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Re: [TowerTalk] stacking 2 different beams

To: towertalk@contesting.com
Subject: Re: [TowerTalk] stacking 2 different beams
From: K4SAV <RadioIR@charter.net>
Date: Mon, 17 Aug 2009 11:12:25 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
OK1DIG wrote: I am considering to stack the 40 m 2 element moxon beam  
below my existing
40 m 4 el  40M4LLDD @ 30m agl.

If you feed these as a stack, getting the phasing proper will not be 
easy.  It will either require a very accurate modeling job or many trips 
up and down the tower.  A more practical way would be to use separate 
feedlines and do the phasing on the ground.  Even if you succeed in 
getting the phasing correct, I think you will be disappointed at the 
results.  You will loose a significant amount of low angle gain versus 
using only the 40M4LLDD.  The combination will fill in the first null 
from the 40M4LLDD but that should be at a high angle (maybe about 45 
degrees), so the combination would only be useful for very close stations.

You could use them as separate antennas and switch between them, but I 
think the only time the lower antenna will be useful will be for very 
close stations (so you wouldn't want it fixed).  When used as separate 
antennas I would also expect to see some amount of interaction, maybe 
some degradation in F/B of the top antenna depending on the directions 
they are pointed.  I would expect to see a more significant pattern 
distortion of the bottom antenna caused by the top antenna.  That would 
require a modeling job.

If you think about using these as a stack, I think you can see what  
will happen.  You will be decreasing the power to the top antenna by 3 
dB and applying that power to a lower antenna that sends most of the 
signal up at a high angle.  However since the lower antenna has less 
gain than the top antenna, it's gain at low angles is not enough to make 
up for the 3 dB low angle gain loss in the top antenna.  The effect is a 
lot like creating a stack where the bottom antenna is too low.  Stacking 
identical antennas at good spacings is not the same.

Jerry, K4SAV


Daniel Glanc wrote:
> Hi everyone,
>
>  
>
> I am considering to stack the 40 m 2 element moxon beam  below my existing
> 40 m 4 el  40M4LLDD @ 30m agl.
>
> I am not sure if this is a good idea. I can't model the situation.  My idea
> is to put it @ 15m agl beaming into one direction (NA).
>
> I would appreciate any comments or hints form the community.
>
>  
>
> Thanks in advance 
>
>  
>
> Daniel / OK1DIG ( also OK7M and OL6X )
>
> _____


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