Many antennas have a ground plane built in, but in particular magnetic mounted antennas often kitted with mobile boosters require one be provided.
These antennas are specifically designed for automobile use, and being placed on the roof of a metal car or truck.
The magnet keeps them connected to the metal roof, however, what a lot of folks don't realize is that the metal roof is also serving as the ground plane - an integral part of the antenna's performance.
Most RVs and boats don't have a metal roof - they are fiberglass, rubber or some other material.
A complaint we often hear is that a new booster an RVer just installed isn't working all that well. The first question we ask is - did you install a ground plane as detailed in the instructions that came with your booster?
Included in this Guide:
- Video Overview
- Why is a ground plane necessary?
- Just how much difference does one make?
- How do you create a ground plane?
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We created a short video to supplement the written information on this topic:
Why is a Ground Plane Necessary
A ground plane is a reflective metal surface that the signal from an antenna bounces off of to better meet the antenna’s design goals.
Antennas requiring ground planes are called ground-plane dependent, and having an appropriate ground plane can greatly affect performance. A dependent antenna can become effectively useless without one. You should always ensure that any antenna requiring an external ground plane – typically magnetic-mount antennas – is placed upon an effective reflective surface.
Antennas that do not require an external ground plane are called ground- plane independent antennas. These antennas have the means to reflect and focus the signal without the external ground plane being present. For most ground-plane independent antennas, having a ground plane present will still somewhat enhance signal gain, but it is not required.
How Much Difference Does a Ground Plane Actually Make?
We put our iPhone into Field Test mode (back when iPhones allowed this prior to iOS 11), and took some readings with and without a ground plane. Here are our measurements for AT&T LTE:
- With no booster at all: -111 db
- Booster w/ Ground Plane: -81 db
- Booster without a Ground Plane: -103 db
These tests were completed with the Top Signal Cobra 4G Mobile Cellular booster, and the stock rubber magnetic mounted antenna it comes with.
The difference is significant. -103 and -81 is the difference between barely loading web pages and being able to stream video.
Use a ground plane with antennas that require it!
How to Create a Ground Plane on a Non-Metal Roof
Most RV and boat roofs are not metal, so you'll have to replicate the metal of the automobile roof that these antennas were designed for. Thankfully, it's a pretty easy modification to make - just place a piece of metal on your roof, and mount the antenna to that.
An antenna's ground plane should be at least 1/4 the wavelength of the signal being broadcast. Any larger doesn't hurt, but it doesn't help much more either.
To convert from a radio frequency to a physical wavelength, you divide the speed of light by the frequency.
You'll see that the lower the frequency, the larger the wavelength, and thus the larger the ground plane.
The Quick Answer: For most cellular and WiFi transmission frequencies, a disc of some sort of metal at least 8" in diameter is generally considered sufficient and actually provides some room for error.
Want to Geek Out with us? Let's do some math!
The lowest frequency currently supported by cellular boosters is LTE Band 12, which runs from 699MHz to 746MHz. This works out to a wavelength of 16.9" - 15.8". Using the quarter-wavelength formula, this means that an effective ground plane should extend out from the center of an antenna at least 4.225" to be optimal for LTE Band 12.
Since you really only need four radials and not a continuous sheet of metal, the distance to the four corners from the center of an 8" x 8" metal sheet is 5.7" - meaning that you have plenty of ground plane for the job.
And you are actually ready for the future too. The FCC auctioned off the spectrum used by TV channels 30 - 51 to the cellular carriers - this will open up spectrum potentially as low as 550MHz for cellular use. T-Mobile is rapidly expanding on Band 71 in the 600Mhz range.
550MHz has a quarter wavelength distance of 5.375" - which is still covered by an 8" x 8" sheet.
As frequencies get higher, the ground plane size shrinks dramatically. For example, at 1700MHz (LTE Band 4) - the quarter wavelength distance is just 1.75". And at Sprint's 2.5GHz LTE Band 41 the quarter wavelength distance is just 1.2".
And at the high gigahertz frequencies that will be used for 5G technologies, the ground plane size required will be just a fraction of an inch.
Most magnetic antenna manufacturers will specify the size required. To be absolutely sure, an 8" diameter plate always works.
The plate does not have to be heavy/thick and it only needs to be magnetic if you want to use the magnet on the antenna base to attach the antenna. You can also use aluminum foil or a metal tape in a pinch.
The shape is also not critical: A ground plane can be rectangular or circular, as long as it meets the minimum size measure in all directions.
Here's some potential materials you can easily use as a ground plane:
- Roof shingle flashing (comes precut to 8" x 12" galvanized steel, and usually under a buck at the hardware store!)
- Flat cookie sheets of the correct size
- Pizza pans
- Old circular saw blades
- Paint can lids
.. and many other things can all make good ground planes for most antennas.
For mounting multiple cellular antennas near each other,separate them by at least 4.225" to avoid them having interference with each other (more is always better!).
And if they are both ground plane dependent they can utilize the same piece of metal - as long as there is a minimum of 4.225" of metal surrounding each antenna. (So for two antennas, 8" x 12" is about the minimum size to achieve this.)
Attaching A Ground Plane
To attach a magnetic antenna to a fiberglass or rubber roof, take your metal plate and spray it with your choice of rust-resistant paint, and then simply attach it to your roof with a compatible caulk (Dicor, sealant, etc).
Then place the antenna in the center of the plate, and let the magnet hold it. Then at the side of the plate, secure the wire to the roof with a puddle of caulk.
The advantage of this method of attachment is that if a tree limb hits the antenna, it will simply flip off the mounting plate and be retained on the roof by the caulk puddle. Later, it is a simple matter to right the antenna back on to the plate.
A rigidly mounted antenna on the other hand is easily damaged by tree limbs, however, you can attach the antenna by other means (silicon, adhesive, etc) if your ground plane is not a ferrous metal.
For more on antenna installations:
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