Chrisman also did a study of that very question Dennis. The results
were published in NCJ about a year ago. There are two factors at work -
ground conductivity (efficiency) and the strength of the reflected ray
off the water.
Getting optimum efficiency from the vertical for ground mounted radials
can occur a significant distance from the water - it simply depends on
the conductivity of the ground. A low flat sandy island typically has
good conductivity significant distances from the water (think Jamaica).
Meanwhile, a coral atoll where the "shoreline" is actually a coral cliff
(PJ2T, PJ4, HP1XX) will have poor ground conductivity just a foot or two
- horizontally - from the water's edge, although you may be 30 feet
above the water vertically.
There is typically another 3 DB of gain to be had from the reflected ray
in the direction of the water vs. the direction of the land, if you put
a vertical at the water's edge (salt water - not fresh water). This is
why team vertical puts their antennas on the beach at a point where
there is a water shot to the US for the ARRL DX contest. Chrisman's
analysis shows this effect disappears quickly with distance from the
shoreline. It starts to dissipate at less than 1/2 wavelength from the
shoreline and is all but completely gone at a distance of 1.5
wavelengths.
One more consideration - folks have been cutting 1/4 wave radials for
years to put on or in the ground. The velocity factor of wire on or in
the ground is around .5 to .6 and I hate to think of all the so-called
1/4 wave radials that are closer electrically to 1/2 wave. Once you get
elevated radials up 10 or 15 feet the velocity factor is very close to
1.
For serious installations of ground-placed radial fields - up around 30
radials or more - the "concentric ring" method will provided identical
results for only about half the amount of wire - although that style
radial field is more work to construct. I can send you the reference to
that if you like - the paper is too big to go through this reflector.
The easiest way to improve the performance of a ground mounted vertical
in a neighborhood setting is to make it too long, then tune the
reactance out at the base with a series capacitor to get it back to a
reasonable SWR. This moves the current maximum from the ground up
higher on the antenna - hopefully above the shrubs, houses and such.
This is band-dependent of course. It's easy to do on 160 - and not
possible on 10, although for higher bands like 10 some shrewd guys have
gone to 5/8 wave ground mounted verticals with good results. The 160
ground mounted vertical here is a 75 foot vertical with a double set of
very large T-tops. It is self resonant at 1.2 MHz and works very well -
the current maximum is quite high off the ground. It's matched to r50
j0 ohms at the ground with a simple L network.
73,
Hal
N4GG
-----Original Message-----
From: secc-bounces at contesting.com [mailto:secc-bounces at contesting.com]
On Behalf Of Dennis McAlpine
Sent: Thursday, August 23, 2007 10:26 PM
To: Bill Coleman; K9AY
Cc: secc at contesting.com
Subject: Re: [SECC] Antenna improvements (Was: NAQP)
Tnx, Bill. After reading your comments, I went back to ON4UN's book on
LF operating. He cited a study by A. Chrisman that concluded that four
elevated radials were the equivalent of 120 buried radials if the
elevation was at least 15' on 80 meters. He also said that as the
underlying ground became poorer, the height of the elevation had to go
higher to have the same effect. Maybe I ought to go to 15' and forget
the 10'. Radio Shack, here we come.
Another question, as I start to search for a suitable remote location,
how close to the ocean do you have to be in order to receive the
benefits of salt water? Obviously, if you can stick the antenna right
in the ocean, you have a good salt water ground and all the benefits
that inure. But, how about if your antenna is 100' from the ocean? or
1/2 mile? or 1 mile? In other words, when does the distance away from
the salt water offset the positive effective of being on the salt water?
73,
Dennis, K2SX
-----Original Message-----
>From: Bill Coleman <aa4lr at arrl.net>
>Sent: Aug 23, 2007 9:45 PM
>To: K9AY <k9ay at k9ay.com>
>Cc: secc at contesting.com
>Subject: Re: [SECC] Antenna improvements (Was: NAQP)
>
>
>On Aug 23, 2007, at 10:21 AM, K9AY wrote:
>
>> I have not had experience with multiband verticals, but in the past
>> I have
>> used elevated ground planes for 15, 20, 30 and 40 meters -- all of
>> which
>> performed much better than their ground-mounted equivalents.
>
>There was a key study sited by W8JI on TowerTalk several years ago.
>The AMBC guys had actually investigated using a few elevated radials
>rather than a standard ground-mounted radial system. BC stations
>specify 120 1/2 wave radials, which is way better than most amateur
>installations.
>
>The bottom line on the study was that the ground-mounted system gave
>higher field-strength readings than the elevated system. So, they
>stuck with the ground-mounted system.
>
>So, I think it depends on what you are comparing. If you can only put
>down a few radials, then elevating them makes sense. Putting up a lot
>of elevated radials is harder, since you need to support them all.
>(Although W4WA does have that huge spiderweb of 60 1/4 wave radials
>for 160m at 10 feet...)
>
>The other issue is height. Above 1/4 wave high, a few elevated
>radials work great. Below 1/8 wave, its not so clear.
>
>Consider:
>
>Case A: 4 radials below 1/16 wave versus 4 on the ground.
>
>Case B: 4 radials below 1/16 wave versus 30 on the ground.
>
>Case C: 4 radials above 1/16 wave versus 60 on the ground.
>
>I'd say case A is a definite win for elevated radials, but case B
>favors the ground-mounted system. Case C isn't so clear. The exact
>height of the radials would be the determining factor.
>
>Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr at arrl.net
>Quote: "Not within a thousand years will man ever fly!"
> -- Wilbur Wright, 1901
>
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