that ground planes don’t have to be physically connected to your antenna. Some hams advocate adding a wire instead of relying on just the coupling effect. In most cases this has little or no effect.
The surface area of the magnetic mount plays an important role in its effectiveness. Larger mounts provide greater capacitance, allowing for better performance across various frequencies. The Moonraker Turbo-38 seven-inch diameter mount (below) is a good example. You may have also seen their triple magnet mounts.
To enhance the performance of magnetic mounts, consider using accessories like magnetic mats for increased capacitance and line isolators (chokes) to prevent RF interference. Magnetic mats, such as the Chelegance MAT-50, can improve the performance of your mobile antenna by providing a larger surface area for capacitance. Line isolators prevent unwanted RF currents from affecting your SWR measurements and tuning your screwdriver antenna. Snap-on Mix 31 ferrites or a 240-31 toroid installed on the feedline near the antenna will take care of these issues and make tuning much easier.
Tricks & Tips for Mag Mounts
Magnetic antennas are generally efficient, but you need to do a few things to get best performance. First and most important, clean the mounting area and the bottom of the magnet mount. Dirt, metal filings, and other crud can accumulate, causing damage to the paint and poor contact with the vehicle. Always check the feed cable for damage–and poor connections if you can get access to the inside without damaging the antenna.
Got aluminum? I have a friend who owns an Airstream trailer—it’s an awesome ground plane, but mag mounts obviously won’t attach. Stick-on steel disks are available to attach magnetic antenna mounts to aluminum bodies. Aluminum is a very good conductor and will couple capacitively.
Got fiberglass? You can create a ground plane under the fiberglass, but it’s an involved process and beyond the scope of this article. A better solution would be to use the steel disk mentioned above, along with a ½ wave antenna such as the Larsen NMO150C or Rugged Radios VHF-1/2W-SPR.
The Plane Truth
If ground losses are high, it doesn’t make much difference how good the antenna is because ground losses will be the largest factor in determining efficiency. If ground losses are low, the difference in efficiency between a poor antenna and a good one becomes very apparent. Comparing the signal strength of a 1/4 wave whip alone to one with a ground plane, signal strength may increase as much as 3 dB.
It pays to bond large surfaces like trunks, roofs, and hoods for better RF continuity.
All other things being equal, capacitors conduct better at higher frequencies and capacitance increases with the size of the plates. Applying this to a mag mount antenna installation, the following are generally true:
- Antennas for VHF/UHF will perform better than those for HF, especially on 20m and above.
- Antenna placement makes a difference.
For all antennas, it’s the metal mass directly under the antenna, not what’s alongside, that counts.
