At 01:39 PM 2/14/96 GMT, you wrote:
>Hi all - I'd like to get some opinions on what happens when you point 2
>Yagis in a stack in directions about 180 deg apart.
>This past weekend in WPX RTTY, at times, tried pointing upper YH7 at
>Europe and lower West (oops, that's TH7). In my thinking, you then have
>two stacked Yagis out of phase. If the gain of each was equal, you would
>then get a high angle of radiation as a result, however there's the F/B
>of each antenna to consider.
>Has anyone modelled this? It didn't seem very productive during the
>contest, but this was in the afternoon when EU was past peak and West
>coast was good. Any thoughts? Tnx/Barry
They are not necessarily out of phase just because they are pointed 180 from
each other. Naturally the driven elements are phase flipped with respect to
each other when they are opposite, but the driven elements of the two beams
are not directly above one another when the are pointing in opposite
directions. The phase relationship is a function of the SPACING of the
phase centers as well as the phase flip from pointing oppositely, and this
can be approximated by the spatial relationship (delay) of the driven
elements (plus the extra 180 degrees). In the case of the TH7s with dual
driven elements, half way between the two elements is reasonable to assume
for the phase center.
But this is really irrelevant since the signal towards Europe is that of the
antenna pointed towards Europe plus the vector sum of the signal from the
other antenna. Vector sums require phase and amplitude information, but
since you are off the back of the second antenna, the amplitude is very low
and the sum of the two signals is basically that of the Europe antenna,
since no matter what the phase of the second antenna the contribution will
be very low and have negligible effect on the first antenna.
This arrangement is often used to spray signals in multiple directions--very
handy for CONTESTING. On 40M CW during the CQWW signals were coming in to
Colorado from Europe during the entire JA run until local sunrise (just
after noon in Europe). The ability to spray in both directions was critical
for success on this weekend. CQ in both directions, answer whoever comes
back. If they are weak switch to their direction on receive to enhance
signal to noise ratio.
To estimate what is happening with your signal when antennas in a stack are
pointed in different directions you can imagine looking at your array from a
long ways away. How big is the signal coming in that direction from the
antenna pointing towards you (also a function of that antennas height and
the angle of take off angle required for that path). Then how big is the
signal from the second antenna (amplitude), and how much further does it
have to travel (phase). This little game will give you an estimate of what
is going on. If the antennas have reasonable patterns and they are pointed
in widely differing directions you essentially have one antenna with half
the power (since the other half is in the other antenna, so you'd be 3 dB
down. (6 dB down compared to what you'd have if both antennas were pointing
at you and you were seeing both halves of the power.) But this may be a
reasonable trade off if there are lots of signals to work in the other
direction as well.
Actually it is a little more complex than all of this (isn't life like that)
because of the changes in mutual coupling, etc. But for a first
approximation (read guess) you can spray two directions knowing that 1/2
your power is going in a second direction and that power is having little
effect on the half that is going in the first direction.
(Trying to keep this simple and without using a blackboard makes it a little
tougher--hope my simplistic explanation helps.)
73 John W0UN
John Brosnahan
La Salle Research Corp 24115 WCR 40 La Salle, CO 80645 USA
voice 970-284-6602 fax 970-284-0979 email broz@csn.net
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