Ground plane sizing for radio, transponder and PilotAware

Transponder, radio and PilotAware antenna fitting in a non-metallic skinned aircraft

There is often talk around the hangers about poor radio signals and how some aircraft can be heard from great distances while other fail to make contact with a ground station even nearby. One of the issues often overlooked is a missing or wrongly sizes ground plane. There are many myths about the purpose of a ground plane and I hope in this short article to give you guidance on the correct size and purpose of a ground plane.

Before we look at the ground plane lets set expectation on maximum distance you can be heard.

The civil air band radio is in the VHF (Very High Frequency) band and under normal weather / tropospheric conditions, it requires radio line of sight between the aircraft and the ground station; thus, the maximum range is the distances to the radio horizon from the aircraft to the ground station, for simplification we are assuming the ground station is at sea level.

The radio horizon distance is given by the formula: D= K√h

Where: D = distance in nautical miles (NM), h = height of aircraft’s antenna above ground level and K = 1.23 when h is expressed in feet

So, when we are flying at 2,000ft the maximum range to a sea level ground station is 55.00NM or at 3,000ft 67.34NM

The same is true for transponders, CAP1391 devices e.g. SkyEcho and PilotAware transmissions, however the limited power and antenna arrangement of CAP1391 and PilotAware means the range is less.

What is a ground plane is and why we need one?

Keeping this simple, it provides a ¼ wave monopole antenna (also know as a Marconi) with its required counterpart allowing the flow of the electrical current which generates the electrical and magnetic waves that make up the RF signal. The earth acts as a type of electrical “mirror,” effectively providing the other quarter wavelength making it equivalent to a vertical dipole. This in turn helps present the correct impedance to the transmitter, thus allowing the maximum power transfer into the antenna, which results in the most efficient transmission of the signal. Most antennas sold for Microlights and light aircraft are ¼ wave.

Does size matter?

I’m afraid the answer is yes it does! Bigger is generally better, but again there are exceptions and a minimum size for the efficient working. The minimum size is a radius of ¼ the wave length of the signal being transmitted.  In basic RF theory we were taught that bigger is better, although the benefits of size are less important above 3 times the wave length. In aviation while we are flying along we want our signal to be picked up both above and below the aircraft, the larger the ground plane the bigger the blind spot we will have on the underside of the ground plane.

Is the shape important?

No, a ground planes can be square, circular or many different shapes. Say you had a metal aeroplane, the whole fuselage is the ground plane for your radio antenna, and your transponder antenna. So long as you have at least ¼ wave length in all directions, then the shape doesn’t matter too much. However as we want the lightest ground plane we can get, a circle is the optimum shape and supports equal propagation of the signal in all directions.

What should the thickness of the ground plane be?

Any, you can use foil tape or discs of different thicknesses so long as it is mechanically robust enough and conductive.

Can it be a mesh or does it have to be solid?

It can be a mesh (the tighter the mesh the better) or even multiple radial wires, typically the more the better 8 or 16, but for our purposes a solid circle is normally the easiest and best option.

What should it be made of?

Copper, however it can be any material which is a good conductor. Aluminium is a good conductor and can be used, however you need to be mindful of the galvanic effect between a stainless-steel antenna and the ground plane. This may lead to connection issues in the future, thus I would recommend a thin copper disc.

Why do some avionics companies state to use a square ground plane?

Well I assume this is just convenience, so long as the diameter of the circle below can fit within the square it will work fine. Just make sure the antenna is in the middle!

What does all of this mean in practice?

Minimum diameter of ground planes in aviation should be:

N.B. Always round the ground plane size up not down.

Transponder: frequency 1090MHz, Wave length 275mm, ¼ wave length 68.75mm

Minimum ground plane diameter 137.5mm

Radio: lowest frequency 108MHz, Wave length 2775.85mm, ¼ wave length 693.96mm

Minimum ground plane diameter 1,387.9mm

PilotAware: external antenna fitting, frequency 869.5MHz, Wave length 344.79mm, ¼ wave length 86.19mm

Minimum ground plane diameter 172.39mm

Antenna Theory Is Complicated

Antenna theory is complicated, but if you have a ground plane with the correct minimum size and the antenna mounted in the middle this will be one less problem.  But let’s quickly prove this is fact and not just some text book exercise. As an extreme lets consider an antenna without a ground plane vs the same antenna with a correctly sized ground plane.

An external PilotAware antenna without a ground plane is put under test using a low cost VNA (Vector Network Analyser)

As can be seen opposite the resonant frequency of this antenna is not 869.5MHz as it should be! It is showing a resonant of over 1GHz. The impedance and VSWR for the antenna was also way out, in fact the reflections from the imbalance could damage the transmitter or at best the radiated signal will not be optimal.

Let’s add a make shift ground plane out of kitchen foil of the recommended size above.

opposite you can see the effect of adding the ground plane. The resonant frequency has shifted down to be close to the 869.5MHZ that we are looking for, the VSWR was down to acceptable limits and the impedance was close to 50 ohms.

Hopefully this graphically demonstrates the importance of having a correctly sized ground plane!

One last Fact! The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi