Posted by
Don Wilhelm on
Mar 14, 2020; 1:44am
URL: http://elecraft.85.s1.nabble.com/Off-Topic-Your-advice-suggestion-about-antenna-tp7658892p7658908.html
If I recall correctly, the original question in this thread was about
the effectiveness of radials over rocky ground. We have gone far astray
from that consideration. I am trying to bring it back to 'ground zero'.
As I recall, the question was about a vertical whose feedpoint was above
the ground - rocky or not.
Let me add some conceptual thoughts.
Does anyone remember the Ground Plane vertical that was popular back in
the 1960s and 1970s (and is still commonly used on VHF today)? A 1/4
wavelength vertical radiator with usually 4 1/4 wavelength radials from
the base. It was often mounted on a roof, and the radials sloped
downward along the roof providing a feedpoint impedance near 50 ohms as
opposed to 35 ohms if the radials are perpendicular to the vertical member.
That antenna has a nice low angle of radiation, which is good for far
away stations. A dipole has a higher angle of radiation, and will
outperform the vertical at near-in distances. A high dipole (at least 1
wavelength high) will also have equal low angle radiation to the vertical.
To work effectively, the radials must be arranged symmetrically, so each
pair will cancel the horizontally polarized radiation. Actually only 2
radials are required for that, but they must be arranged 180 degrees
from each other. For best performance, the radials should be tuned to
the center frequency of interest - just like tuning a dipole.
This is what is known today as a vertical with raised (or elevated)
radials. It is ground independent in contrast to a typical vertical
with buried radials which is quite dependent on ground conductivity.
In comparing a vertical with a dipole, remember that a good vertical has
its best radiation at a low takeoff angle, while a dipole will have its
best radiation at a much higher angle. Listening tests will make the
dipole have much better performance when there are a lot of stations in
the 400 to 800 mile radius of the station, while the vertical will
'reach out further' while suppressing the strength of the nearby signals.
So all this talk about how many dBi is nonsensical unless a specific
angle of radiation is being stated as well. A good vertical has better
low angle radiation than a dipole, and rejects high angle signals. A
dipole that is low in terms of wavelength has a high angle component of
radiation.
That means to me that a dipole mounted at a modest height for 160, 80,
and 40 will provide local as well as moderately distant contacts easily.
A proper vertical will provide more DX contacts, and weaken the
receive signals for more close-in stations - it is all in the angle of
radiation - how many miles does the signal travel before it is reflected
by the ionosphere? It is all a matter of physics and geometry.
73,
Don W3FPR
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