[TowerTalk] XM240 to Moxon

[DF3KV]

Hi Clay,

Exactly!

My answer to Dan overlapped with yours

73
Peter

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Clay W7CE
Sent: Donnerstag, 26. August 2010 17:38
To: towertalk at contesting.com
Subject: Re: [TowerTalk] XM240 to Moxon

  When comparing antennas, you can't just look at the maximum gain, you 
also have to consider take-off angle.

When you add the highest yagi to the lower stack of two yagi, you will 
have a lower take-off angle.  To model this with EZNec, first model the 
lower two antennas and do an elevation plot (over real ground).  From 
the elevation plot  window, select File|Save Trace As ... and save the 
plot to disk.  Now add the top antenna to the stack and do the elevation 
plot.  From the plot window select File/Add Trace and select the file 
you just saved.  You will now have both plots drawn in different colors 
and you can see the difference in gain at specific take-off angle.  As 
you go to very low angles you will see that the stack of three antennas 
has substantially more gain than the lower two antenna plot.  When you 
add extra traces to the plot window they are listed in the upper left 
corner of the window.  If you  click on the name of one of the plots, 
the relative gain between that antenna and the primary antenna (the one 
you simulated before adding traces) will be shown in the bottom right 
corner of the window (e.g. -1.82 dBPrTrc, dB relative to the primary 
trace).  If you haven't used this EZNec feature before, play with it.  
It's a great tool.

Also, at typical heights above ground, yagis have a lower take-off angle 
than dipoles.  If you follow the above procedure you can see the 
difference in gain between the two at a specific take-off angle.  At low 
take-off angles, the difference is usually a lot more than the maximum 
gain from the models would indicate.

All of the above assumes flat terrain at your location.  To get a better 
idea of relative performance at specific take-off angles, use a terrain 
modeling program such as HFTA.

73,
Clay  W7CE

On 8/26/2010 5:03 AM, DF3KV wrote:
> Yes, I have a three stack of 5L KLM yagis.
> The lower two are hard wired phased and fixed direction U.S.
> If I add the highest yagi to the stack I almost always get 10db higher
> differences reported from west coast stations.
> The EZNEC simulation shows just +2,31db
>
> 73
> Peter
>
> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Thomson
>
> ##  Does anybody else see similar effects... whereby gain differences are
> way nore than what
> software predicts?   The diff between a rotary dipole on 80m.. and a 2-el
is
> another one that baffles
> me  [same deal on 40m].   It's like throwing a light switch.
>
> later... Jim   VE7RF
>
>
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