From: Brad Bradfield QLF
Subj: VERTICAL GROUND SYSTEMS
I posted the following to the QRP-L last year sometime, and it was also
published in the newsletter of the NWQRP club. Although I worked in the field
of broadcast engineering (and particularly antennas) for many years, in all
areas of question, I will always defer to the real gurus, such as those listed
in the bibliography at the end.
With nothing personal intended, too many amateurs tend to base their
statements too much on subjectivity ("My antenna works great!"), and too
little on objectivity ("I measured X and found Y.) I remember a discussion in
a forum at a hamfest a number of years ago with a well known 160 meter DX'er.
He went to great lengths describing all the headaches he went through trying
to make his Gamma-matched 1/4 wavelength tower load properly without the tuner
at the base catching fire (actually happened!) and so on. He was doing
everything seat-of-the-pants with nothing more than an SWR bridge (Yuk!) to go
by, and had no idea what the real impedances and other parameters were that he
was working with. When I asked him something and used the term "shunt fed"
tower, he jumped back at me indicating that this was not a shunt fed tower.
Well, sorry DX dude, but a grounded antenna fed like this is certainly shunt
fed. In reality, a gamma-matched yagi is actually shunt fed, but this guy's
mind was made up irregardless of what the real experts said. Oh, well. No
objectivity. Sorry about the length, but I hope this adds something to the
currently ongoing thread. Please note references to the bibliography at the
end.
=============================================================================
VERTICAL GROUND SYSTEMS
Even though this isn't QRP related per se, it is a topic that continues to
come up on the QRP-L and in other conversations, so let me attempt to give
y'all some information to gnaw on. Ok now, everybody put away your knives,
guns and flame throwers. Hand grenades are ok as long as I have time to
through them back at'cha. Keep your small (or large) thermonuclear devices at
home. Antennas and related subjects are a lot like automatic transmissions;
everybody has one, but very few people really understand them. It's kind of
like a little bit of black magic right in your own back yard.
First off; verticals REQUIRE a low impedance ground return for PROPER
operation. Now unless your vertical is installed offshore in the ocean (I've
seen broadcast stations like that,) or the lawn in your back yard is actually
one big sheet of copper (not with my wife!), this probably means radials.
Everybody's heard some amateur saying "My Cush-nut-Mos-Gain vertical loads
fine with just a ground rod and no radials so I must not need them after all."
In the words of the immortal Colonel Sherman T. Potter; "HORSE FEATHERS!"
Just because it LOADS moderately well doesn't mean that the radiation
efficiency is any better than an empty can of beans. You could be QRPppp
without really intending to be. Ground rods are fine for DC or lightning
grounds, but are typically very ineffective as RF grounds.
The following table will show the relative improvement in radiated field
intensity with an increasing number of 1/4 wavelength radials. Each entry is
referenced to an antenna with two radials.[1,4]
NUMBER FIELD
OF INTENSITY
RADIALS IMPROVEMENT
(n) (dB)
------------------------------------
2 REF
15 2.7 dB
30 3.4 dB
60 3.9 dB
113 4.2 dB
The following table will show the relative improvement in radiated field
intensity with radials of increasing length on a 1/4 wavelength vertical
antenna with 113 total radials. Each entry is referenced to a radial length
of 0.125 wavelength.[1,4]
LENGTH FIELD
OF INTENSITY
RADIALS IMPROVEMENT
(Wavelength) (dB)
------------------------------------
0.125 REF
0.25 2.1
0.375 2.5
0.412 3.8
Looking closely at these numbers, it becomes readily apparent that you rapidly
approach a point of diminishing returns, beyond which additional radials or
radial length is probably a waste of time and money for the amateur.
Common practice in the broadcast industry is to use 120 to 240 1/4 wavelength
or longer radials, often interspersed with a number of shorter radials. They
are typically #8 or #10 AWG bare copper, and are plowed into the ground. If
you're building a broadcast station, you will, of course, allow enough real
estate to install these radials. If you're sitting on a normal city lot,
however, you won't have this luxury. In that case, install as many radials of
whatever length is possible. And you can always bend them to get around
objects and/or to make them longer. While any distortion in the circularity
of your ground radial system will, in fact, cause some distortion in the
radiated pattern of a vertical, in amateur usage, you'll probably never notice
the difference.
Another interesting fact is that the field intensity improvement with an
increasing number of equal length radials is more pronounced with an antenna
that is physically short to begin with. Increasing the number of 0.412
wavelength radials from a quantity of two to a quantity of 113 gives the
following results at various antenna heights.[4]
HEIGHT FIELD
OF INTENSITY
TOWER IMPROVEMENT
(Wavelength) (dB)
------------------------------------
30 7.38
60 4.58
90 3.82
Ground losses increase rapidly with frequency. The higher the frequency, the
more ground system required to minimize losses. That's why you'll never see a
VHF or UHF ground mounted vertical with buried radials. This is obviously an
extreme example, but true. At frequencies up into the 7-10 MHz range, a
ground mounted vertical with buried radials will work very well. While ground
mounted verticals with radials are commonly used in the amateur service as
high as 28 MHz, an elevated vertical with a counterpoise or metal ground plane
will be a more efficient radiator above the 7-10 MHz range.[1]
I can find no hard data on the relative efficiency of insulated versus bare
wire for buried radial systems. I've used the insulated stuff myself, so know
it works, but don't know anything about the relative merit of one over the
other.
I won't go into any one method of burying radials. Everyone has his own trick
or can come up with one. At HF, any depth beyond 2-3 inches is not required.
Terminate your radials around the base of your vertical by soldering them to
a copper bus strap or heavy copper wire. If you're shunt feeding your
vertical, a heavy bus strap or wire is required between this radial
termination and the tuner ground, as heavy currents will flow at this
point.[1] Even if you intend to operate only at QRP power levels, the loss at
this point needs to be minimized. Actual wire size of the radials is not of
great importance. They should be of a large enough size to survive whatever
method of installation you choose.
BIBLIOGRAPHY
For more information, read the following:
1) The Amateur Radio Vertical Antenna Handbook by CAPT Paul Lee, N6PL
CQ Publishing, Inc
[Outstanding reference, but may or may not be still in print]
2) The Complete Broadcast Antenna Handbook by John E. Cunningham
TAB Books
[Out of print?]
3) Antenna Engineering Handbook by Johnson and Jasik
McGraw-Hill, 2nd Edition
Chapter 25
[Expensive! >$100]
4) Ground Systems as a Factor in Antenna Efficiency by Brown, Lewis and
Epstein
Proc. of the I.R.E., June 1937
[This is probably the best classical work on ground radial systems, and is
still regularly quoted today. Find a library that has it.]
5) The ARRL Antenna Book
15th Edition, 1988
Pages 3-13,14 and 8-22,23
6) Radio Antenna Engineering
1952
Edmund A. Laporte
7) Broadcast Engineering Handbook
National Association of Broadcasters
8) Radio Handbook
22nd Edition
Bill Orr, W6SAI
Chapter 24
This list is certainly not all inclusive, it is simply what I commonly have on
the reference shelf at my desk. Another excellent reference is the articles
written by Jerry Sevick, W2FMI, in QST in July 1971 and March 1973.
==============================================================================
72's es 73's,
Brad, WB0CGH
******************************************************************************
Brad Bradfield, PE Electrical Design Engineer
(H) 817-321-2960 Texas Instruments, Inc.
(W) 972-462-6230 (Soon to be: Raytheon/TI Systems)
Real men talk with their fingers!
QLF@MSG.TI.COM
WB0CGH@WO5H.#DFW.TX.USA.NA
ARRL Life Member QRP-L #377 SMIRK #4906 IEEE(M) ARS #72
Collector of wireless and landline Morse keys and accessories.
******************************************************************************
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