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Re: [TowerTalk] small antennas

To: "W4ZW" <w4zw@comcast.net>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] small antennas
From: "W7CE" <w7ce@curtiss.net>
Date: Sat, 19 Aug 2006 00:15:44 -0700
List-post: <mailto:towertalk@contesting.com>
>
> Thanks for the clarification, Joe.  As to the comment I made about the
> SteppIR being noisier on 20 compared to the A4S, I really have no
> explanation for it.  The reverse should be true, and I first put it down 
> to
> the notion that the SteppIR was a "hotter" antenna and the lower noise 
> level
> on the A4S was because it was more lossy, but the signal levels on both
> antennas with a switched A/B test are virtually the same, the only
> difference is in the background noise level.  Often I will switch to one 
> the
> A4S's to copy a station that is in the noise.  I've discussed this with 
> Mike
> at SteppIR and he says the SteppIR should have a better signal to noise, 
> but
> it clearly doesn't here at my QTH.  All the tribanders are fed with 9913
> which I replace every two years and the runs are quite short because my
> shack is on the 3d floor of my house and two of the yagis are roof mounted
> above me.  The SteppIR is on the bottom of the mast with a Cushcraft 402CD
> about 14' above it.  The A4S is about 8 lower than the SteppIR on the 
> other
> corner of the roof.

Jon,
Mike has mentioned your A4S vs SteppIR S/N situation and we were both 
baffled.  I think the description of your antenna farm has probably answered 
the question.  Having a 402CD 14' above the SteppIR (less than 0.3 
wavelength) will most likely reduce the F/R performance of your SteppIR. 
For example, I have found that installing a 2M beam within 0.6 wavelength 
(4') of a 6M beam will start to seriously impact the F/R and forward gain of 
the antenna.  Whenever one or both of those numbers goes down, your S/N 
ratio will suffer.  I haven't modeled your exact scenario, but in general a 
lower frequency antenna that is mounted too close vertically to a higher 
frequency antenna will reduce the F/R of the higher frequency antenna, 
sometimes significantly.  I don't have a good NEC-2 or EZNEC model of the 
402CD, but I'll try some short 2 element 40M antenna models that I do have 
mounted 14' above a SteppIR and let you know what I find.

What is the height above ground of your 3 element SteppIR and the Cushcraft 
A4S?  Height above ground can also impact the F/R of the antennas and 
different antenna designs can react differently at the same height.

73,
Clay  W7CE

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