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@contesting.com
> [mailto:towertalk-bounces@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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