[TowerTalk] Two 15m yagi stacking...and SteppIRs

[Dick Green WC1M]

I've done a fair amount of modeling for my 3-stack of 4-el SteppIRs (only
the top one is in the air so far -- the rest will be mounted when the snow
melts.)

I think you're theory is correct, but may not make much of a difference in
practice.

First, I don't think the difference in gain is relevant for anything except
comparing your station with others. I'm not a modeling expert, but it
doesn't appear to me that the gain affects the pattern at all in HFTA. In
other words, even if there's more gain on 15m and 10m than shown in HFTA,
you'll get that same relative difference at all stack spacings. If a 5-el
15m stack is one dB louder than a 4-el 15m stack, it'll be one dB louder any
way stack the yagis. I don't know if that's just the way it works, or if
HFTA uses a simplified model. 

Second, the difference really is only about 1 dB on 15m. Probably not enough
to compromise other variables.

Third, on 10m the difference could be as much as 2.5 dB, but you have to
look at the actual gain of the SteppIRs with a program like EZNEC, not the
theoretical gain. My models indicate that the 4-el SteppIR is within .5 dB
of theory on 20m and 15m, but my 10m models show some variation from theory
depending on the mode. Gain results are different depending on whether you
use the SteppIR element lengths from Ham mode or General mode (albeit with
the version 3 firmware; I think the settings are the same in the latest
firmware.) It may be that for some frequency setting SteppIR has traded some
gain for F/B, which is notably inferior on 10m due to the excessive boom
length.
 
Even if the SteppIRs behave according to theory, it may not help much with
stack spacing. Depends on how big your tower is, distance to the guy
anchors, and whether you're rotating all of the SteppIRs. In my case, the
guy placement constrained spacing of the SteppIRs. Essentially, I had to put
the TIC rings just above the guy brackets to get enough turning clearance
for the middle SteppIR. Another constraint was the 40-2CD at the top of the
tower. I wanted maximum separation from the top SteppIR because models show
significant interaction on 15m and 17m when the antennas are less than 8
feet apart. Even with 14 feet of separation I'm seeing slight interaction on
15.

If you have a taller tower, then you might have more choice in stack
spacing. But as I recall, the spacing between guy sets shouldn't be more
than 40 feet max, so there's always some constraint there. Also, you want
the bottom SteppIR to be in the 30-40 foot range for best performance on 10
and 15 (well, that's true for New England to EU... perhaps it's a different
number for HI.)

I started with nominal spacing of 90'/60'/30', which has proven effective
elsewhere (e.g., NT1Y.) Then I adjusted for desired guy placement, middle
SteppIR clearance, performance, etc. I settled on 96'/64'/34', all on TIC
rings, with the 40-2CD at 110' turned with a T2X.

Hope this is helpful.

73, Dick WC1M
 
> -----Original Message-----
> From: Cqtestk4xs at aol.com [mailto:Cqtestk4xs at aol.com] 
> Sent: Saturday, February 24, 2007 12:17 PM
> To: TOWERTALK at contesting.com
> Subject: Re: [TowerTalk] Two 15m yagi stacking...and SteppIRs
> 
>  
> In a message dated 2/24/2007 4:25:59 A.M. Greenwich Standard 
> Time, jimlux at earthlink.net writes:
> 
> The best  way to answer these questions is to get a copy of 
> HFTA, which comes with  the ARRL Antenna book. It lets you 
> model the pattern of stacked antennas,  varying spacing, 
> etc., and factoring in your terrain, as well.  You  just 
> select the kind of antenna (e.g. 5 element beam), 
> 
> 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ~~~~~~~~~~~~~~
>  
> But there is a rub to doing that, isn't there?  I think the 
> program  assumes a standard length boom for 3 el and a 
> standard length for a 4 el and so  on.  
> If one uses the 4 element STeppIR for 10 will it not behave 
> more like  a 6 element in a stack since it has a boom which 
> is what a usual 6 element  has?  I believe the boom on the 
> SteppIr is around 32 feet and the usual 6  el 10 meter is around 27.
>  
> What this means to me is the antenna will "behave" more like 
> a 6 el in stacking since the boom length is a primary 
> consideration in stacking  distance.  
> The same concept is true for 15 for the SteppIR, with the 
> boom  somewhere between what a 5 or a 6 element standard Yagi 
> would have ...32  ft.
>  
> Because of the above reasoning, I have input the 4 element on 
> 20, a 5 el on
> 15 and a 6 el on 10 on the HFTA program in computing stacking 
> distances...not the standard 4 element for all bands.
>  
> I'd like to hear from others on this idea, since I am in the 
> middle of modeling the three high stack of SteppIRs on that 
> HFTA program.  I have a  wide range of available heights and 
> want to do it right the first time.
>  
> Bill K4XS/KH7XS
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