----- Original Message -----
From: "Edward Sylvester" <navydude1962@yahoo.com>
To: <towertalk@contesting.com>
Sent: Sunday, July 30, 2006 2:56 PM
Subject: Re: [TowerTalk] Stacking tribanders
Here's what I've come up with. There is no ideal separation distance, as
we're dealing with multiple bands. The emphasis in my case will be 20m.
I'm will be using 2 antennas.
The bottom antenna will be a Steppir 3 ele 6-20m antenna at 35-40'. The
top antenna will a Steppir 4 ele 6-20m antenna at 70'. These will be fed
through the Array Solutions Stackmaster II, thereby eliminating the need for
precisely cut feedlines, so long as they're the same length.
I will be using a crank up Hygain HG-70HD. How does this sound?
Feasible?? Objections welcome!
73,
Ed NI6S
_______________________________________________
Hi Ed,
I've done a lot of models for the folks at SteppIR and have done extensive
modeling of stacked SteppIRs. I think you may be disappointed with your
current plan. My models show that 4 over 3 SteppIRs at heights below 90'
will only give you about 1.7 dB improvement over the single 4 element. Most
of my models have been based on placing the top antenna at 90', and I'd
actually expect the stacking performance to decrease as you move the stack
closer to the ground. Also, the driven elements on the two antennas are not
vertically aligned, so you'll need to account for that by adding a phase
delay (something like 60 degrees if I remember correctly) to the 3 element
antenna whose DE is much closer to the center of the antenna. The total
gain for the 4 over 3 stack is only about 0.5 dB better than a 3 over 3
stack. After a lot of analysis, I've decided that 3 over 3 at 90'/55' (35'
spacing) gives the best performance and flexibility for my setup (my goals
seem to be about the same as yours). This spacing seems to work well for
20-10M and actually works ok on 6m also, although there are a couple of high
angle lobes present since the spacing is so large. I should also mention
that the 3 over 3 stack outperforms the single 4 element by about 1 db when
the antennas are fed in phase and gives you the take off angle flexibility
that you don't get with the single antenna. The 3 over 3 stack also has the
benefit or reducing the wind loading at the top of the tower by 3.6 sq ft.
when compared with a 4 over 3 stack.
In my models, I've also noticed that the F/R suffers when the antennas are
stacked. However, I've been able to re-optimize the element lengths based
on the stacking distance and height above ground and can get F/R numbers
that meet or exceed those of a single antenna. In some cases, the change in
the reflector and director lengths are significant for best stacked
performance.
Of course local terrain will impact your results, so YMMV.
73,
Clay W7CE
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