The work of Moxon, and others, has shown that elevating the radials just a
few feet reduces ground coupling and ground losses considerably.... For 80
meters, a 3 foot elevation of the radials reduces ground currents >90%...
For my 160 array, the pair (as in 2) of elevated radials are about 7' at
the low point.... attaching and then removing, etc. an 8' lead to a copper
ground stake does not change the measured antenna impedence even 1 ohm...
(wet ground, rich pastoral soil, considered to have excellent
conductivity)... this indicates, to me, that the pair of radials have coupled
to the vertical to an extent that they are screening it from ground with
high efficiency....
i.e. in the equation
Effieciency = Ra / (Ra + Rg)
the ground current due to Rg must be small....
Even after having reviewed the literature on this, I have to admit I was both
impressed and a bit surprised... Like most, I have the mental image of 120
radials as being the ideal... OTOH, we don't fuss because a dipole antenna
has only 2 elements! ... So by analogy, if the radials are the other half of
the dipole, and are elevated above ground and are resonant, then 2 should be
sufficient ( only 1 radial allows the vertical to see too much ground on the
side opposite the single radial)...
Denny
>From millersg@dmapub.dma.org (Steve Miller) Sat Dec 7 13:53:49 1996
From: millersg@dmapub.dma.org (Steve Miller) (Steve Miller)
Subject: Elevated verticals
Message-ID: <m0vWNCP-0002BGC@dmapub.dma.org>
> The work of Moxon, and others, has shown that elevating the radials just a
> few feet reduces ground coupling and ground losses considerably.... For 80
> meters, a 3 foot elevation of the radials reduces ground currents >90%...
I have serious doubts about such a claim - please cite the references.
> For my 160 array, the pair (as in 2) of elevated radials are about 7' at
> the low point.... attaching and then removing, etc. an 8' lead to a copper
> ground stake does not change the measured antenna impedence even 1 ohm...
A single, electrically short ground stake will not significantly reduce
ground losses (except perhaps in seawater). This comparison simply indicates
the change in Rg is small compared to Ra between the two cases.
> (wet ground, rich pastoral soil, considered to have excellent
> conductivity)...
Copper, silver, etc. have excellent conductivity - soil does not. This
difference is not subtle.
> this indicates, to me, that the pair of radials have coupled
> to the vertical to an extent that they are screening it from ground with
> high efficiency....
> i.e. in the equation Effieciency = Ra / (Ra + Rg)
> the ground current due to Rg must be small....
Your test indicates the 2 radials plus ground stake reduces ground loss
about the same as the 2 radials alone. What is missing in order to
determine efficiency is the input impedance with just the ground rod and
a reasonable model of the vertical over perfectly conducting ground
(i.e. a good estimate of Ra with no ground losses)
> Even after having reviewed the literature on this, I have to admit I was both
> impressed and a bit surprised... Like most, I have the mental image of 120
> radials as being the ideal... OTOH, we don't fuss because a dipole antenna
> has only 2 elements! ... So by analogy, if the radials are the other half of
> the dipole, and are elevated above ground and are resonant, then 2 should be
> sufficient
Your analogy assumes the 2 radials (or dipole) are the other "half" of
the (vertical) dipole which is incorrect. The presence of the earth still
plays a role in forming the image.
> ( only 1 radial allows the vertical to see too much ground on the
> side opposite the single radial)...
How much ground does the vertical see at right angles to the 2 radials?
Quite alot I would say....
Not a flame, just electromagnetics.
73,
--
Steve Miller N8SM
millersg@dma.org
http://www.dma.org/~millersg
>From HWDX09A@prodigy.com ( ROBERT REED) Sat Dec 7 13:38:19 1996
From: HWDX09A@prodigy.com ( ROBERT REED) ( ROBERT REED)
Subject: Elevated verticals
Message-ID: <199612071338.IAA23306@mime4.prodigy.com>
I recently moved from a good size piece of land to a mobile home park.
By selecting a lot adjacent to the woods I knew I could at least
stick up an elevated vertical. Wanting a multiband array was making
the actual choice a bit of a problem.
I eventually decided on a CushCraft AP8-A which went together easily
in only 1 hour. Cordless drills with nutdriver attachments out to be
standard amateur gear.
I had a Rohn tapered top section about 3 feet in the ground with a
mast that was to place the base of the vertical and radials at about
12 feet.
To my surprise ALL bands were way low in frequency. Shockingly low as
if there was a major problem with the antenna. As all bands were
effected it didn't appear to be a blown trap unless it was the 10M
one. Radial were removed, balun removed. No change in the resonant
frequencies to speak of.
A telephone call to CushCraft on Monday revealed that the antenna
tuning took into effect lots of ground losses and needed to be no
more that 2 inches above ground which placed the feed about 1 foot up.
A quick pipe in the ground and moving the vertical over confirmed
approaching decent SWR's on all bands.
The decision regarding ground mounting or elevated became the next
question. How easy would it be to retune the AP8-A ? Having a bad
heart and relying on help when having to work on gear causing the
arms to be above heart level wasn't making me happy.
Rather than waiting weeks past the SS it was decided to go with a
ground mounted system. Thinking of how many radials was the next
question. Hw badly would the ground detune and wipe out my efforts ?
The plan quickly came to using more than the normal ground system.
Simple but different. Overkill to some but listen to my receiver on
80 compared to others who can't hear what my ground mounted vertical
does.
The basics of the radial system became a piece of wire that would not
disappear laying on the ground as it coupled to the ground. The
mobile home park had just my answer. An abandoned roll of 3/4 inch
MATV hardline.
Quickly 2 fullsize radials were cut for 75M and another pair for 40M.
Dragging them through the woods and a hose clamp and braid at the
vertical base gave me the start of the system. The 4 more radials for
20M are cut but still not installed.
The antenna the tuned nicely for all bands.
The results are very much textbook. 10M is poor as a ground mounted
antenna would be. We're not talking a good selection here. 20M is
reasonable but for the SS the angle was so low I barely had a chance
to catch some sections before the band went out. 40M worked very well
on daytime local mults and even at QRP work several west coast
stations. The biggest thrill and pleasure to me was seeing the
performance on 75M. Again textbook I had to wait for the propagation
to lengthen to my low angle. I don't think KS7T still believes I
broke his occaisional QRZ's while chatting with a QRP power.
The results are as I said textbook. The vertical performed exactly as
should be expected on each band and with this simple but different
radial system a real surprise on 75M. I have since listened on 75M at
night and every night hear DX stations perfectly that others have
obviously not copied callsignes or questions.
Of course a vertical for SS would not be the choice. A couple of low
dipoles would have been less frustarting especially on evening 75M
Q's from local mults. Breaking KQ2M in CT took 20 minutes and he was
60/over.
Had I been less concerned looking for mults and having a better
respect for the score I was working on I probably would have called
many more stations. I passed up many as breaking pileups QRP for the
first time for me was frustrating. I'm used to being able to run a
freq or S&P on one call in the SS from the old station. I probably
let 100 or more good signals slide by and from the claimed scores see
myself drop out of the Top 10 QRP with a really simple station at
present.
____
73, Bob Reed, W2CE, ex WB2DIN
1991 Route 37 West - Lot 109
Toms River, New Jersey 08757
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