Bob,
It is very important that the antennas, feedline (electrical) lengths and
alignment of the antennas in the vertical plane (for a vertical stack) be
identical for a stack to perform optimally.
To the extent any of these parameters is not identical, you lose
performance. Critically, if the antennas are not identical, the electrical
lengths will not be identical and therefore, the beam will not form its
desired in-phase pattern and the full stacking gain will not be achieved.
Different electrical length feedlines will do the same thing.
I use identical antennas with equal electrical length feedlines. I achieve
equal electrical length feedlines by using a TDR (time domain reflectometer)
to measure the cables. I use TIC RingRotors to ensure the antennas are in
near-exact vertical plane alignment. [Disclosure: I am a TIC authorized
factory representative]. But, I am also a happy TIC customer!!
73,
Bill, N3RR
http://www.erols.com/n3rr
http://www.erols.com/n3rr/n3ringrotor/index.htm
Robert L. Shohet wrote:
> Tnx to all who responded to my guy wire spacing question.
>
> I have wondered how important it is to stack similar antennas or if it
> does not matter at all.
>
> If I wish to stack 3 15 meter beams, how advantageous is it to make sure
> that all three antennas are the same design, boom lenght, manufacturer,
> match etc.?
>
> For example, does it matter if the bottom one is a widespaced 4L
> Cushcraft on a 27' boom with a gamma match, the middle is a 5L Hygain
> 155CA and the top is a homebrewed 4L on a short boom with gamma match?
>
> Ideally I would like the amplifier loading to be roughly the same for
> all combinations of antennas so do I need to make sure that all are
> identical or is the most important thing to make sure that they all
> resonate about the same frequency?
>
> What happens if I wish to have two resonant about 21.05 and the other
> resonant at 21.25? Does the difference in resonant frequency screw-up
> the overall pattern and loading when in a stack?
>
> I know that these sound like basic questions to many but I feel it is
> better to ask and give the project lots of thought BEFORE anything gets
> bought, built and put up!
>
> Thanks for your help.
>
> 73
>
> Bob KQ2M
>
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