There are two reasons to stack yagis or LPDA's:
1. To increase gain by using a phased array. Maintaining a specific
wavelength spacing is important.
2. Taking advantage of the different take off angles that antennas at
different heights have, due to their interaction with the ground. Depending
upon the ionospheric conditions, the band, the distance between stations,
etc. sometimes a lower antenna is better than a higher antenna. Often the
antennas can be switched so that all, any two or only one antenna is active.
This is commonly called "space diversity". Spending an hour with a program
such as HFTA will provide some insight into why stacked antennas are the
mainstay of contesters.
Steve, W3AHL
----- Original Message -----
From: "jim Jarvis" <jimjarvis@optonline.net>
To: <towertalk@contesting.com>
Sent: Sunday, January 24, 2010 2:42 PM
Subject: Re: [TowerTalk] stacking logs
> OK, I vowed not to add further to this thread. I lied.
> Curiosity has the better of me.
>
> Where the logs are stacked horizontally, they would seem not
> to be ideally spaced for vertical angle pattern, over frequency.
>
> The installation on the DX eng'g website... N9IN or wherever it was...
> has the
> logs at 44, 74 and 116 feet. That's a 30' and 42' spacing between the
> pairs,
> and 72' between top and bottom. There may be some frequency dependent
> relationship in this spacing, but it isn't falling out in my feeble brain.
>
> The spacings don't work out with any of the free-space pattern spacings
> with which I'm familiar.
> Maybe modeled over ground, there's some sense there?
>
> In any case, if anyone is able to explain the concept, I'd be interested
> to hear it.
>
> N2EA
>
>
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