Yes excactly, that´s the question.
One example: I remember once when I was attending a
antenna seminar at the Swedish Telecom one antenna
guru professor was talking about how vertical antennas
worked, in this case example was a VHF vertical. Usually
VHF verticals was constructed with 4 ground planes (or rods).
Now this professor said that 2 groundplanes was enough and
the other two was just waist of material, i e the antenna
would still produce a uniform pattern, we are now talking
about a vertical antenna many many wl above ground so near
field is not an issue.
If this is true it looks like decreasing near field loss
in the desired direction of radiation would be a good idea,
i e adding more radials in that direction, however I have
never seen any measurements to confirm this.
73 Jim SM2EKM
----------------------------------------
Jim Lux wrote:
Interesting point you raise...
In the near field, the radials are mostly there to provide a "return
path" or image for the current in the vertical. The question would be
whether that "image" gets bent or warped if the radials are not evenly
spaced. Since they are, by geometry, very much closer than 1/4
wavelength apart, there's a lot of mutual interaction, and one would
intuitively expect the current to adjust itself to be even (spread over
all the available radials)..
Example.. if you had 20 radials over 180 deg, and 40 radials over the
other 180 deg, the current in each radial on the 20 side would be twice
that on the 40 side, but, the virtual image formed below the radials
will probably be exactly the same. There's probably some limit (for
instance, if the radials are exactly tuned, and there's not many of
them, so they aren't strongly coupled to each other)
At 06:16 PM 12/18/2003 +0100, Jan Erik Holm wrote:
Yes but 4 wl long you will start to "modify" the
far field and brewster angle would get smaller.
What you then did would not be relevant for anything
but your QTH, i e your type of far field...conductivity
etc etc.
If we consentrate only on the near field I still
wounder if you really can benefit much from haveing
more radials in one direction (i e 5 or 10 degree sector
forexample), I´m not convinced.
If so you could stretch the example further. Lets say
someone just likes to work one direction, he then
puts lets say 90% of the radials in that direction,
well well....no I´m not convinced.
73 Jim SM2EKM
-------------------------
Richard Karlquist wrote:
I put up a 20 meter vertical with closely
spaced radials 4 wavelengths long spread over about
60 degrees centered on Europe. I A/B'ed it
with an ordinary vertical. The vertical with the
long radials had an advantage of perhaps 3 dB
vs the ordinary vertical toward Europe (a
just discernable difference). In other directions,
it had no advantage. Not what I would call
gangbusters.
Rick N6RK
(A report on this and other vertical experiments
is on my web site www.n6rk.com)
_______________________________________________
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