Ground Plane Notes

Throughout this web site, I (overly) emphasize that it is the mass under the antenna, not along side, that counts. The reason I do is because ground plane losses are the largest single factor in any equation relating to efficiency, input impedance, and particularly common mode currents with respect to RFI issues.

Since this subject is so important, especially in a mobile setting, I think it prudent to present a different slant on the issue of ground planes, more correctly called image planes.

What follows, except for a few minor punctuation changes, is the exact text of an e-mail I received from Dr. James D. Van Putten, Jr., W8QT, Physics professor emeritus, of Hope College, in Holland, Michigan. A man I admire, and a man I'm proud to call a close friend.

I have been thinking about how to help persons who cannot seem to understand the problems associated with RF currents flowing on control or other lines. Here is a thought.

Electric fields begin and terminate on charges. By convention the field lines run from positive charges to negative charges. A positive charge does not have to be a positive electron (although they exist) but equally effective is the absence of a negative charge. Solid state physicists call these "holes". An example of the absence of an electron is seen in capacitors. Think of a capacitor as two parallel plates separated by an insulator. When electrons flow onto a plate, they repel electrons on the other plate which flow out into the circuit. This creates an absence of electrons on the plate from which the electrons have been repelled resulting a positively charged plate. The electric field in the capacitor then runs from the absence of electrons on one plate to the electrons on the other.

A similar effect is present in antennas. When the transmitter drives electrons onto one wire of a dipole antenna (creating a surplus of charges), it draws electrons from the other wire (creating an absence of electrons). The electric field runs then from the absence of electrons on one wire to the excess of electrons on the other. This flow of electrons surges back and forth at the frequency of the signal being transmitted. The surging electrons are the current in the antenna wire.

When one uses an antenna that is essentially only one wire such as a mobile antenna or a vertical antenna, the transmitter drives electrons into that wire while drawing electrons from whatever other conductor is connected to the return line to the transmitter. This is often the outside of the shield of a coax or any other wires associated with the antenna or its tuner. In an automobile antenna, this can be the body of the car. Unless specific actions are taken to prevent the current from flowing on outside of the coax or on associated wires, the current will flow back and forth over the leads to the transmitter and any equipment connected to it. This RF can cause nips to the lips from microphones as well as malfunctioning in equipment connected to the transmitter. Prevention of the wayward current flow can be prevented by the judicious use of ferrite chokes.

The mental image of the electric field lines running from the absence of charge to real electrons can also help one to understand why radials or ground planes are necessary on any vertical antenna. There must be a conductor upon which the transmitter impresses electrons and another from which it draws electrons. This can be metal, sea water or conductive soil. As the power loss is proportional to the resistance of that conductor one can easily see why multiple radials or sea water is one reason that these increase the efficiency of a vertical antenna system.

Center fed, vertical half wave antennas require a more sophisticated analysis. In such an antenna there is a both a conductor on which to induce charge and a conductor from which to draw charge. However the electric fields associated with a vertical antenna must also obey all of Maxwell's Equations which require image currents to flow in any nearby conductor. These vertical image fields behave somewhat differently than horizontal image fields. The importance of this difference is that all vertical radiators require a conductive area in the near field around the antenna itself.

Jim, W8QT

There are several articles on this web site that explain how ferrite chokes work, and where and how to install them. However, with the above in mind, it is important to point out just what happens to the antenna current when a choke is used.

If you took time to read aforementioned article on how ferrite chokes work, you'd know they look like an inductance in series with a resistor. Thus, whatever RF energy they choke off, is turned into heat instead of a radiated RF signal. Some of this wayward current flow is unavoidable, and the motor leads of a remotely tuned antenna are an example. This said, if you have to choke off the coax as well, this is a classic example of an inadequate ground plane under the antenna.

As I point out several places within these pages, ground straps are not a substitute for an adequate ground plane. Just try and keep this thought in mind; the ground plane of a mobile antenna should be directly under where the coax shield connects. If it isn't, or there's a ground strap there instead, you have a less than stellar installation.