Antenna Thoughts

I have been thinking about antenna radiation patterns. This after doing some computer modeling of various dipoles and verticals. Ground effects have considerable say, as well as height above ground, when considering the resulting radiation pattern. Here are some thoughts as to why that is.

A dipole in free space has a donut shaped radiation pattern with the wire perpendicular to and going through the hole in the donut, so we are told. We normally mount the dipole horizontally. Most installation instructions caution you to try and install the dipole one quarter wavelength above ground on the lowest frequency of interest. Ever wonder why that is?

Well, one reason is that at a quarter wavelength above ground, the dipole is a perfect match to 75 ohm transmission line. The other reason is that at a quarter wavelength above ground, the reflected wave (that part of the radiation being reflected back up from the ground) is IN PHASE with the radiation being emitted above the dipole. This IN PHASE condition will occur for any multiple of half wavelength traveled by the reflected wave.

Note that the reflected wave has to travel down to the reflector before it can be reflected. Then, when it is reflected, it has to travel back up to the antenna before it can have an effect on the radiation pattern. So it travels a half wavelength even though the antenna is only a quarter wavelength above the reflecting ground.

So a support at 45 feet above the reflector will provide an in phase condition for a 10 meter application because 45 feet is three half wavelengths. Or a round trip of six half wavelengths for the reflected portion of the RF.

This 10 meter example also demonstrates the difficulty in ensuring that the radiation reflected from the ground travels a multiple of half wavelengths before it reaches the radiator. With so many multiples it does not take much to be off. I have not seen many 10 meter antennas mounted 7feet (approx 1/4 wavelength) above ground.

At higher frequencies, like 10 meters, the reflection problem is neatly solved by using beams. By providing reflectors and directors in the horizontal (or vertical) plane the pattern is skewed toward the favored direction making ground reflections less significant.

When the radiation is IN PHASE, it ADDS! (Unfortunately, OUT OF PHASE radiation cancels or subtracts). That is why you see a dipole rated at 2.5 dbi gain. Meaning nearly double the gain of a dipole in free space. Actually, it means gain over a point source radiator in free space. The point source would have a spherical radiation pattern. I suppose the lower half of that sphere would be reflected up by the ground and allowed to add the same way the lower portion of the donut pattern of a dipole does, but a point source does not exist in reality, a dipole does.

An actual doubling of power would equate to a 3db increase. So I guess the 2.5 (or 2.7)db gain makes allowance for an imperfect ground. (not a perfect RF mirror).

Before you rush out to measure the height of your dipole supports you might want to know that height above ground means height above ELECTRICAL ground. The reflected RF does not care where your lawn ends. It is more effected by where your water table starts.

Water tables vary in depth below the surface over time and seasons. So knowing were it is now may not be useful later. Then consider what might happen when it rains. Wouldn’t a wet lawn bring the electrical ground closer to the surface? Since it is unlikely that we will resort to variable height antenna supports, we need to strike a happy medium.

The electrical ground being below the surface of the lawn actually works in our favor because now we can lower the antenna supports and still get our quarter wavelength seperation between radiator and ground reflector.

In my area the electrical ground appears to be somewhere between 10 and 15 feet below the surface. I came to this conclusion because there is a creek running alongside my property about 300 feet to the west. The surface of the water in the creek is 10 to 15 feet below ground. My 75 meter dipole has its apex at 55 feet and appears to work fine.

‘Fine’ means I am happy with its performance. I have no idea if it would improve things by changing its height. I subscribe to the theory that is best to leave a sleeping dog lie. As long as you are sure he is not really dead.

As high as possible is not always a good solution. If you happen to be unlucky enough to hit 3/8 wavelength above electrical ground, the reflected RF will be OUT OF PHASE (3/4 wavelength round trip) and have a cancelling effect on the radiation leaving the antenna at the top. At 3/4 wavelength (round trip) you also loose that magic 75 ohm match. If you are using coax, losses in the feedline combined with the cancelling effect at the antenna may set a new poor performance record.

Optimizing a simple antenna system is not easy. Most people just give up and go to a beam to ensure success of some degree. Fiddling with height above electrical ground can be frustrating. Note that beams do not have to be made of expensive tubing. Wire works too if the supports are available.

Three db is only half an S unit in signal strength. Going to a two element beam can easily get you that 3db and more. Or you can always increase your transmitter power by 2X and gain that 3db. Note that increasing transmitter power to overcome antenna shortcomings does nothing for the received signal.

The antenna is the only place I know where you can improve your ability to hear signals as well as improving other stations ability to hear you. All by improving only the antenna. Biggest bang for the buck? Maybe not, but it will more likely occur with an investment in the antenna.

The following link will take you back to the main antenna page.

This link will take you back to the K5DKZ home page. You can also get there by selecting K5DKZ under the Pages heading in the green sidebar.