> It goes without saying that both halves of the antenna (radials and the
> vertical) must be present in order for the bottom-fed vertical monopole to
> radiate.
A vertical will radiate without any ground, and actually radiate pretty well
if the common mode feedline currents can be controlled and loading losses
minimized.
> But I maintain that any far-field radiation from the radials is
> way down from the radiation from the vertical. It only exists at all
> because in the real world, nothing is perfect and there will always be
> losses, imbalance, and imperfect cancellation of fields.
Radiation comes from charge acceleration, or time-varying current over
distance (ampere-feet). If radials have common mode current, they will
radiate.
> Maybe this is not a good comparison, but there is also a strong field in
> the immediate vicinity of a transmission line made from two parallel
> wires.
> But as long as it is straight and balanced, the field rapidly decays as we
> move away from it.
You can't do that with radials, radials work because of common mode current.
There is always space between the opposing radials, and that means some
direction where things do not exactly cancel. It is always a matter of how
much, how far away, and what the overall effect is.
> I have always believed otherwise; that while there is certainly a field
> around a symmetrical elevated counterpoise, that field actually very
> rapidly decreases as we move farther away from it, as is the case with the
> field surrounding a transmission line that consists of two straight
> close-spaced parallel conductors.
It is a "phase and level thing" that makes radiation go away, not just
distance. After all, distance from the antenna is the same as the distance
from the radials. :-) If a certain point is significantly closer to one
radial then the other, there isn't much cancellation.
> Having said all this, I doubt that the radials on a VHF ground plane
> antenna, many wavelengths above the earth, have much coupling to the earth
> itself. Maybe a counterpoise radiates, after all. :-)
Model a 1/4 wave groundplane with four 1/4 wave radials and look at it.
Without a feedline, perfect radials work very well. Add a feedline without
decoupling for common mode to the perfect system, and the system becomes a
mess.
Absent feedline issues, three radials start to show pattern distortion at
various viewpoint elevations, and two radials can get pretty nasty at
certain elevations.
The worse thing is probably keeping the feedline tame, and it gets worse
with smaller or fewer radials (no matter what type they are).
> I don't have a problem with understanding that both legs of a 90 degree
> center-fed dipole (such as with one element vertical and one element
> horizontal) radiate. But everything I've ever studied seemed to indicate
> that in the case of a symmetrical counterpoise (balanced, equally spaced
> wires and with the monopole directly under their apex) parallel with the
> earth and with a sufficient number of radial, >99% of the far field comes
> from the vertical monopole.
I'd be careful about that number if you are talking about two radials. I
don't know what the exact ratio is, but I do know pattern distortion can be
significant even without a feedline with two radials. With four radials
things look pretty good, if we keep the feeder out of the radiating system.
Of course that is at a very large distance, where everything looks like it
comes from one point in space. Near one of the radials compared to the
others, there will be significant radiation fields unless there are many
radials dividing the current. It all comes down to levels and phase at any
point.
73 Tom
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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