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Re: [TowerTalk] 80m vertical size calculation

To: towertalk@contesting.com
Subject: Re: [TowerTalk] 80m vertical size calculation
From: jimlux <jimlux@earthlink.net>
Date: Wed, 13 Sep 2017 12:11:15 -0700
List-post: <mailto:towertalk@contesting.com>
On 9/13/17 8:41 AM, V. Sciucka wrote:
Thanks Gary K9RX and Charlie N1RR, 2nd question is clear now.
Charlie also gave formula for length 246/f(mhz) =  element (feet) which I
assume includes length to diameter ratio or this ratio is not so much
important.


the classic 246/f is the free space half wavelength. You'll see other numbers too (e.g. 234..) (see "Where Does 468 Come From?" at https://www.eham.net/articles/23802 for an interesting article by N0AX, which also mentions the 492)
The length of a resonant (defined as where the reactive term of the 
feedpoint impedance) is slightly less than a half wavelength in free 
space.  How much less is weakly dependent on the length/radius.
length/radius ratio has a fairly strong effect on the "slope" of the 
feedpoint reactance and resistance (more on the reactance than the 
resistance), so it has a large effect on the VSWR bandwidth.

For a vertical, (or a low horizontal dipole), the soil properties will have a significant effect on the "resonant" length - in general, making it shorter than free space resonant (which in turn is shorter than free space 1/4 or 1/2 wavelength)
Here is where modeling is your friend - almost any version of NEC can 
quickly run through some variations of soil properties, diameters, etc. 
and you can get an idea if you are worried about millimeters or meters.



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