Below is a question posted on Part15.us, and my comments in response. Not sure if readers here also read Part15.us...
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Q. Must ground radials be placed symmetrically? I seem to recall reading something about that. I'd probably be able to get them 120-150 degrees around my antenna, out into the yard running away from the house.
A. Radial ground wires provide a low resistance path for the currents induced in the nearby earth from radiation by the antenna. This is important to have, because the resistance in this connection is in series with the r-f current in the antenna. The current flowing through the ground resistance consumes power that could otherwise be radiated by the antenna, which reduces the radiation efficiency of the antenna system.
Here are some numbers showing the effects of a radial ground system on received field strength, based on carefully measured data. Frequency is 1.7 MHz, radial length is 80 feet (0.137 wavelengths). The reference field is that produced by a 1/4-wave vertical with about 120 symmetrically arranged radials, each 0.41 wavelengths long.
Number of Radials > % of Ref. Field
2 > 9.5
15 > 16.8
30 > 20
60 > 22
So using 30 radials in this situation increases distant field strength to more than double that when using 2 radials. To double field strength when using 2 radials, the transmitter power would need to increased by four times. (All comments assume other things are equal.)
The most important benefit of the ground system is to reduce ground losses for the antenna system. Asymmetric placement of the radials around the base of the radiator isn't ideal, but if the radials that can be installed minimize ground losses for a given installation, that will maximize the radiation efficiency of the antenna system for that situation.
The feedpoint impedance of the antenna system will change after installing a radial ground system. The resistance term should be lower, and the tuning/matching circuits of the transmitter may need to be optimized for maximum power transfer for the new conditions.
BTW, a cold water pipe ground or "ground stake" is not a very effective MW ground no matter what the length. There is little r-f current induced in the earth beyond about 1/2-wavelength from the radiator and most of it is flowing near the surface, horizontally back toward the radiator. So the only lengths that collect any ground currents from the antenna are within that area.
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Q. Must ground radials be placed symmetrically? I seem to recall reading something about that. I'd probably be able to get them 120-150 degrees around my antenna, out into the yard running away from the house.
A. Radial ground wires provide a low resistance path for the currents induced in the nearby earth from radiation by the antenna. This is important to have, because the resistance in this connection is in series with the r-f current in the antenna. The current flowing through the ground resistance consumes power that could otherwise be radiated by the antenna, which reduces the radiation efficiency of the antenna system.
Here are some numbers showing the effects of a radial ground system on received field strength, based on carefully measured data. Frequency is 1.7 MHz, radial length is 80 feet (0.137 wavelengths). The reference field is that produced by a 1/4-wave vertical with about 120 symmetrically arranged radials, each 0.41 wavelengths long.
Number of Radials > % of Ref. Field
2 > 9.5
15 > 16.8
30 > 20
60 > 22
So using 30 radials in this situation increases distant field strength to more than double that when using 2 radials. To double field strength when using 2 radials, the transmitter power would need to increased by four times. (All comments assume other things are equal.)
The most important benefit of the ground system is to reduce ground losses for the antenna system. Asymmetric placement of the radials around the base of the radiator isn't ideal, but if the radials that can be installed minimize ground losses for a given installation, that will maximize the radiation efficiency of the antenna system for that situation.
The feedpoint impedance of the antenna system will change after installing a radial ground system. The resistance term should be lower, and the tuning/matching circuits of the transmitter may need to be optimized for maximum power transfer for the new conditions.
BTW, a cold water pipe ground or "ground stake" is not a very effective MW ground no matter what the length. There is little r-f current induced in the earth beyond about 1/2-wavelength from the radiator and most of it is flowing near the surface, horizontally back toward the radiator. So the only lengths that collect any ground currents from the antenna are within that area.
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