A Point To Ponder

Our local weather recently dumped over 2 feet (.6 meters) of rainfall in just about 12 hours. Then, over the next few days, we had more rain. To put it mildly, we were floating for nearly 5 days. The ground is definitely soaked, and even though at this time, nearly two weeks later, mud still rules my yard and the surrounding neighborhood.

How does this pertain to antennas? Simple. My antenna had to be retuned! My particular antenna is a four wire inverted V quadpole, fed by twinlead. Two of the elements are cut for 40 meters, and the other two elements have been carefully cut to no particular length at all. The center of the dipole is about 40 ft. (12.6 meters) up and the ends vary from about 6 ft. to 12 ft. (1.82 to 3.6 meters) above ground. I use an old 275 watt Johnson antenna Matchbox to tune the antenna on all the other bands I operate use. After taking care of the other disasters that take place when a monsoon of this magnitude hits, I went to the ham shack and decided to get on the air. I was rather surprised to see my VSWR had jumped up beyond the measurement capabilities of my meter.

I returned the antenna and continued on operating. Several days later I noticed that the VSWR had started to creep up again. So the antenna was retuned again. As the lake in the back yard began to vanish, the VSWR kept changing. Now the lake is gone and the mud is receding, at least on the surface. The VSWR continues to change on a daily basis and constant retuning is still taking place, but not as drastic as originally took place during the highest moisture levels. Of course, this is due to the conductivity of the earth changing in relation to the gradual drying out of the soil.

Before the rains took place, we had been in a rather dry spell and the VSWR had not varied too much on a daily basis. But I have noticed fluctuations in the reflected power throughout the days and weeks. I have a 15 ft. (4.56 Meters) ground pipe made of copper pipe in the ground about 13 ft. (3.952 meters) in the ground. I also know there is a subterranean river under my house that is about 17 to 20 ft. (5.16 to 6.08 meters) below the surface.

Now, one of the things I learned is that ground is not always ground. What defines ground is where ground conducts—or reflects. If you put an antenna up over sand, where is ground? Sand does not conduct unless it is wet, or does it? The black clay, that is predominant here obviously, has no such definite conditions of conductivity since it can vary in moisture content as a result of the varying soil composition. Sand is either wet, damp or dry. A solid rock formation is also going to be dry, be it granite, flint, marble or any other material. Someone living on a rock formation will need a ground system of radials to get a vertical to work, unless you are using a ½ wave vertical. or other ground independent vertical antenna.

So now here’s the question. If you live on a rock called a mesa, which means table, or other rock formation and you are at an altitude of 5,000 ft. (1520 meters), is your antenna that high above mean sea level? Where is RF ground? If you are able to put an antenna in the top of a 300 ft. (91.5meter) tall tree, does the tree become your ground? It has moisture in the trunk and leaves. So where do we stand with this type of installation? Do people install antennas in the top of such huge trees? Well one person I talked to years ago, had done so. I seem to remember he had a ½ wave, 160 meter vertical along the trunk and a curtain array on 75 meters. He had an outstanding signal on 75.

But that does not answer the basic question—was the antenna actually 300 ft. (91.5 meters) high or what? Another question arises from the location directly on top of a salt marsh. This is where another antenna, a vertical this time, was sitting on a pipe stuck in the mud and about1 ft. (.304 meter) above sea level at high tide. No radials, just a trap vertical on a pipe stuck in the mud with between 1 and 5 ft. (.304 to 1.52 meters) of salt water surrounding the shack and antenna for a radius of about 100 yards (30.4 meters). That ham also had a very outstanding signal on the two bands that I talked to him on, 75 and 40 meters, over a 1,200 mile circuit. It would have been interesting to find out if the SWR changed when the tide ebbed and flooded.

Another ham, with an excellent signal, lived on top of a rock pile (small mountain) in west Texas, at an altitude of 2,400 ft (729.6 meters). He had a radial system installed under his vertical as well as a few strips of wire mesh under the length of his dipoles. The sandy surface was only about 3 ft or so (1 meter) deep over solid rock. Water had to be brought in for drinking because of the arid location, and there was absolutely no water to be found anywhere under the solid rock. Did he make a ground or was he suspended in mid air as far as RF was concerned?

Determining the exact conductivity of the ground surrounding an antenna is not going to be easy except in the case of an antenna located over salt water, dry sand, or solid rock due to the many variations in soil composition found in soil all over the world. Of course, there are devices to be built or bought that can measure ground conductivity. It would be interesting to see what variations in antenna VSWR would take place on a daily basis, due to changes in such conductivity, on the lower frequency bands in particular. This is due to the fact that these antennas are usually closer to the ground in relation to wavelength. By this I mean a 75 meter dipole at 40 ft (12.6 meters) is going to be closer to the ground RF-wise than a 10 meter beam on top of a 65 ft. (19.76 meters) tower. That is just at 2 wavelengths on 10 meters, whereas the 75 meter antenna at 40 ft. (12.6 meters) is less than .2 of a wavelength. So, the closer the antenna is in relation to the ground surface, the greater the ground losses.

Being located near a large body of water makes a good location for an antenna—but does the VSWR change directly with the change in water levels? Has anyone made any measurements? It would be interesting to see how fluctuations in ground moisture can affect DX contacts. Does anyone want to undertake an informal study this? Winter is coming and a lot of areas will be covered with snow here in the Northern Hemisphere and it would be interesting to see as how snow fall affects DX working conditions.

If someone anywhere would keep VSWR logs on a daily basis and how it was affected by local climatic conditions, I will be willing to bet the information would be interesting to analyze. Of course you would also have to keep note of propagation conditions in general. But I think this type of information would add a lot to propagation data that does not seem to have been considered before. This would be by its very nature, a long term study. Something like this is so complicated, precise conclusions could not be drawn from just a few weeks or months of data, but from several years at least. It would be worth while to undertake though and should be of a worldwide effort. Even if a person would like to just satisfy their own curiosity, that is a good enough reason.

Now how can the level of moisture in the earth determine DX ranges? Simple. The radiated signal from a horizontal antenna bounces off of the earth doesn’t it? So if the RF is going to bounce off of an RF reflector, that would have to be something RF sees as a reflector—which would be a lot of water under the ground—or does it? Now we have some points to ponder—to soak or not to soak, that is the question.

A last comment. Many years ago, I was operating one of the MARS stations at Ft. Benning and got a call from a ham on board a U.S. Navy submarine. I don’t remember which boat it was, but he was using a URC-32 rig into a 36 ft telescoping whip at about 500 watts. He had a very stout signal, 20 over 9, and the signal level stayed constant during the contact. The boat was in port, docked and undergoing refitting before returning to sea. Sitting in salt water would give a very good ground plane. At the time, we just talked about our particular services and what we did. My antenna was a dipole up 90 ft. (27.36 meters) and on top of a hill surrounded by swamps. I think he was in Groton at that time, over 34 years ago. Point is, both of us were surrounded by water, him by salt water and me by swamps. Both of us had excellent signals both ways. There was always a good ground no matter where we were located at Ft. Benning. Water was everywhere, both above and below the ground. I always managed to get a good ground no matter where I was in the field. I used to get on 20 meters AM with a good signal from a 16 ft. (4.86meter) military mobile whip. Of course, operating from an 8 ton metal armored personnel carrier helps particularly if it sinks into very damp soil.

The illustrations, Figures 1 and 2, are simplifications of what your horizontally polarized antenna sees when you are transmitting. This is also very much simplified as the radiated energy is much more dense than depicted. Basically the reflected energy bounces off of the water present in the lower levels of the layers of the different strata present. The reflected waves then either add or subtract from the energy of the directly radiated waves, depending on the phase of the reflected energy. The amount of cancellation or addition is also dependent on the power the reflected wave has after reflection..

Figure 1 shows the direct radiated energy and how the reflected waves bounce up and intercept the direct energy from the antenna. Figure 2 is a representation of how the different layers might be found as you dig or drill down through the earth, such as for a mine or oil well. This is also very much simplified, as the different layers are not usually as sharply outlined, but usually blend into each other gradually.

All of this amounts to a lot of points to ponder. Does your ground conductivity determine how efficient your antenna is and if so, just how much.? Does the depth of the water table affect your signal strength at long ranges? Does a man-made ground plane help stabilize the location of RF ground? I am sure that there will be plenty of comments on this. Send them in the form of an article and let the world know what you have found out.

Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF

Last Updated : 28th March 2024