Radiation from an antenna

 There are lots of questions about radiation from antennas or wires specifically. It is not well understood by many amateurs. There are several ways to try and explain it without using high level math. There are also many variables for any particular installation. However, regardless of the installation, radiation is caused by and is proportional to the current on the wire. I and other engineers use the term Meter—amperes. To try and simplify a complex topic,  it is the integral of rms current over the length of the wire.  Another way is to say it is that its the area under a plot of rms current against distance along the antenna. If you look at a plot of current on a halfwave wire, regardless of where it’s fed, you see a current minimum at the two ends ( high impedance points) and a maximum in the center ( low impedance point). Most of the radiation is from that center part where the standing wave ( not to be confused with the standing wave on a transmission line) of current is high. Radiation from the ends of such dipole is much less. That’s why you can bend the ends of a dipole down and it does not reduce the performance too much. Roughly it’s that middle third that does the most. It’s also why you can build a shortened dipole and put traps or loading coils out near the ends and not reduce performance too much. 

In general as long as you construct an antenna to put the high current portion high and in the clear your antenna will probably work pretty good. A half wave of wire radiated broadside and has a broad pattern. A null will be off each end. Operation at a harmonic will change the directivity and give more nulls. 

A non resonant wire will work just as well as long as the current maximum is placed correctly for the direction you desire. So a halfwave straight wire up in the clear fed at the end by a single wire such that the overall length is not resonant on the frequency of use becomes a random wire end fed antenna. It works almost exactly the same as a halfwave end fed or a center fed halfwave because the major part of the radiating is done by the high current portion of the antenna. That high current peak will always be 1/4 wavelengths back from the open or far end of the antenna wire. I always recommend learning current patterns on an antenna and the ARRL antenna book is a great resource. I have one published in 1947 and many others over the years. They are all good as the fundamentals have not changed.

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