[TowerTalk] 80m vertical size calculation

[jimlux]

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.