Control Your Pattern Projection
Plot a World Map
As an example, years ago when I was stationed at Fort Benning, a W4 located in the area showed me his unusual setup. I noticed that his rotor indicator had a large plexiglass overlay with an antenna pattern over the world map instead of an indicator needle. I expressed some interest about why it was done this way. He explained that by seeing the antenna pattern enabled him to rotate the beam slightly one way or another thus placing an interfering station in the null of the antenna pattern. Of course this didn’t work when the station was directly in line with his point of contact. Also, you must know the relative location of the other station(s).
Steer the Pattern
Being able to steer your pattern gives an enormous advantage for sending the signal toward the appropriate direction (offensive) and using the null to block interference (defensive). A case in point is shown in the world map illustration.
An Example
A ZL station near Wellington was interested in maintaining a sked with his sister, a war bride who now lives in Oregon. Over the years, the antenna farm has been changed many times in order to maintain communications with Oregon. Finally the antenna farm ended up being a pair of phased verticals with a quarter wave spacing and 90 degree phase shift. This was the only antenna that would fit on the roof of his flat and not irritate his landlord. By careful measurements with compass and ruler, the direction was laid out and the antenna was erected (Pattern 1).
The resulting pattern covered the South Pacific and the West Coast of the U.S. with a very stable signal to his sister. Without anything on the reciprocal bearing of the major lobe, the deep null behind the array did not cause any handicap. For other communications, a pair of dipoles, oriented 90 degrees to each other, performed well.
Sometimes, on the higher frequencies, even a rotary dipole can make a difference. For instance, let’s suppose a JA in Tokyo wants to do some 20 meter DX chasing. Being limited to the type of antenna and power allowed adds difficulty to his project After careful thought, a dipole is made out of bamboo and disguised as a trellis when not in use. Plastic plants cover the wire taped to the bamboo rods. When it is time to go on the air, the long rods are dropped to the horizontal position, and the mast is pushed up over the roof and rotated by hand to the desired direction. By knowing the pattern of a dipole is a figure 8, the antenna can be oriented in the direction of whatever country is desired. Then, only luck is needed to get the rare one through the pile-ups (Pattern 2).
Another Example
There is no denying that a directional array can make a difference. Now suppose that a W4 in Florida has a four-element beam at 60 feet. Looking at Pattern 3, with the beam pointed east, the major lobe is going across the Middle East and into Japan. This will enable a QSO between the JA in Tokyo and Florida to take place, assuming propagation is good between Florida and Japan via long path. For now, we will assume it is a good long path circuit and a QSO takes place.
During the course of the contact, a VK near Perth calls the W4. Now the VK can’t hear the JA in Tokyo, even if conditions would allow it, because the JA is in a null of the VK’s antenna (Pattern 4). Now let’s assume both the W4 and the VK are using identical four-element beams. If they both aim at each other, neither will hear the JA station and the W4 will almost vanish in Tokyo.
Now to make things a little more interesting. A K5 in Cricket Leg, Texas shows up on frequency, and he is using a vertical. Well, since the tri-band vertical has a circular pattern (Pattern 5), this ham can hear everyone but the W4 because he is off the backside of the W4’s beam. Gets complicated doesn’t it?
A Real Experience
In my own experience, I had a three-tower phased array on 40 meters. When the array was phased to place Europe in the null of the array, the SWBC signals went into the noise level. When the array was aimed at Europe, signals were full scale on the S-meter. I was able to punch a signal though to the East Coast with 20 over signals in Maine running 100 watts out of a Heath SB-101. Again, an understanding of the antenna pattern allowed me to use the array to best effect.
Angle of Radiation
So far we have not talked about the vertical angle of radiation, which also plays a major role in producing the desired signal results. In simple terms, a low angle of radiation is good for DX and a high angle is good for local contacts. The reason for this is that, in the course of getting from here to there, your signal has to bounce off of the earth and one or more ionized layers of the atmosphere. Each time it does this, some of the signal is absorbed by both the earth and the ionosphere. Do this enough, and you loose all of the signal through absorption. Looking at Figure 2, you will see that the high angle of radiation antenna causes its signal to bounce many more times in comparison to an antenna that radiates a low angle signal.
Look for Local Objects
Sometimes, despite all of the work and study put into an antenna, it does not work the way we want it to. Most likely, there is usually something in the immediate field of the antenna causing the problem. By either re-radiating an out-of-phase signal or absorbing too much RF from the antenna, an obstruction can cause the signal to go bonkers. The first thing to do in this case is to search the immediate area, at least within 1/8 wavelength of the antenna, for a metallic object that may be of the same or close to the same polarization. Power lines can cause a dipole to become directional or not work at all, depending on how close the antenna is to the power line. Fences can do the same thing.
Another Experiance
Another personal experience in this vein took place when I put up a 20 meter vertical on the roof with poor results. After several weeks, I realized that a mast supporting my two-meter Ringo was less than 10 feet from the vertical. After moving the mast, the vertical worked just fine. The two-meter mast also resonated on 20 meters, I found out later. A little foresight would have prevented that problem and saved a lot of work.
A Common Problem
Judging by the number of letters received, others have had and are having similar problems with unknown-object interference. If you follow the suggestions in this article, you should be able to find that antenna nemesis.
A note about the patterns in the illustrations. The patterns for the two beams (3 & 4) are totally imaginary and are used for demonstration only. They are not to be confused with any other beam, living or dead. The patterns for the dipole (2) and the vertical (5) are correct and assumes the field is free of object interference. The pattern for the pair of phased verticals (1) is also a representative pattern typical of quarter-wave spaced verticals.
Keep in mind the patterns in real life have no boundaries other than those limited by the propagation. An antenna pattern will extend to infinity no matter what the pattern looks like. The boundaries on the diagrams are there to represent the concept of properly using the antennas pattern to your advantage.
In Figure 2, only one radiated wave is used in order to simplify the illustration. In real life, a high-angle radiation antenna will have some low-angle radiation signal, but usually not enough to help. However, installed properly, a low-angle radiation antenna will be devoid of high-angle radiation.
A vertical on 75 meters is not going to give very satisfactory results for local ragchewing, say within 1000 miles. But an inverted V will do very well and even allow you to do some DX chasing. For point to point communications, the antenna system must be examined carefully in relation to the distance to span and for elimination of potential interference in your path. On the ham bands, interference is a way of life.
Help Missionaries
Here’s an example of planning a system. About 15 years ago, a friend of mine asked if I could help him design a system for a group of missionary workers in the South American countries. They had enough hams in the group to handle the radios, but needed cheap and reliable directional antennas. Beams were out in many cases because of the lack of ac power. The transceivers ranged from Ten-Tec Argonauts to elderly Hallicrafters and Collins ac-powered rigs located in the larger towns.
After a lot of thought, a multiband multi-antenna directional wire antenna system was designed. By using wire verticals hung from trees and phasing them, 40 meters was made useable. At the various locations here in the U.S., a combination of phased verticals and phased dipoles were used. This got rid of the SWBC stations from Europe. Later on, phased inverted V antennas were added. At the village level in the jungles, dipoles were used for inter-area communications. By using the phased vertical wire antennas, in conjunction with a good ground system, the 5-watt Ten-Tec rigs were able to report directly to the missionary centers here on either SSB or CW on 40 meters in the evening and at night.
Later on, 20, 15, and 10 meters were added in the same manner, with wire antenna arrays consisting of phased verticals, phased dipoles phased inverted Vs. Some wire YAGI antennas also were used. For the most part, the antennas were put up using a compass to aim in the right direction. A few were put up by using the North Star and the Southern Cross for relative reference and educated guesswork.
Since the missionary groups depended largely on donated equipment, the use of the directional wire arrays was an answer to their prayers. This selection of a simple antenna system, easy to build and erect, did the job for years until the warring factions caused the missionary groups to vacate some of the countries.
The South American story once again proves that antenna systems do not have to be expensive and complicated for most of our needs. And a careful consideration of the intended use enabled the missionary radio network to do as well as it did. Care and planning will increase your chances for projecting a successful signal pattern for the desired use—and that is what the antenna game is all about!
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 19th March 2024