Ed,
First, you are correct that even if you didn't try to use the two antennas
in a stack, you would definitely see a benefit in having two antennas at
different heights in respective of vertical takeoff angle control. High
antenna for low angle stuff and lower for closer in. If you are into
contesting, you could also have them pointing in different directions. There
is a lot of benefit in having two stacked antennas even if you don't use
them in a stacked configuration.
However, I have to disagree with the responses you got about phasing
difficulties. It seems to me that since Steppirs are probably directly
driven antennas, the only phase difference will be as a result of the
different amount of horizontal spacing between the DE of the 4 element and
the 3 element. The solution is to simply make one cable longer by the
difference in the DE positions.
Since my knowledge is only based on theory and not actual practical
experience, perhaps others will find flaws with my logic, but I am always
eager to learn.
Just for fun, I modeled a 4 element and 3 element 15 meter yagi at 70/35
feet using 4NEC2. The 4 element at 70 by itself shows 13.8 dBi gain at 10
degrees and the stack gives 15.5 dbi gain at 11 degrees. That does not sound
bad to me. I didn't even bother to try to even out the phasing. It might
even improve by a few tenths of a dB gain once the transmission line lengths
were tweaked.
According to Cebik, it is usually recommended to put the higher gain antenna
at the top for highest gain. Likewise, for 10/15/20 operation the optimum
distance is closer to 40 feet than 30 feet, so you will have to decide on a
tradeoff between having better on gain on 20 meters (40 foot spacing) and
higher height for the bottom yagi (lower spacing).
You should try using one of the modeling programs like 4NEC2. They are fun
and very useful.
--
Gerald Boutin, VE6WA
Subject: [TowerTalk] Stacking Dissimilar Antennas
From: Edward Sylvester
Date: Fri, 18 Aug 2006 21:30:26 -0700 (PDT)
Sometime back, I posted an inquiry regarding having two dissimilar antennas
on
the same tower. In this case, I would have a 4 ele Steppir at 70' and a 3
ele
Steppir at 35-40'.
Initially, I figured there would be an advantage to this, but after much
feedback from the group, concluded that this would not be worthwhile,
primarily
because of phasing issues.
But if I don't combine the antennas and simply have them completely
separate,
is there an advantage here in terms of vertical take off angles because of
the
different heights of the antennas (i.e. lower antenna better for short path
and
higher antenna better for long path and short path to farther places)?
Or am I just spinning my wheels and reaching a point of redundancy?
Your thoughts are appreciated.
73,
Ed NI6S
--
Gerald Boutin, VE6WA
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