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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