HI, Grant!
A couple of things to think of.
First, as reported in the IEEE paper mentioned earlier, increasing the
number of elevated radials to more than four , yields a rather limited
benefit, This work was done by professional antenna engineers with plenty of
equipment and resources. The point of the investigation was the development
of a viable replacement of degraded buried radial systems under broadcast
towers with elevated radials and did involve measurement of radiated field
intensity.
As for our 160 antenna with elevated radials - think of it as a ground-plane
antenna! In the case of the inverted-L the vertical monopole element is
simply bent at a convenient height to take advantage of available supports.
In the case of the tee, this can be very convenient for supporting the upper
end of the shortened monopole. Additionally, in the case of the TEE any
residual horizontally polarized radiation from the flat-top of the TEE will
be pretty much cancelled by the equal and opposite currents flowing in
opposite directions in the flat-top wires.
Unless you are very space limited, the major advantages of RESONANT radials
are that the current maxima in the radials will occur at the antenna
feed-point. That allows the driving-point impedance to approach pure real in
a well-behaved manner - being something less than 70 ohms. (More like 35-50
ohms if you have a good radial system. Short radials introduce a reactive
component into the antenna feed that needs to be dealt with. I expect that
using many short radials acts more like capacitive coupling to the
underlying soil and its losses to provide and "image" for the vertical
monopole.
Dig, if you wish, but there is nothing "magic" about dirt! And it can be a
bit lossy!
I think you are on a good track! I'll be interested to see how it turns
out! I'm very much in favor of your elevated resonant radial approach! It
has worked very well for me! The closest I ever came with a "direct ground
"system" was in another location where I had a very tall pine on dam (that
I owned) of as good-sized lake! I drove an 8-foot copper ground rod at the
edge of the lake, at the base of the inverted-L, and using my canoe, I ran
several quarter-wave radials out into the water. Worked pretty well! - With
a fairly well behaved driving-point impedance! I could see the drop in the
driving-point impedance when I ran the radials out into the water. (Turned
out that some copper sulfate had been added to the lake to kill off some
vegetation! I expect that helped!)
Since then, I've done quite well with elevated resonant radials and an
inverted L - Until a hurrlcane tilted the tall oak that was supporting the
far end of the inverted L and we had to have it taken down.) My "elevated
radials ran around the perimeter of my lot and had 90 degree bends in them
60-70 feet from the antenna feed point!
Good luck!! "If you build it, they will come"! :)
Regards,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Grant
Saviers
Sent: Thursday, December 13, 2012 5:28 PM
To: Dennis W0JX
Cc: topband@contesting.com
Subject: Re: Topband: Elevated Radials Questions
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
> _______________________________________________
> Topband reflector - topband@contesting.com
>
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