[TowerTalk] tower height increse.

[Pete Smith]

At 04:29 PM 5/10/03 -0400, Barockteer at aol.com wrote:
>In this height range (~70 feet), given take off angles and vertical
>propagation angle info from N6BV, I calculated that every 10 feet was 1 db,
>on 14 MHz meters or higher.
>
>That's why for my setup, I added 10 feet of tower, put the tribander at the
>top, and put the 2 element 40 on a side mount below the tribander. That way I
>gained 1 dB on the top three bands, at the expense of ~ 0.5 dB on 40.

I modeled a tribander at 70, 80 and 90 feet against the arrival angle 
statistics for my area (Washington DC) and got the following results (the 
numbers are weighted average gain in dBi, over flat terrain):

Height          20m     15m     10m

70'             9.9     10.4    11.1
80'             10.2    10.6    10.9
90'             10.3    10.6    10.7

Note that higher is actually worse on 10m.  The antenna at 90 feet has a 
major null at 10-12 degrees, a high-probability arrival angle.  The same 
trends were evident for a change from 60 to 70 feet, except that the 
improvement on 20m. was a little higher (.6 dB).

By contrast, a 2-element 40m yagi yields:

70'     5.8 dBi
80'     6.7
90'     7.4

Here the increase in gain per 10 feet of additional height is much closer 
to what Tony quoted, although it tails off as the increase becomes a 
smaller percentage of the total height.

Terrain can make a huge difference, too.  I've modeled the terrain of a 
well-known New England contester in the direction of Europe, and the 
modeling clearly shows that a tribander at 50 feet would be significantly 
better than one at 70 feet, particularly on the higher bands.

I'm not knocking Tony's decision -- it probably made sense for his QTH and 
terrain.  But I think the only general answer is, "It depends."  You have 
to evaluate what you want out of the installation, your location and the 
terrain that's involved, before you can make an informed decision.

73, Pete N4ZR
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