Antenna Signal Patterns
One of the first challenges new hams have is to develop a basic understanding of antenna patterns. To take a polar coordinate plot of an antenna, and connect the vertical pattern of the same antenna and tie the two patterns together to make a 3-D pattern of the results, can be difficult. However, this visual aid can result in putting up an antenna with the probability the radiation pattern will go where you want.
Objects Absorb Your Signal
The patterns represented in the books are under ideal conditions, no interfering objects, such as power lines, phone lines, etc. In the real world things are not so ideal, and antenna patterns are distorted by many things in the near field of the antenna. Anything metallic and within 1/4 to 1/2 or even 1 wavelength of the antenna can distort the pattern and sometimes absorb so much of the transmitted signal that you can’t even get out of the back yard.
Unfortunately, there is no way to predict what and how much an object will interfere with your transmitted signal. One case that will certainly cause loss of signal is where a vertical antenna is less than 1/4 wavelength from another vertical metallic object that is connected to a very solid ground. This situation can result in the transmitted signal being either reflected or directed by the other vertical object, depending on the spacing from the vertical antenna. If it is too close, there will be a lot of absorption of the transmitted signal, and a great reduction will take place in your transmitted signal.
The same thing will happen to a dipole too close to power lines, and if there is too much coupling, there will be TVI, BCI, and more cases of interference to things unknown than you can imagine. Phone lines can do the same thing, not to mention the wipe out of a solid state phone if the induced current is too high. So, for the most part we just have to grin and bear it, and endure our antenna patterns being distorted.
Some Signal Patterns
So on to the diagrams. In Diagram A, a polar plot of a phased vertical array is shown and the array consists of two 1/4 wavelength verticals spaced 1/4 wavelength. The phase shift between the two verticals is 90°. The resulting pattern is called a cardioid pattern because it is shaped like a heart. This simple array gives a gain of 3.4 dB over a single vertical, with a very deep null to the rear of the array. Signals that arrive directly to the rear simply cannot be heard, nor can the transmitted signal be heard. This is a good example of a unidirectional array.
In Diagram B, the array has been sliced down the middle, and the pattern is shaded so it may be seen. All of the angles of radiation have been exaggerated and the pattern has been extended past the ground wave terminator. This is for clarity. The only thing not shown is the deep null to the rear of the pattern. It is easy to see this pattern, all you have to do is to look at the heart-shaped jewelry a lot of the women are wearing, particularly the fat heart shaped necklace items. That is what a 90° 1/4 wave phased array pattern looks like, or as close to it as anyone will ever see, unless one has the special ability to see down into the BC band.
The next pattern in Diagram C is that of a single vertical antenna, 1 /4 wave tall. It is also sliced through the middle and the viewpoint is from ground level. This pattern looks like a doughnut. Not too complicated so far.
Diagram D is that of a 1/2 wave vertical. It is flatter, because a 1/2 wave vertical has a lower angle of radiation than a 1/4 wave, making it a slightly better antenna for DX. The 1/2 wave vertical has its current node in the center of the antenna, which removes the current node from the influence of things in the immediate vicinity.
Keep in mind that these patterns are not to scale, but are used only to help you understand what antenna patterns look like.
Now, in Diagram E, there are two different antennas. Antenna no. 1 is a dipole with something causing the pattern to not be symmetrical.
There is one good lobe on one side and the other side has several lobes with much higher angles of radiation. This happens more times in real life than you can imagine, but without accurate measurements, usually can’t be proved beyond “I get out better to the North, East, West, etc., than anywhere else.”
Diagram F is of a bi-directional antenna such as a dipole. The minor lobes of the antenna as well as the major lobes are shown. The vertical angle of radiation is also shown.
Diagram G shows the same thing for a unidirectional antenna, such as a beam. Remember, these are representative diagrams, not of any particular antenna.
Diagrams H and Diagram I show the difference between a pair of in phase and out of phase antennas.
Diagram J is the pattern of a pair of in phase antennas, spaced 1/2 wavelength. This is a broadside array, as the signal comes off of the array at a 90° angle.
Diagram K is of two verticals that have a 180 phase shift between them. This is an endfire array because the signal comes off of the antenna along the axis of the array. In both cases, each antenna has equal power being fed to it. In other words, if 100 watts is being fed to the array, each antenna is getting 50 watts.
Combining the two patterns by switching the antenna phase, a directional array with two different patterns can be made. The broadside array has a narrower pattern than the endfire, with a little more gain than the endfire array. This is an easy antenna to construct for DX on 40 or 75 if you have the room, and it works well.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 19th March 2024