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Re: [TowerTalk] 40 and 20 at 15 ft apart

Subject: Re: [TowerTalk] 40 and 20 at 15 ft apart
From: Jim Lux <>
Date: Tue, 30 Aug 2011 05:58:43 -0700
List-post: <">>
On 8/29/11 6:37 PM, "Felipe J. Hernández" wrote:
> Really?
> So no 90 deg off for that combination? May I ask how far where they apart?
> I guess is the right approach to then  model the antennas exactly..

For an initial interaction study, you could work with a fairly 
simplistic model (i.e. you might not need tapers, and you certainly 
wouldn't need to model every bolt and nut).  You run the model driving 
the antenna you're interested in, and look for current in the other 
antenna parts.

However, if you're looking to see whether your 30dB F/B is degraded to 
28dB, then you need a better model.

> As I understand the stacking situation, the current drop of multiband yagis, 
> like the Force12 series,
> have all the elements electrically insulated from the boom which provides 
> very nice stacking possibilities
> with minimal interaction.

One way to think about it is that if you have a "elements connected to 
the boom" sort of Yagi is that the boom looks like a dipole with a bunch 
of capacity hats.  So a physically short boom might be electrically the 
"wrong" length.

One thing to watch out for is that the forward gain/beamwidth might not 
change very much, but you might kill the nulls off the back and sides. 
The "other antenna" acts to change the tuning of the elements a bit and 
throws of the current phasing/magnitude.  A 2 or 3 element antenna isn't 
going to be as sensitive, but a 5 element superdirective Yagi is going 
to be "pickier" about what's in the vicinity.

  The older design tribanders like the KLM KT34's had the parasitic elements
> electrically attached to the boom and it is almost impossible to achieve a 
> low interaction regardless
> of height/orientation.

That seems plausible (see my idea of "capacity hats" above), but I 
suspect that it's not a general rule.  It might just work out that way 
because most antennas for a given band tend to be of similar length.


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