Almost anything can work for an antenna
Considering just wires. RF current flowing in a wire will radiate a signal. There are well known basic relationships for current and the wire length.
I like to go back to the basics. I have a Radio Engineering Handbook by Frederick E. Terman. I consider it a great reference. Terman was a Professor of EE and Dean of the School of Engineering at Stanford University.
In his book Terman includes a formula for the strength of a radiated field. Copies out of the book are below.
In an effort to not make this too complicated there are only two variables in the formula that directly relate to the antenna and that is the magnitude of the current I and the length of the antenna or antenna segment l. Other items are the phase of the current, distance from the antenna, elevation angle, the constant pi (3.14159), frequency, time, c the velocity of light, and wavelength.
So we see that simply increasing current and length over which current flows is all that increases the field strength.
If you want an antenna to radiate more, simply increase the current or increase the distance over which that current travels or both. Looking at a half-wave of wire that has somehow been supplied with RF current, it matters not how that was done at this point, we will see that if we break up the wire into segments each segment will have some current value. Assuming each segment is the same length, the segments near the center will have high current and the segments near the ends will have low current. From this formula we can easily see that the most radiation will come from the segments in the center of the wire and less and less radiation from the segments as we move out toward the ends.
For this reason it is very important to know the current distribution on your antenna. These high current sections need to be high and straight if possible. This is simple with a half wave antenna, the high current is near the center of the half-wave wire. The center third at least should be high and as straight as possible. The ends don’t matter nearly as much. They can be bent up or down even zig zagged.
Of course it is best if it can be high and straight for the whole half wave!
Above I have just discussed the simple half wave case. All radiation from a wire or wire follows this same formula regarding radiation from an antenna segment. However with radiation from wires longer than a half-wave long and various loops you need to add the radiation from each segment taking into account the geometry of the antenna. For example take a loop made up of four half wave wires formed into a square. This would be a two wave loop. Each side of the loop will have a side that radiates just like a half wave dipole. Let’s number each side 1, 2, 3, and 4 going around the loop. Note that sides 1 and 3 are parallel and separated by 1/2 wave . A receiving station off to the side some distance will be broadside to 1 and 3 and will receive radiation from side 1 and side 3 ( neglect the other sides for a moment). Sides 1 and 3 will have in phase currents. So what will be the phase relationship be when radiation reaches the receiving station? Let’s put the receiving station 1 wave from the antenna side 3 that puts it 1 1/2 waves distance from side 1. Radiation from both sides starts in phase. Radiation from side 1 travels 180 degrees farther than from side 3. That means radiation from these two sides tends to cancel. If the receiving station was off of side 2, it would receive radiation from sides 1 and 3 in phase and that radiation would add together. The same would happen to radiation from sides 2 and 4. It would tend to cancel broadside off sides 2 and 4 but add up broadside to sides 1 and 3. This is an over simplification of course because the actual phase relationships are very complex to sum up. Basically the pattern for a full wave loop would be a clover leaf and have radiation off all 4 sides slightly greater than a dipole and radiation off the corners would be down about 10 dB from the broadside. Computers can easily handle the math.
If two half wave wires were placed end to end and fed in phase any receiving station broadside would receive radiation from each wire completely in phase. End to end spacing could be adjusted to optimize the geometry to give maximum gain.
Now let’s consider a full wave loop. Each side being a quarter wave. In this common case we have adjacent sides in phase and opposite sides out of phase. Essentially it would be like two horizontal dipoles in a V shape with the ends of one V dipole touching the ends of the other V dipole. The currents in these opposite dipoles would be out of phase. This provides a rather complex summation problem that results in a pattern that has two lobes of maximum gain ( both less than the dipole max gain)and has two shallow nulls (much less than the deeper nulls of a dipole). It is possible that those who like the loops like them because of the shallow nulls. With the same amount of wire slightly better performance can be had with two dipoles at right angles.
The same can be said for any loop.
Much longer loops will have larger major lobes and will then exceed the gain of the half-wave dipole in certain narrow directions. They will also have more and deeper nulls and more minor lobes. Satisfaction with these larger loops will be determined by the necessity of communicating in a specific direction and of course the proper height above ground to determine the elevation angle.
It appears a full wave loop may be the best overall loop size as it is within 1 or 2 dB of a dipole on its fundamental frequency and it’s null is about 3 dB less than a dipole. As far as wire is concerned one full wave loop takes the same amount of wire as two dipoles. The loop has the advantage of being used on other bands, but has the disadvantage of needing a tuner and possibly having excessive feed line losses if not fed with open wire line.
The cleaner winner in my book is two half wave dipoles at right angles. Simple and easy, fed with coax and no tuner required.
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