>
>I would like to stack my 105BA on the same mast above my 204BA at the
>top of a 75ft Rohn-25 tower. Does anyone have a feel for the minimum
>distance between the two yagis to avoid/minimize interaction?
>
>The mast I am using is the Rohn 1/8" wall 2' dia 10ft length supported
>by a TB2 at the tower top and a oak-block bearing 2ft below. The bottom
>6ft of the Rohn mast has a 1.75" x 1/4 wall 50kPSI tube inserted and
>bolted in place. The rotor in shelved 5ft below the top and I will be
>using an extension tube to join the mast to the rotor. The length of
>this tube can vary to allow the total mast height above the tower
>bearing to range from 5-8ft. I would like to extend the mast out of the
>tower the minimum amount to provide adequate spacing of these two
>yagis. Your feedback appreciated.
>
>Thanks es 73,
>
>Bill N4ALG
I think the real answer is, of course, to stack them as far as possible for
minimum interaction. It sounds like you have already determined what the
mast will be so I would put one just above the tower top and the other at
the top of the mast. I would also calculate to make sure this configuration
will not bend the mast under the winds that you expect. I believe the
modeling software you can get should predict the interaction of the two
antennas.
About 20 years ago, Wilson made some interlaced single-band Yagis that were
on the same boom so had zero stacking distance. My experience with a couple
of these owned by friends was that the higher frequency antenna, which was
generally "inside" of the lower frequency antenna did not perform well at
all while the "outside" antenna seemed to do OK. Separating these antennas
vertically by just 3 feet or so by mounting the elements on separate booms
made a large difference and I believe 10 feet would have been much better.
About the same time, I read an article somewhere about a ZS that
experimented with the interlaced Yagi concept by taking two Yagis for
different bands that were already proven to work well when isolated from
other antennas and mounting them with almost no vertical separation. He
actually temporarily laid one boom on the other and slid the top boom back
an forth thus varying the relative horizontal positions of the antennas with
respect to each other. He claimed to have found large performance
differences when doing this, so maybe the vertical separation is not the
only variable that needs to be considered with antennas of different bands
that are closely stacked. This causes me to wonder if the performance of
the interlaced Wilson antennas couldn't have been improved by moving the
elements around some on the boom . . .
Again, I would expect some good modeling software to give you the answers to
these questions, but, unfortunately, antenna modeling software is not
something I have ever had the time to play with. There are lots of guys on
here who have, however, and maybe some of them can give you some advice.
Stan wtni@teleport.com
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