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From: PALLE PREBEN-HANSEN@datacentralen_di_cpr
To: cq-contest@oampc12.csg.mot.com at INTERNET
To: kurscj@oampc12.csg.mot.com at INTERNET
Subject: Re: Beam Interaction - FYI
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Hello,
Stacking a beam for the same frequency within a stack seems like
looking for troubles. Also I think the 2 elm Cushcraft 402CD is not
full size and has some kind of high-Q loading, making is more
sensitive.
In the good old days (pre computer age) a stacking distance of 2/3 of
the boom length, but no less than 1/2 wavelength, was considered
resonable.
Today we know better how to calculate stacking distance of similar
beams for max. gain. It is a function of the 3 dB opening angle of the
beam measured in the stacking plane (= opening angle in the vertical
plane (H-plane) for a vertical stack of horizontal polarized beams).
At this distance interaction is usually not severe.
At HF stacking is not so much done for max gain as for filling in the
vertical radiation pattern. Still the beams requires a reasonable
seperation to avoid interaction. Your 20 m beams are most likely not
too far from the calculated optimum stacking distance.
I have stacked beams for six. The seperation is only little smaller
than for max gain and the interaction, when they are rotated
individually, is minimal on the SWR.
At 40 m your seperation is 1/4 wavelength and on 20 m more than 1/2
wavelength.
It is said, that stacking beams for different bands requires a
seperation of half the max-gain-stacking-distance of the beam for the
higher frequency. Interlacing stacked beams for different bands is not
good, but what else can one do when the credit card will not give
rotating poles for all 5 bands .... ..
Stacking beams is great for contesting and switching BLU (Both-Lower-
Upper) or BIP/BOP (Both In Phase/Both Out of Phase) could be added.
Individual rotators is great, and so is one linear to each of the
beams in the stack!
Has anyone programs for calculating a stack feed out of phase? TA by
K6STI and YTAD does not handle this. Well, I am building a BIP/BOP
switch even without computer modelling.
73, Palle, OZ1RH (OZ9EDR-OZ5W contest team) Internet: pph@dc.dk
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