Radiation from Antennas

Evaluating radiation from antennas is not widely understood. My intention is to attempt to simplify this misunderstood concept.

I will use the term wire but it could actually be any conductor normally used in antenna construction. I will also use the term amp or ampere to be current flow. 

Radiation from a piece of wire (antenna) is simply proportional to the amount of current flowing in that wire. It is also proportional to the length of that wire. If the wire length is measured in feet and the current in amperes we can do some comparisons. 

I will start very basic to introduce what may be a new concept to many. First of all , if one amp of current flows in a piece of wire one foot long that will result in less radiation than if we have one amp flowing in a wire two feet long. I think that is easy to understand. We have a term for current flowing in a wire. That is feet-amperes. If you take the length of the wire and multiply it by the amount of current you have a result in feet- amperes.

A wire one foot long with 1 one amp will radiate the same as a wire two feet long but only carrying one half of an ampere. Many text books will use the equivalent term Meter-ampere. It matters not what system you are in as long as you are consistent. In the above example the wire two feet long carrying one amp of current is said to have two feet-amperes. It stands to reason that if we supply one amp to a wire, the longer that wire is the more feet-amps we will have, and consequently the more radiation we will have. That is true to a certain extent but we need to set some boundaries on this or we might think only very long wire are the best antennas.

We are talking about radio frequency current and radio frequency radiation. We must include the wavelength in our calculations or at least limit our thinking to wires on the order of very short up to about a half wavelength long. For wires longer than a half wavelength long it gets a bit more complicated. We will not go into that now as my intention is to keep this as simple as possible. 

The term feet-amperes and meter-amperes are commonly used terms. There is another similar term that is more general and takes into account the wavelength. 

One wavelength is the distance the radio wave travels in one cycle of current flow. In engineering we say that one cycle is 360 degrees. It makes a lot of our calculations very easy if we use degrees. For example one wave length is 360 degrees, one half wavelength is 180 degrees and one quarter wavelength is 90 degrees. In other words we can use degrees as a measure of wire length. A wire that is 360 degrees long is one wavelength long, a wire 180 degrees long is one half wavelength long. Using degrees a a measure of length is very convenient. The term feet-ampere and meter- ampere is not as general as is degree-amperes. You may see any of these used in the antenna and radio engineering literature. Not so much in the amateur radio literature. 

So if you have a short antenna, say 45 degrees long and it is supplied with one amp of current we say that we have 45 degree-amperes. An antenna 90 degrees with the same current has 90 degree-amperes. Thus we conclude that the 90 degree long antenna radiates more than the 45 degree's antenna because there are more degree-amps in the straight wire. 

This product of length times current is also called a moment. I will not get into that here as that is an advanced topic or concept.

I will say that if you draw a straight line representing an antenna a half wave or 180 degrees long and then draw half of a sinusoidal curve above it  you will see that the curve and the straight line enclose a certain amount of area. The area of that curve is significant. It represents the length of the line times the current or feet-amps, meter-amps or degree-amps depending on what units you use to measure the length. A larger area under that current curve the more radiation you will obtain from that segment of the antenna.