I have never isolated any antenna from the mast since 1965 but if it makes you happy then go for it.
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Antenna mast isolation
- Thread starter Christopher17
- Start date
I started looking into the concept of isolating vertical antennas from the ground and their support structures years ago based on something i read in what i consider to be a great resource on antennas, The Department of the Army Technical Manual TM 11-666
titled 'Antennas and Radio Propagation' published in 1953.
Instead of adding to any confusion by editorializing what i read, i will just quote directly from the book the parts that made me start considering this as part of my antenna installations.
This section comes from the chapter on vertical antennas, and is entitled 'Counterpoise'.
"When an actual ground connection cannot be used because of the high resistance of the soil, or because a large buried ground system is not practicable, a counterpoise may replace the usual direct ground connection in which the current actually flows to and from the antenna through the ground itself.
The counterpoise consists of a structure made of wire erected a short distance off the ground and insulated from the ground. The size of the counterpoise should be at least equal to and preferably larger than the size of the antenna.
The counterpoise operates by virtue of its capacitance to ground. Because of this capacitance, the ground currents which flow normally and usually are collected by conduction now are collected in the form of charge and discharge currents.
The end of the antenna which normally is connected directly to ground now is connected to ground through the large capacitance formed by the counterpoise.
If the counterpoise is not well insulated from ground, the effect is much the same as that of a leaky capacitor. Leakage currents flow between the counterpoise and ground so that a poorly insulated counterpoise introduces more losses than no counterpoise at all.
The shape and size of the counterpoise are not particularly critical. In some field antenna installations, a type of grounded antenna is used in which a large portion of the antenna is folded into a horizontal position. the counterpoise used with such an antenna has the same shape and approximate dimensions as does the antenna itself. This counterpoise is mounted directly under the antenna at a height of about 8 to 12 feet off the ground.
When the antenna is mounted vertically, the counterpoise is made to have any simple geometrical pattern such as those shown in figure 114 (square and hexagonal shapes are shown with radials extending out horizontally with wire connecting them like spider webs)
Although perfect symmetry is not necessary, the counterpoise should extend for equal distances at all angles from the antenna.
The area covered by the counterpoise should be as great as possible, although very little is gained by extending the counterpoise more than half a wavelength from the lower end of the antenna.
in some VHF antenna installations on vehicles, the metal roof of the vehicle is used as the counterpoise for the antenna."
No i do not consider this to be proof of why someone should isolate their antenna from its mast.
I do think it provides a good explanation of what a counterpoise is and does.
I don't want to type another book right now LOL, but i believe in another section they talk about how the higher up the antenna is, the less this effect matters, but don't quote me on that.
LC
titled 'Antennas and Radio Propagation' published in 1953.
Instead of adding to any confusion by editorializing what i read, i will just quote directly from the book the parts that made me start considering this as part of my antenna installations.
This section comes from the chapter on vertical antennas, and is entitled 'Counterpoise'.
"When an actual ground connection cannot be used because of the high resistance of the soil, or because a large buried ground system is not practicable, a counterpoise may replace the usual direct ground connection in which the current actually flows to and from the antenna through the ground itself.
The counterpoise consists of a structure made of wire erected a short distance off the ground and insulated from the ground. The size of the counterpoise should be at least equal to and preferably larger than the size of the antenna.
The counterpoise operates by virtue of its capacitance to ground. Because of this capacitance, the ground currents which flow normally and usually are collected by conduction now are collected in the form of charge and discharge currents.
The end of the antenna which normally is connected directly to ground now is connected to ground through the large capacitance formed by the counterpoise.
If the counterpoise is not well insulated from ground, the effect is much the same as that of a leaky capacitor. Leakage currents flow between the counterpoise and ground so that a poorly insulated counterpoise introduces more losses than no counterpoise at all.
The shape and size of the counterpoise are not particularly critical. In some field antenna installations, a type of grounded antenna is used in which a large portion of the antenna is folded into a horizontal position. the counterpoise used with such an antenna has the same shape and approximate dimensions as does the antenna itself. This counterpoise is mounted directly under the antenna at a height of about 8 to 12 feet off the ground.
When the antenna is mounted vertically, the counterpoise is made to have any simple geometrical pattern such as those shown in figure 114 (square and hexagonal shapes are shown with radials extending out horizontally with wire connecting them like spider webs)
Although perfect symmetry is not necessary, the counterpoise should extend for equal distances at all angles from the antenna.
The area covered by the counterpoise should be as great as possible, although very little is gained by extending the counterpoise more than half a wavelength from the lower end of the antenna.
in some VHF antenna installations on vehicles, the metal roof of the vehicle is used as the counterpoise for the antenna."
No i do not consider this to be proof of why someone should isolate their antenna from its mast.
I do think it provides a good explanation of what a counterpoise is and does.
I don't want to type another book right now LOL, but i believe in another section they talk about how the higher up the antenna is, the less this effect matters, but don't quote me on that.
LC
Had no idea this was a controversial subject , do you want your mast to be part of the antennas counterpoise or not? doesn't really matter to me, just wanting to see a good example of an easy way to accomplish this for my antenna which I figured out now thanks to Sp5it
If there is antenna current flowing along the support structure and/or the outer shield of the feedline then there are not enough radials to decouple the feedline from the antenna. Under these conditions antenna radiation efficiency is extremely poor and a large amount of power being delivered to the feedpoint is being eaten up in the inadequate antenna current ground return circuit resulting in ground resistance losses and not being radiated.
https://www.worldwidedx.com/threads/calling-all-antenna-experts.260109/#post-748158
https://www.worldwidedx.com/threads/calling-all-antenna-experts.260109/#post-748158
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