Posted by
Geoffrey Mackenzie-Kennedy-2 on
Jun 15, 2006; 2:22pm
URL: http://elecraft.85.s1.nabble.com/VERTICALS-tp390943p390946.html
Ron AC7AC wrote:
There are two issues with ground-mounted verticals - either one that is
actually 1/4 wave long or one that uses traps to 'disconnect' the unneeded
length on the higher bands.
1) On the higher bands, the active portion of the antenna is that part
closest to the ground. So, on the higher frequencies most vulnerable to
absorption by surrounding objects, the active part of the antenna is most
likely down among such objects.
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Good point Ron, and a fact of life that is very seldom mentioned in the text
books as far as HF antennas are concerned.
If it is not possible to put up a vertical taller than a 1/4 wave in among
small trees and shrubs, in my experience better results can be obtained if
the antenna is turned upside down so that the high current portion is at the
top, and the bottom end a few feet above ground. This way the antenna has a
better chance of looking over the vegetation. The T antenna is an example of
this scheme, and can be voltage fed at the bottom with a parallel LC 'tank'
with link or tap for the coax feed. Personally I prefer to feed this type of
antenna, or vertical 1/2 wave dipoles, at the high current point which is
1/4 wave plus a few feet above ground using coax. The centre of the coax
connects to the top of the vertical, the braid to the horizontal 'radials'.
If the coax is run alongside the vertical, the antenna's radiation pattern
can be and usually is affected, with the main lobe being moved to a higher
take-off angle. To avoid this problem the coax can be run inside or tied to
the vertical element, and where the coax feeder leaves the bottom of the
vertical it is coiled so that the braid of this coil forms the L of a high
impedance LC trap tuned to the operating band. A simple choke coil could be
used, but is not so effective since it has to present a very high impedance.
A neater method can be used with vertical centre fed 1/2 wave dipoles. The
top 1/4 wave section is made from wire (Flexweave is good) with its 'bottom'
end connected to the centre conductor of the coax feeder, the braid is not
connected to anything. The bottom 1/4 wave part of the dipole uses the
outside of the coax feeder's braid. The RF current flowing on the inside of
the feeder loops over to the outside of the braid at the open end of the
coax, and continues to flow down the outside of the feeder untill told to
stop.This is done by introducing a high impedance trap, again using a coiled
length of the feeder as part of a trap, approximately 1/4 wave down from the
feeder's open end. The velocity factor of the coax is not involved in
working out the length of this 1/4 wave section because only the electrical
length of the outside of the coax braid is of importance, but the jacket and
diameter of the coax do have some effect on electrical length although they
tend to cancel in practice. At the feedpoint the coax must be well supported
by the top 1/4 wave wire so that the coax's centre conductor is not under
stress.
Anything to get rid of flapping feeders!
73,
Geoff
GM4ESD
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