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ANTENNA NOTES FOR A DUMMY

Restricted Space Antennas

by Walt Fair, Jr., W5ALT
 

The RF Ground

One often hears about the need for a ground or ground system. There are recommendations to run 1/4 wave counterpoise(s) and do other things to ensure a good RF ground is available for optimum performance. In this section, let's take a look at what a ground is and isn't and whether one is needed or not. In general, there are 3 separate uses for a ground system in the typical ham shack: safety ground, lightning ground and RF ground. We will evaluate in detail only the RF ground here, after briefly taking a look at the other types of grounds.

The Safety Ground. First, we'll take a look at the safety ground system. In most houses and buildings one of the wires in the normal electrical wiring is connected to ground for safety purposes. Certainly we do not want to risk anyone being electrocuted by touching the chassis of any of our radios or other appliances. The best way to ensure that doesn't happen is to connect the grounded electrical wire to the chassis. Then if someone touches the chassis, they are at ground potential and no harm is done. This ground system should be part of the electrical wiring of the building. If it isn't, that problem should be fixed before going any further! All electrical codes require a functioning ground system as a normal part of home and building electrical wiring.

But then we run an additional ground wire for our radio equipment. Normally this additional ground wire is connected, either directly or indirectly to the chassis of the radios, tuner, amplifiers, etc. So we now have the situation where the chassis of our equipment is connected to ground through 1) the house electrical system and 2) our additional ground wire. One could certainly ask the question: Why 2 separate connections for the same thing?

Under normal circumstances having 2 ground connections will be unnoticable. Problems can occur, however, if the house ground comes loose. In that case, the entire house would be grounded through the radio ground connection. While that might be better than having no safety ground at all, that probably is not the purpose most hams had in mind. Of course, the proper action is to fix the house ground! In that case, the additional ground is not needed for safety purposes.

In any case, the issues surrounding a safety ground are covered by the building electrical codes. For further information on safety grounding, consult the electrical codes and guidelines for house and building electrical wiring.

Lightning Ground. Another use for a ground connection is to divert lightning which may strike an antenna to the ground, thereby by-passing problems in the shack. However, if the antenna is connected to our rig and our rig is connected to the ground, then all of the current from lightning striking the antenna must pass through the antenna, feedline, radio and ground connection. Most likely it will burn up lots of things on its way there!

The subject of a lightning ground is an entirely different matter, since its purpose should be to bypass the current from a lightning strike away from our house and equipment. This subject is important, but beyond the scope of this dicussion. Additional information is available in various books on antennas and from companies that specialize in lightning arrestors and diverters.

RF Ground. So, now we have ensured that our equipment is grounded for safety and lightning purposes. What is the reason and utility of providing an RF ground? Let's take a look at 2 fairly common ham situations.

Balanced Transmission Line. In the first situation, we will consider that the transmitter is using a feedline to a remote antenna. If we look at the RF circuit of such a station setup, it would look similar to the following figure:


Using this figure as a guideline, it is apparent that the current from the transmitter, It, will be equal to the current from the antenna, Ia, plus the current going to the ground, Ig. If our transmission line is balanced, however, we know that the current on both feedline conductors is equal. So if It = Ia, then obviously Ig = 0. And if Ig = 0, then we can disconnect the ground wire and not observe any difference.

For this situation, the purpose of the RF ground is to make up for deficiencies in the balance of the antenna. Any excess (i.e. common mode current) will be bypassed to ground instead of going back to the rig and radiating from power cords, etc. But, it should also be apparent that whatever current goes to ground represents energy that is produced by the transmitter and is not radiated. That represents an inefficiency in the system. Although the ground may appear to solve some RF feedback problems, it does so at the expense of antenna system efficiency. It would be better to get rid of the common mode current by improving the feedline balance. In this case the presence of the ground should have no effect on a properly installed antenna system.

End Fed Random Wire. The second situation is where our antenna consists of an end fed wire. The RF circuit for this setup is shown in the following figure. As can be seen there is no direct return path for the antenna current, It, so therefore the return path is through the ground connection and thus Ig must be equal to It.


So, what happens if we disconnect the ground? According to the figure there would be no return path for current, we would have an open circuit and the system would not work. In practice, though, there would always be some sort of return path, through our house wiring, even through someone's fingers touching the case. Of course we probably don't want our house wiring or our bodies to be part of our antenna system! In this case, the RF ground is absolutely necessary to avoid problems. So what do you do when a good RF ground connection is not available?

The answer can be seen by comparing the preceeding two figures. We can see that the need for the RF ground is due to the lack of a return path and due to not having a balanced antenna/feedline system. So if we can't get a good RF ground, we can convert the antenna to a more balanced system by adding the missing antenna element. Some people choose to call it a counterpoise or artificial ground, but as can be seen, it is really just making up for the lack of a current return on an unbalanced antenna.

Conclusions? So what can we conclude from this evaluation? Basically it appears that if we have a balanced antenna feed line with no common mode currents, there is no benefit to having a good RF ground. It is only when the antenna transmission line is unbalanced that an additional return path for the current is needed and an RF ground can supply that. However, it is generally agreed that the ground is the most inefficient part of any antenna system, so whatever curents flow to ground represent inefficiencies in the antenna system.

My recommendation is that when possible a good RF ground is a nice backup, just in case something goes wrong. However, checking the current flowing on transmission lines and in the ground connection should be done periodically. If the currents are large enough to be noticeable, then something should be done to reduce them.

In the cases where one cannot install a good RF ground, such as in the upper floors of a multi-story building, don't dispair. It simply means that we're forced to take care of the common mode currents and make sure our system is balanced. In general, using a counterpoise or artificial ground wire can appear to help, but the best solution is to take care of the real problem, which are due to common mode currents caused by an unbalanced system.

 

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