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'The Resonant Point' no longer to be e-mailed as of 2026?

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ThaDood:
Due to the high restrictions sending high volume emails, this  will be our last Resonant Point article this way. Please go to our web site for the latest article.
www.isotronantennas.com


THE BEST WAY TO OPERATE HF:

EASY INSTALLATION
EXCELLENT PERFORMANCE
DURABLE CONSTRUCTION
CC&R FRIENDLY (XYL ALSO) 
I have been publishing this technical article since January of 2013. For the past 46 years I have manufactured the Isotron Antennas. This also involved helping many with their very difficult installations. Some of the information will be from what we learned solve these problems. Some information will be right from a text book or credible article.

You are welcome to contact me at wd0eja@isotronantennas.com with question you may have.

 THE AFFECTS OF GROUND IN THE REACTIVE NEAR FIELD

What in the world is this? It sounds like I know something. Don't be fooled, this can be simple even though it can be made quite complicated.

We are talking about the earth (dirt) directly below the antenna.

What is the affect on the antenna per different heights?

This is in respect to a dipole type antenna. This will also include the Isotron for those that are using them.
Waves radiated from the antenna directly downward reflect vertically from the ground and, in passing the antenna on its upward journey, induce a voltage in it. The magnitude and phase of the current resulting from the induced voltage depends on the height of the antenna above the reflecting surface.

The total current in the antenna consists of two components. The amplitude of the first is determined by the power supplied by the transmitter and the free-space feed-point resistance of the antenna. The second component is induced in the antenna by the wave reflected from ground. This second component of current, while considerably smaller than the first at most useful heights, is by no means insignificant. At some heights, the two components will be in phase, so the total current is larger than is indicated by the free space feed-point resistance. In other words it will be less than the 50 ohms needed to match the radio. At other heights, the two components are out of phase, and the total current is the difference between the two components. In other words higher than 50 ohms.

Changing the height of the antenna above ground will change the amount of current flow, assuming that the power input to the antenna is constant. A higher current at the same power input means that the effective resistance of the antenna is lower, and vice versa. In other words, the feed-point resistance of the antenna is affected by height of the antenna above ground because of mutual coupling between the antenna and the ground beneath it.

The electrical characteristics of the ground affect both the amplitude and the phase of reflected signals. For this reason, the electrical characteristics of the ground under the antenna will have some affect on the impedance of that antenna, the reflected wave having been influenced by the ground. Different impedance values may be encountered when an antenna is erected at identical heights, but over different types of earth.

Here are some examples of radiation resistance of a dipole type antenna as it varies in height above ground.

HEIGHT-WAVELENGTH (FEET)     RADIATION RESISTANCE-OHMS
0                                                                                 0
.1                                                                              20
.2                                                                              65
.3                                                                              96
.4                                                                              90
.5                                                                              70
.6                                                                              59
.7                                                                              66
.8                                                                              80
.9                                                                              82
1.0                                                                            72

In real life the radiation resistance will be somewhat higher. This is due to the earth absorbing some of the signal resulting in some loss and not reaching the antenna. This gives you an idea why antennas vary with each installation and why some installations work great, while others not so good.
                               
73,
Ralph WD0EJA

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