---------------------- Forwarded by Tyler G Stewart/BENN/CEC on 04/15/97
02:31 PM ---------------------------
From: Tyler G Stewart on 04/15/97 02:38 PM
To: K7LXC@aol.com
cc:
Subject: Re: [TowerTalk] Stacks of tribanders (Document link not
converted)
For tribanders, the spacing is always going to be a compromise. Assuming
20 is the lower freq in question, around 35 feet is the minimum acceptable
spacing (1/2 wavelength) to provide reasonable gain advantage on 20. Since
I wanted to stack 3 tribanders on separate rotators maximum directional
diversity, this required at least a 100' tower. This spacing on 20 also
gives you the optimal 3/4 wavelength spacing on 15, and 1 wavelength
spacing on 10, which isnt too bad, especially for 10.
Since I was planning on putting up another bigger tower with stacked 20's
with 50' spacing, I was more concerned about the 15 meter performance of
the tribander stack over 20 meters. However, 3 stacked at 1/2 wave is
still a little better (on computer) than 2 at 3/4 wave spacing. Note
however that I find the lowest yagi (at 38 feet) of little use on 20 by
itself. It's almost always several S units lower than the top and/or
middle yagi's on 20. However, in the stack on higher angles, you can
notice some difference, and it's still useful to spray some signal to the
West when beamin EU with the top two, etc. On 15 and especially 10, the
lowest beam works quite well by itself, and especially on 10 with a "dead"
band, I'll point all three in different directions to scare up some QSO's.
In all fairness, for most DX work, the top beam by itself is very close if
not as good as the complete stack. It's a small percentage of the time
where the bottom 2 may play better than the top two, etc. However, I
expect as the sunspot numbers rise, so may the angles, so that the lower
beams may play better in a couple of years. If I could only stack 2 beams
but had the height, I might consider just leaving out the bottom yagi.
Having the second of 2 at 70 feet vs 50 feet or so would be a big advantage
on 20 if you wanted to point in two separate directions, since the lower
one by itself will work much better at 70. This would hurt the gain on 20
slightly as a stack, but improve the 15 meter performance.
For tribanders, it just doesnt make sense to use anything other than a
broadband transformer matching system with complete selection diversity
such as the WX0B Stackmatch sold by Dunestar, which is what I'm using.
It's important to have all combinations selectable as well as rotatable if
it's your only antenna system and you contest.
73, Tyler K3MM
K7LXC@aol.com on 04/14/97 11:55:41 AM
To: towertalk@contesting.com
cc: AA7EA@aol.com (bcc: Tyler G Stewart/BENN/CEC)
Subject: [TowerTalk] Stacks of tribanders
TowerTalkians --
Recently I received some interesting thoughts both tried and
theoretical on spacing for a tribander stack from N4KG, K1VR, N6BT and
others. I would like to get more information on the topic.
Here's my scenario; I want to have a stack of 2 or 3 tribanders
(probably TH7's but don't know if the particular antenna is relevant). The
tower height can be from 80 to 120 feet.
1. What spacing do you like and why?
2. Is it a real installation or one that you've modeled that looks good?
3. How do you switch antennas or do you have to (want to)?
Comments on these questions are appreciated and any general comments
on the topic will be gratefully received.
73, Steve K7LXC
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