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[TowerTalk] More 4 TH6'S

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Subject: [TowerTalk] More 4 TH6'S
From: (Leeson)
Date: Mon, 15 Dec 1997 23:07:54 -0800
Gerald, W5BA, asked
>How close is <too> close for stacking monobanders?

There are several schools of thought on spacing of close-stacked Yagis
for different bands.  I had a 12' stack with the 10m in the middle, and
it wasn't very good on 10.  When I modeled it, it seemed to have lost
3-4 dB, a huge penalty, and it acted that way.  Down it came!

Now I have two towers, 20 and 10 on one, 40 and 15 on the other, with
30' spacing, but this is a special feature of the terrain slope that
allows (actually dictates) 10 and 15 at 30' and 20 and 40 at 60'.  I
don't see any interaction at all with this spacing, and get the 11 or 12
dBi (freespace) gain I worked to get with the 6 el 15 and 7 el 10.  I
use a big ring rotator on 10, and K5IU rotating tower on 15/40.

In a close stack, as in an interlace, the lower band directors act as
reflectors on the higher bands, playing havoc with the higher band
pattern.  One possible cure is to use a multi-band director on the
lower-band Yagi, either trapped or open sleeve, so it acts as a director
on the higher frequencies as well.  I've modeled this a bit, and it
seems promising.

The other problem is that you typically want the lower frequencies to
have higher physical height to get the electrical height you need, but
this is not great mechanically (and for some reason, esthetically).

But N6BT has worked with N6TV on Bob's close stack, which I can tell you
works great.  I believe 80 through 10 are sqeezed into a 14' height. 
Bob or Tom would be the source of better information.  My own experience
is that spacing closer than 10-12' can really compromise the higher-band
Yagis in a Christmas tree stack.

If I were to choose for a single tower station, I would put a
shorty-forty (I'm having good luck with a Cushcraft on yet another
tower, possible in part because wife Barb's an enthusiastic ham, too) on
the top of the mast, put the longest-boom, best bandwidth tribander I
could find at the tower top, and play with another, possibly smaller
tribander side- or ring-mounted halfway up.  I'd look forward to the
actual real-life measurements being made by N0AX and K7LXC, or go by the
experience of folks like N6BV, K6LL and N7TR, among others.  But you owe
it to yourself to see if you can get the info on the close stacks that
N6BT has put up, so you can decide for yourself.  If you can come up
with a good duobander (I have some preliminary designs I've been trying
to get back to so I can post them), you might go with a taller tower and
then sidemount the 10m monobander below the interlaced antenna, or even
go with a tribander at half-tower height.

There doesn't seem to be any magic answer, and it's conditioned by
terrain and by the uses you intend (I stayed low because I didn't want
to sacrifice SS performance if I could help it).  In auto racing, you
try to duplicate the setup of the guys who are winning, and when you
master that you then hope to add something that puts you ahead enough to
win yourself.

I think the elevation angle flexibility and compression of a stack of
two tribanders has an awful lot to recommend it (I think that many of
the situations where the lower antenna is best arise from nulls in the
pattern of the higher antenna, not necessarily high-angle arrival).

Good luck, sorry I can't be more specific than to say 10' seems to close
to preserve bandwidth and pattern, but 30' has worked for me.  But then,
there's the N6TV short stack hanging right in there...

Dave. W6NL

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