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[TowerTalk] element length criticality

To: towertalk <towertalk@contesting.com>
Subject: [TowerTalk] element length criticality
From: Jim Lux <jimlux@earthlink.net>
Date: Tue, 14 Apr 2015 06:45:34 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 4/14/15 5:44 AM, Peter Voelpel wrote:> Why is a difference of a 1/16" of element length a disaster with an antenna
> to cover 430-438MHz?
>

a long boom Yagi with many elements has fairly critical lengths to get the side and back lobes down. Forward gain isn't strongly affected, though.

10 MHz BW is 1%.

1/16" (actually 0.125" total element length) out of a 12" element is also 1%

If you're looking for -20dB or -30dB side/backlobes then 1% kinds of errors can be a problem.

These kinds of designs are optimized for one frequency (432, usually) and while they might show decent feedpoint match for a fairly wide frequency, their pattern probably isn't very good 5MHz away from the design frequency.


Element length in long boom yagis is like surface smoothness in a dish. Huge dings in a dish don't change the gain much, but boy, do they create sidelobes.

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