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Re: [TowerTalk] C31XR versus SteppIR

To: "Peter Voelpel" <>, <>
Subject: Re: [TowerTalk] C31XR versus SteppIR
From: Jim Lux <>
Date: Sun, 13 May 2007 11:31:35 -0700
List-post: <>
At 10:53 AM 5/13/2007, Peter Voelpel wrote:

>-----Original Message-----
>From: Jim Lux []
>At 02:51 PM 5/11/2007, Peter Voelpel wrote:
> >>Those figures are EZNEC simulated
> >Which figures?
>The figures given by Optibeam on their homepage
> >In general, decent NEC models of different antennas should be
>intercomparable. "decent" being the operative word.. It
> >takes a fair amount of skill and work to generate good models, especially
>if there are tapered elements, traps, or lumped >components.  The other
>thing that will definitely be an issue for a model is anything where the
>feedline or other stuff
> >is part of the antenna, whether deliberately or not.
> >But, for comparisons of run of the mill Yagi-Uda arrays, with attention to
>segmentation and such, one should be able to
> >get comparative data within 1/2 dB or so.
>Try to convince Steve.
>Seems he is not a believing in antenna simulation.

I'm a believer in modeling (A good part of my job 
depends on modeling), but not necessarily in model builders.

>But I don´t believe in gain figures which are not measured in the main lobe

And there's a key problem.  Most of us don't 
operate antennas (particularly for HF) in 
anything resembling "free space", so you have the 
antenna pattern overlaid on the "ground 
reflection" pattern.  Add to that the fact that 
many antennas, at typical heights, have their 
main lobe (with ground effects) substantially 
above the desired take-off-angle for the signal 
of interest, and you're set up for all sorts of inconsistencies.

You have the gain at the main lobe (generally too 
high elevation).. the gain at the signal's angle 
of arrival (not known with any degree of 
accuracy, and you're in a part of the pattern 
where 1 degree makes a big gain difference).. and 
the gain measured in a test scenario, essentially at zero elevation angle.

I think the empirical measurements in this case 
serve as an extremely valuable "reality check"... 
you can model the antenna, and if the number you 
get from empirical measurements is 10dB 
different, something is going on (unaccounted for loss in the antenna, etc.)

Modeling serves an extremely valuable service in 
assessing the "sensitivity" of the antenna to 
changes in configuration.  If changing the 
spacing of an element by a few cm or it's angle 
by a couple degrees causes radical changes in 
modeled performance, that does not bode well for 
an amateur installation, where the elements are 
waving around in the wind, birds sit on the antenna, etc.

Jim, W6RMK 


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