I also found the 2005 thread "tuning elevated radials" on this
reflector quite informative.
One thing that stands out is that I may be better off with more than
7 shorter than 130' radials.
Note - if the radials are not "tuned", that is do not supply the
"missing half" of the resonant antenna, the vertical and "T" will
need to be adjusted to make up the difference. There may be some
advantages to using shorter radials and a longer/taller vertical/T
in that the current node will move up the vertical (getting it
farther away from the lossy "dirt") and the E/I ratio at the feed
point will change raising the feed (not radiation) resistance which
may make the antenna easier to feed.
N7LF's work also shows that more elevated radials are better than
fewer. Since the losses are a function of the square of the field
intensity, spreading the E field more evenly over a larger area
reduces losses by decreasing the peak field intensity.
73,
... Joe, W4TV
On 12/13/2012 5:27 PM, Grant Saviers wrote:
Thanks for the comments and pointers. The land around the antenna is
mixed grass and forested islands so on the ground radials would be
partially buried and partially on the surface. Digging through the
trees and clearing the brush is not something I want to do. Also, based
on prior experience with verticals on metal roofs, I'm a real fan of
elevated radials.
I am relying on the credibility of the N6LF QEX series for how well/not
well elevated radials will work (Mar - June 2012). I realize this work
was all analysis with EZNEC PRO, but it seems to be the similar to
results of others I've read. Googling "K5IU elevated radials" I did
find the 2008 N6LF article which has the experimental data as well. His
analysis shows there isn't much difference in losses with more than 4
radials between 0.15 and 0.27 wavelengths long. I've heard conventional
wisdom is to tune radials for resonance, but the analysis for 4 or more
radials elevated > than a couple of feet seems to indicate it is a lot
of work for little benefit.
I also found the 2005 thread "tuning elevated radials" on this reflector
quite informative.
One thing that stands out is that I may be better off with more than 7
shorter than 130' radials.
Grant KZ1W
On 12/13/2012 12:06 PM, Dennis W0JX wrote:
Grant, you should consider putting in an additional 23 radials and put
the radial system on or in the ground. This will eliminate any
possible detuning by the big metal building and interaction with the
RX 4 square. You said that your vertical T will go up to 85 feet.
However, by elevating the radials 10 feet, your effective vertical
distance is 75 feet which will allow you to shorten the top hat wires
a bit. As an alternate, you could put down 1/8 wavelength radials on
the ground but more of them and have a good system too.
If you must go with an elevated radial system, I recommend that you
read the articles by Dick Weber, K5IU, who strongly advocated elevated
radials shorter or longer than 1/4 wavelength. If shorter, then the
radials are loaded with a small coil. If longer, then they are tuned
out with a capacitor. W5UN uses shortened elevated radials on his 160
meter 4 square with great results. They are about 70% of a
quarter-wave in length.
73, Dennis W0JX/8
Milan OH
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