[TowerTalk] Lengthening a vertical antenna

[Roger Parsons]

I am constructing a vertical array, where the driven
element is a base fed half wave vertical. Because it
is close to resonance and closely coupled to the
parasitic elements the drive impedance changes
dramatically with a small frequency change. This is
not very desirable. EZNEC tells me that if the driven
element is moved away from resonance, say to 5/8 wave,
that the drive impedance becomes much more amenable
and changes much less with frequency. For many reasons
I do not want to raise the height of the array so that
the top is 5/8L above ground rather than 1/2L. Making
the driven element alone longer so that it pokes out
of the top of the other elements totally changes the
radiation pattern of the array as one would expect.

I know that the experts deprecate linear loading, but
I am considering an open wire stub of 1/8L at the base
of the driven element so that its electrical length
appears to be 5/8L, although its physical length would
still be 1/2L. My thoughts were to take the stub up
vertically with say 1' spacing from the driven
element, mostly because this would be the easiest way
to do it mechanically. I am aware that I will still
need an ATU, but that is not a problem provided that
it can cover a reasonable frequency range without
retuning.

Apart from the fact that EZNEC may be lying to me (as
the elements are close to ground and I may not be a
very good EZNEC driver), can anybody see anything
wrong with this approach or suggest a better one?

Thanks in advance.

73 Roger
VE3ZI




	
	
		
___________________________________________________________ALL-NEW Yahoo! Messenger - all new features - even more fun!  http://uk.messenger.yahoo.com