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Restricted Space Antennas

by Walt Fair, Jr., W5ALT

Basics Concepts I

Before we get too far into the subject of antennas, it is useful to quickly review a few basic concepts and terms.

What is an antenna? Perhaps some people visualize an antenna as a wire or bunch of wires or aluminum elements, but that isn't the definition of an antenna - those are types of antennas. One dictionary defines an antenna as:

"A metallic apparatus for sending or receiving electromagnetic waves."

When we get to its most basic form, an antenna is simply something that radiates or receives RF energy.

Notice that the definition says that an antenna is a metallic object. While it may be possible to make non-metallic objects radiate, we won't concern ourselves with that here. Note also that the definition doesn't say what kind of metal or what shape it has. If we are trying to get on the air from a space-restricted location, then we shouldn't limit ourselves either. As will be shown, the variety of things that can be used to construct antennas is mind-boggling. The rule when constructing homebrew antennas is to let your imagination run wild. If it's metal and if it radiates, then it is an antenna. Of course we would like it to radiate RF energy efficiently and in the right directions! Figuring out how to do that is the purpose of these notes.

The Law of Reciprocity. In the literature on antennas there is an often referred to concept called reciprocity. Without getting into a lot of theory, reciprocity says that an antenna works the same for transmitting or for receiving. While the law of reciprocity is well known and proven, that doesn't necessarily mean that a good transmitting antenna is also a good receiving antenna. The problem is not violation of the law of reciprocity, but a difference in design objectives.

A receiving antenna is designed to deliver a distant signal to a receiver. In principle, there is always a tiny bit of signal everywhere, so the limitation on receiving is noise level. If a signal is weak, we can simply amplify it. But if the signal is below the noise level, amplifying both the signal and the noise will not help - it will still be below the noise level. The only thing important in a receiving system is the signal to noise ratio. The important point is that the desired signal and any received noise may have very different characteristics. For example the signal may come from Europe or Asia, while most of the noise comes from much nearer, maybe our own house or neighborhood.

A transmitting antenna, however, is designed to deliver a signal to another location. We cannot be responsible for the amount of noise received on the other end and usually have no control over it. What happens nearby is not much of a concern, unless it endangers someone or causes problems for our neighbors. So a transmitting antenna that transmits a good signal in a certain direction will most certainly receive a good signal from that direction, too. That's due to the law of reciprocity. If it also receives a lot of noise from another direction, then it may not be a good receiving antenna, though. That doesn't violate the law of reciprocity, since the antenna probably delivers a good signal to the noise sources, too, but no one really cares.

Conservation of Energy. One concept that is rarely formally stated in most amateur antenna texts is the principle of conservation of energy. In essence, conservation of energy says that energy is not lost nor created. As applied to an antenna, whatever energy is fed into the antenna is conserved, so an antenna cannot create energy nor can any losses be overcome without using up some energy. For our purposes that means that the energy fed to an antenna will be either converted to heat due to resistance in the antenna itself or it will be radiated. Period. There is no other place for the energy to go.

What that means for practical purposes, is that almost anything will work as an antenna, if you can feed energy into it. Hams have used rain gutters, light bulbs, bedsprings, tin cans and many other bizarre objects as antennas. The surprise is that they all work! Of course, they may not work very efficiently, but they do radiate - at least to some extent. That's good news for those limited to small quarters. It means that we can surely find something to accept RF energy from our transmitter and if it accepts energy, what isn't lost in resistive losses (heat) will be radiated.

Radiation Resistance. Radiation resistance is one of those terms that's tossed around quite a bit and quite often misunderstood. In electronics, the component that dissipates energy is the resistor and it works by changing electrical energy to heat. Whatever part of the energy that is changed to heat is lost for electrical purposes. In an antenna, there is physical resistance in the wire and other parts of the antenna, but there is also another source of energy loss. Hopefully some of the energy (most of it) is radiated. From the viewpoint of your transmitter, you have no way of knowing whether the energy was radiated or dissipated in heat. That's why a dummy load can be substituted for an antenna for testing purposes.

For convenience the radiated part of the energy can be represented as if it also were a resistance, called radiation resistance. Note that the radiation resistance is not a real resistor, but simply a convenient form for representing a loss of energy from the antenna. In a real resistance, the lost energy is converted to heat. With respect to radiation resistance, the energy isn't converted to heat, but simply radiated.

Ground. Another basic concept in antenna performance is the concept of ground. Normally we amateurs think of an antenna and a ground as two very different things. After all, the antenna is up in the air and the ground is down below. The ground is pretty solid, we walk on it, drive ground rods into it, etc. Unfortunately things are not that simple. Most practical antennas are positioned above the earth's surface and the radiation from the antenna interacts with the ground. This has two major consequences: some of the radiated energy is reflected off the ground and some of the energy is absorbed or attenuated by the ground. Of course the part that's reflected is still useful, but the part that is absorbed does us no good at all, even though it is radiated from the antenna. As a result, except for theory, it is necessary to think of the antenna and the ground as a system.

Note that there are other uses for the term "ground." There is a safety ground and a lightning protection ground and a ground for DC current. In the context of antennas we are concerned about RF energy. All of those concepts are important, however, here we will only be worried about the RF ground that affects antenna performance.

Transmission Lines. Most newcomers don't think too much about transmission lines or feed lines. After all, the plug on the radio says "ANT", not transmission line! However, with an antenna in the back yard and the radio in the house, it's obvious that something has to connect them. So we go to the local radio store and the clerk tells us to take some coaxial cable and connect one end to the radio and the other end to the antenna. That seems simple enough, so we buy it, go home, connect it up and (usually) it works. That's the way transmission lines should work!

It would be nice if a transmission line took the energy that the transmitter sends out and put it straight into the antenna. Unfortunately, real transmission lines are not invisible, but also have losses and change some of the characteristics of the energy on the way from the transmitter to the antenna, so we need to worry about the transmission line, too. In addition to the antenna and the ground, we also have to consider the transmission line as part of the overall system.

We will treat transmission lines in more detail later in these notes.

Next - Basic Concepts 2