On 80 meters wonder if you, at low TX power, have checked the RF at the
end of your radials with a field strength meters ?
Do you have a perimeter wire around the ends of you radials ?
Fellows,
This is a complex issue. Despite what we hear or imagine an antenna or
ground does, there actually isn't an antenna in the world that is really
ground independent in the true literal meaning of those words.
"Ground independent" is most commonly used to mean the antenna does not need
an earth terminal. That does not mean it is insensitive to ground, earth, or
ignores things we might call grounds.
There are occasions where "ground independent" is used to mean an antenna or
counterpoise not being influenced by earth or coupled to earth, but that
pretty much is always "quack science" unless the antenna is many wavelengths
above earth.
Fields are mathematical or theoretical descriptions of force on charges by
other charges. There are three fields, electric, magnetic, and
electromagnetic. If we use a field strength meter in an attempt to quantify
intensities in the near field, the measurement is critically dependent on
the type (field impedance) of the FS meter. If you have a little probe with
a detector, it primarily sniffs out the electric field. If it is a closed
loop, it sniffs out the magnetic field.
An antenna always responds to both fields, although it responds as a ratio.
If you really took either the magnetic or electric field to zero, the
antenna would go stone dead. An antenna always has a certain "field
impedance", because it always has both electric and magnetic fields in some
proportion. Of course radiation always has a certain proportion of electric
to magnetic field that is never extreme, and depends on the media the wave
propagates through. This all means we can measure close to an antenna or
instrument with something we call a field strength meter until we are blue
in the face, and we almost always wind up with no idea what the measurement
means in relationship to coupling between two things, like an antenna and a
radial. It pretty much is a waste of time.
The way we can measure coupling is to actually measure coupling. Then we are
actually measuring the effects we want to know, and not looking at a small
part of a big picture and not knowing what it really means.
Interactions here at my house with my antennas in the near field means
nothing to anyone else, unless we have the very same installations with all
the same spacings and even the same surroundings.
Not understanding this is generally behind why some antennas make promises
impossible to keep as a blanket statement or guideline. Someone reads
something, thinks it applies to every case or to most cases, when it only
applies to a specific case.
I can't think of a single receiving antenna that can be placed within a few
wavelengths of other structures or systems and be guaranteed to not
interact, let alone interact with something .05 wavelengths away. Small
closed loops, including flags and pennants, are "ground independent" in the
sense of not requiring a ground connection to function, but in order to be
"ground independent" in the sense of not coupling to earth, nearby radials,
things 1 wavelength away, and on, they would also have to not radiate
(receive). If it receives, it will receive from the mess of things all
around it from the dirt below it to the wires down the road.
The only way we know is to actually build it, or put it out by itself a long
distance from anything else.
73 Tom
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