Jeff, that appears to be an article by N6LF and not N6BT. I wasn't able
to download the article, but on N6LF's website
(http://rudys.typepad.com/files/qex-ground-systems-part-3.pdf) he shows
the results of an experiment with elevated radials on 7.2 MHz. He
compared four elevated radials to 64 ground mounted radials and found
the elevated system had equivalent gain if the radials were elevated a
minimum of four feet above the ground. I know I had at one time
considered an equivalent 160M vertical and was planning on elevating
them at 18 feet.
73, Joe
K2XX
Jeff AC0C <mailto:keepwalking188@ac0c.com>
Friday, September 23, 2016 3:00 PM
I see this a lot - need to get the elevated radials up xx feet or
whatever. But I don't know how to square that with the N6BT QEX data
which shows a improvement in efficiency as the radial is lifted up
just slightly from the ground (per QEX Mar/Apr 2012 page 39, figure
16, it's 0.0005 WL approx) - with the slope of improvement flattening
out beyond that.
Either the QEX data is wrong, or it's not applicable for some reason
in the special case of a 4-square (the article is about a single
vertical with elevated radials), or the claims of "moving the radials
up made a big difference" are subjective.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message----- From: Jim Brown
Sent: Friday, September 23, 2016 1:46 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 4 square for 80
Excellent post, Tor. I'm not so sure that I agree with this paragraph
though. While elevating radials DOES reduce the effective height of the
radiator, and thus radiation resistance, elevating radials reduces
ground losses for the same number of radials. In essence, efficiency is
the voltage divider ratio between Rr and Rg + Rr, where Rr is the
radiation resistance and Rg is the ground loss. So it can be a tradeoff,
depending on what's practical at your QTH and how good your soil is.
Based on my 160M experience (4 radials up about 20 ft on a full size
vertical), I'd guess that 8 ft is pretty good for 80M. Those 4 radials
at 6 ft had way too much ground loss. On advice from N6BT, raising them
to 20 ft helped a lot. The feedpoint for those verticals is about 2-3 ft
high, and the radials rise quickly as they go away from the feedpoint in
a "gull wing" configuration.
73, Jim K9YC
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Jim Brown <mailto:jim@audiosystemsgroup.com>
Friday, September 23, 2016 2:46 PM
Excellent post, Tor. I'm not so sure that I agree with this paragraph
though. While elevating radials DOES reduce the effective height of
the radiator, and thus radiation resistance, elevating radials reduces
ground losses for the same number of radials. In essence, efficiency
is the voltage divider ratio between Rr and Rg + Rr, where Rr is the
radiation resistance and Rg is the ground loss. So it can be a
tradeoff, depending on what's practical at your QTH and how good your
soil is.
Based on my 160M experience (4 radials up about 20 ft on a full size
vertical), I'd guess that 8 ft is pretty good for 80M. Those 4 radials
at 6 ft had way too much ground loss. On advice from N6BT, raising
them to 20 ft helped a lot. The feedpoint for those verticals is about
2-3 ft high, and the radials rise quickly as they go away from the
feedpoint in a "gull wing" configuration.
73, Jim K9YC
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RT Clay <mailto:rt_clay@bellsouth.net>
Friday, September 23, 2016 2:15 PM
Interesting comments re 4-squares. I currently have up a "wire
compromise in the woods" 4 square for 80 that seems to work quite
well. Here's what I have done:
In the center of the 4-square is a 69 foot R25 tower with
non-conducting guys. On the tower are two 15m yagis and a 6m yagi. My
4-square verticals are wires. They go vertical for 48 feet and then
head towards the top of the tower- like inverted L's, except the
horizontal part of the L slopes upwards. There are ropes tied out to
trees to keep them up. Radials are on the ground. I use a Comtek
hybrid coupler.
I detuned the tower by dropping a wire from about 20 feet up on a
sidearm. This loop is tuned to resonance with a capacitor. I see about
a 1 ohm difference in the resistive part of each vertical's Z with the
detuning in vs. out.
As far as performance, the 4-square is always better than a dipole
(nearly flat) at 90 feet for dx stations. For the USA west coast the 4
square is also quite a bit better than the dipole (the dipole favors
NE/SW). Even for working W1 from my qth (MS) I often use the 4-square.
Various comments:
The reason I chose inverted-L-like elements was that they are easy to
make exactly identical, and I get the maximum height possible. With
this geometry, the pull-out ropes going to trees don't have to all be
at the same height.
In my opinion, if you are height limited, you want to stay away from
elevated radials. If you elevate your radials, you are giving up
valuable vertical distance which will further lower the feedpoint Z
and increase ground losses.
I don't think the advice to model the central tower is very helpful.
Modeling the typical tower is nearly impossible. Just getting the
effective diameter of a triangular tower is nontrivial. Plus add
stacked yagis, insulated/noninsulated yagi elements on top, etc. Just
plan on detuning anyway.
I thought that the hybrid-type controllers (at least the Comtek I
have) were really meant to work at 50 ohms. Then for a fullsize
vertical, 36 ohms + some ground loss is not too far from 50. Obviously
they seem to work ok over a reasonable range.
Tor
N4OGW
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