Vertical ant gain vs No radials
Wes Stewart wrote:
On Fri, 27 Jan 2006 16:13:29 +0000, kc1di wrote:
[op snipped]
Hi John,
I once had a 30m Vertical ground mounted with 120 radials and it worked
great.. but it work quite well with the 40 or so I had at first also..
the other dynamic that is at work that you hardly ever hear about is
that as you increase the number of radials say from your 32 to approach
100 or so you also need to increase their length so it approaches
1/2wave for each radial.
You hear about it if you read Devoldere's "Low-Band DXing." [g]
He quotes a rule of thumb developed by a fellow member of my DX club,
Eric, N7CL, who says that radial length and number should be such that
the tips are not more than 0.015 lambda apart. So if you use 1/4
lambda radials you need 104 of them to be within 0.1 dB of theoretical
max gain (over average to good ground). Half this number would reduce
the gain by about 1/2 dB.
Of course right above this text in the book is a table that shows for
120 radials the optimum length is 0.5 lambda. So the same book, on
the same page has two different "rules."
Personally, my "rule of thumb" is that the radials need be no longer
than the physical height of the radiator (1/4 lambda) and their
number follow Eric's rule.
as a rule of thumb you should keep adding radials until you see no
further change in antenna input impedance. At that point it is not
worth the effort or expense to add more.
The problem with this is that the derived number is dependent upon the
resolution of the instrumentation. If you can't see a 5 Ohm change,
you quit early. If you can resolve 0.1 Ohm, you keep adding wire,
even though the efficiency improvement will be insignificant.
some point well take Wes,
73 Dave
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