Jim,
The concept of sloping the booms of two vertically stacked LPDA's is to
maintain proper phasing between the wave fronts from the two antennas as the
frequency changes and the active region moves along the boom so they
optimally reinforce each other in the near field and far field regions. If
the antennas are too far apart or too close the gain decreases somewhat.
But with proper spacing / phasing, the minor lobes can be reduced
significantly, improving F/B ratio.
It has nothing to do with take off angle. That is primarily a function of
the far field interaction with the ground terrain for the first 2-4
kilometers.
Steve, W3AHL
----- Original Message -----
From: "jim Jarvis" <jimjarvis@optonline.net>
To: <towertalk@contesting.com>
Sent: Sunday, January 24, 2010 3:31 PM
Subject: Re: [TowerTalk] stacking logs
>
> Sorry for extending this, but another thought came up.
>
> (Doug...aren't you glad you got this one started??)
>
> (Gene... the tilted log post was mine, at first... )
>
> So here's the thought, with respect to tilted logs:
>
> The full length of the log isn't really active at any given frequency.
> Only a short portion of the boom, and maybe 2 or 3 elements have real
> current in them. Therefore, is the tilt of the boom all that
> appreciable, in terms
> of takeoff angle?
>
> A tangible example: With a 16' boom log (is the t8 16 or 18', I've
> forgotten? no matter.)
> ... the booms would need to slope at 30 degrees. If the last 2 elements
> were primary in forming
> the pattern at 14 MHz, and they were around 4' apart..... the slope
> would result in the forward elements
> being only 2' lower (or higher, depending on if top or bottom log) than
> the rear element.
>
> Is this really significant in the takeoff angle? Intuitively, it
> wouldn't seem to be the same thing as
> a yagi with the director 16' in front of the reflector.
>
> What am I missing?
>
> N2EA
>
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