The return path is often a subtle part of an antenna system, and is not as prominent as, say, the feed lines or the antenna arms. However, the return path is a crucial part of the design, and needs to be factored in to ensure good performance.
What is the Return Path?
A monopole, for instance, behaves rather similarly to a dipole, only instead of two arms, there is only one arm, namely, the monopole proper. The monopole is mounted such that it projects above a ground plane. The ground plane acts like an arm of a dipole, and works to complete the monopole antenna. The radiating signal tends to flow heavily along the outside of the ground plane. Furthermore, current must, of course, flow from the ground plane to the feed. This is where return path comes in.
The so-called return path is where the current flows on the ground plane to the transmitter or receiver. The “shield” side of the antenna feed, whether the feed wire is a coax cable or a PC board microstrip, is also part of the return path.
Antennas with Large Ground Planes
For antennas using literally the ground under our feet as the ground plane, return path is not likely to be much of an issue. Of course, the ground connection should be good; a metal rod driven far into the earth is usually acceptable, especially if several such connections are made.
The ideal earth ground is a large body of water. For many other antennas, ground planes are not going to be much of an issue. An antenna mounted on a large metal object will often use the metal object as a ground plane. Again, as long as the connection between the antenna and the metal ground is electrically good, everything will likely work out as it should.
But what about small ground planes, especially on PC boards?
PC Board Ground Planes and the Return Path
PC board ground planes can pose interesting problems. Most PC board ground planes are not exclusively intended for the antenna. Typically, the ground plane will also be part of the circuitry of the device the antenna is housed in. This in and of itself isn’t much of a problem. However, if the ground plane is a part of the device’s circuitry, the shape of the ground plane will likely be irregular to accommodate the specifications. This, in turn, can lead to trouble.
As mentioned above, the vast majority of the radiating current with an antenna using a ground plane tends to flow along the edges of the ground plane. If the edges of the ground plane are heavily notched, the return path is lengthened, as the current then flows along the sides of the notches. Furthermore, these notches tend to add inductance to the circuit, and can thereby detune the antenna. It is worth mentioning that the method of notching the ground plane can be used to effectively lengthen a too-small ground plane. This technique is similar in principle to the zigzags used to “lengthen” too-small antennas.
Building Good Return Paths
For best performance, the ground plane should be as large and regularly shaped as possible. Recognizing that irregularities will likely occur, it is always a good idea to tune the antenna to accommodate its surroundings, including the available ground plane footprint. This can help mitigate the effects of too much inductance in the antenna circuitry.
It is also important to remember that, on a PC board, the ground plane will often act as part of the feed, as well. For a microstrip line, for instance, the ground plane below the feed is a part of the system. Thus, if the ground plane is discontinuous, or too narrow, the feed will no longer work correctly, resulting in mismatch. For PC board feeds it is imperative that the ground plane below the feed is sufficient and continuous.
Overall, it is helpful to keep in mind where the power to and from an antenna must flow with the idea of making the path as straightforward as possible.
ARSI 