K3MT
presents . .
.
The GRASSWIRE
another approach to hidden HF antennas
April, 1997
Deed restrictions got you down? Neighbors intimidating your tower plans?
Need a really easy portable HF antenna? Then the grasswire may be
the answer! Virtually invisible, lightweight, and compact (you can carry one in
your hip pocket), this antenna works! It has been used by K3MT in various
installations for more than 10 years.
Read on - and listen to the "experts" telling you that this is hogwash, that
an antenna like this can't work. But it does. And true experts, who have
taken a decade or more to come to grips with the intricasies of Maxwell's Math,
know why it works.
This antenna will not out-perform a yagi, or a decent dipole up a half
wavelength. Not in gain or signal strength, at least. But it will survive an ice
storm, wind storm, and is practically immune to lightning. And it doesn't need a
large tower or tall support. I deploy one from my hip pocket at times - the
balun to match it is larger than the antenna!
THE GRASSWIRE - IN BRIEF
What is it? Put simply, it is an end-fed, longwire antenna that is
laid right on the grass. Hence the name. The original grasswire used by K3MT in
the summer of 1988 was just 204' of #18 AWG magnet wire laid along the property
line, anywhere from 1" to 6" above the ground. This sketch shows plan and
elevation views of a typical installation. Both an 8' ground rod and optional
counterpoise wires are shown. Use one or the other. Both are not needed.
![grasswire plat]()
These antennas are largely resistive, with values ranging from 150 to 500
ohms or so on average ground. They have been used successfully on the average
soils northwest of Washington, DC, on the sandy soils of the Cape Canaveral,
Florida area, in the rocky, shale soils of the mountains in Somerset county, PA,
and on river bottomland of Allegheny County, PA. One was used with great success
by K3MT/VP9 in Southampton, Bermuda - the object of nightly pileups on 30 m CW
for four nights.
REFLECTION AND THE BREWSTER ANGLE
The skeptic in you will doubt that such low antennas can work. After all, its
image in the ground radiates and cancels out all radiation. True - if the ground
is perfect. But nothing is perfect! The grasswire radiates vertically
polarized off the end of the wire. Extensive monitoring tests with
wires laid along the great circle route toward WWV, and perpendicular to that
line, demonstrate the end-fire nature of the antenna. So why does it work?
When a plane wave reflects from an air-earth boundary, an incoming ray
reflects, giving an outgoing ray. These two, and the line normal to the boundary
plane, form a plane of incidence. Solutions of Maxwell's equations differ
for the case of the E-field being perpendicular to this plane (i.e.,
horizontally polarized), and the case when the E-field vector is in the
plane of incidence. You will probably call the latter "vertical" polarization,
although this is technically not correct. Electromagneticists (a.k.a those who
practice Electromagical effects) refer to these cases as normal incidence
(horizontal polarization) and planar incidence (vertical polarization.)
For the normal incidence case, reflection is nearly total, with a nearly 180
degree phase reversal. Thus very low antennas neither respond to, nor generate,
appreciable amounts of horizontally polarized radiation. But for the planar
incidence case, the reflection varies in strength considerably. At some
takeoff angle (angle between outgoing ray and the ground) the reflection
becomes quite weak, and has a 90 degree phase shift. Near this angle, the sum of
direct and reflected rays will have a magnitude as if the antenna were in free
space! Of course, at other angles, ground reflection largely cancels the direct
ray, and the antenna does not radiate well at all.
A reflection coefficient is calculated as the ratio of the electric
field in the incoming ray to the electric field in the reflected ray. It varies
from one (total reflection without loss) to zero (no reflection at all.) It
depends on the takeoff angle, frequency, and the soil parameters (dielectric
constant and conductivity.) Here are plots of planar incident (vertical
polarization) reflection for typical "good" and "poor" soils.
![reflection, good soil]()
![reflection, poor soil]()
Notice that, at 10 to 25 degrees, the ground reflection is very weak. It also
is shifted 90 degrees in phase from the incident ray. Therefore, radiation from
the grasswire, off the ends will be about the same as if the ground were not
present.
But launching a ray at, say, 15 to 20 degrees takeoff angle, in a direction
toward Europe, can be useful! That's what a grasswire does. It is lossy in all
directions, but least lossy when exciting the ionosphere for a long-haul DX
contact. To demonstrate the point, here's an extract of K3MT's log, for October
of 1988, (ahh, glory! Yes, the SSN was good then!) using a grasswire:
Date GMT CALL his/my RST FREQ Power
OCTOBER
27 1554 SM6DYK 579 / 559 28004 80
1601 SM0LBR 569 / 439 21007 100 RAY - STOCKHOLM
2001 W4JBQ 579 / 569 7029 40 JOE - FT WRIGHT, KY
2141 W8LNJ 579 / 459 28015 80 DAVE - DALLAS, TX
28 0227 W8AO 589 / 569 3547 15 BOB - SILVER LAKE, OH
1720 G3RFE 579 / 559 21016 100 TOM - BARROW
1932 G0CBW 569 / 559 14029 50 MEL
1945 VE2FOU 589 / 559 7032 100 ANDRE - IBERVILLE
2026 KB7UX 569 / 539 21040 100 RUSS - CHINO VALLEY, AZ
2100 I2JIN 589 / 559 14022 40 BOB - COMO
2123 G3JVC 569 / 559 14022 40 JOHN - LONDON
29 2105 WA200JXT 599 / 599 28015 80 ND
Not bad, for a wire on the ground. Notice that contacts were made on 80,
40, 20, 15, and 10 meters. The signal reports are not fantastic. But contacts
were made, and ham radio was enjoyed! Five countries were worked in 3 days. And
the best part of this setup: the neighbors never knew that a ham station was
on the air!
FEEDING THE GRASSWIRE
Since this antenna is largely resistive, a simple trifilar balun is
all that I have ever had to use. This sketch shows how to make a balun that
works:
![balun]()
Typically I pull the insulation off of some indoor telephone wiring cable.
Four insulated #22 copper wires are inside: discard one of these and use the
remaining three. Wind about 16 turns on the core, without allowing the wire to
twist (keep the three conductors parallel at all times.)
Notice that this "balun" really matches an unbalanced antenna to an
unbalanced transmission line. It is basically a wide-band, three-winding
autotransformer. Impedance ratios are as shown on the drawing. Generally it is
necessary to connect the coax to either A2/B1 or B2/C1, and the antenna to B2/C1
or to C2. This may change from one band to another, and usually does.
WINDOM IN THE GRASS
I have elsewhere described a windom antenna. While
it is usually hung from a pole or in a tree, it works when used in a "grasswire"
mode. Just lay it on the ground. Dimensions are repeated here for ready
reference.
![ground-mounted windom]()
When I travel, I take one of these made of #22 insulated hookup wire. Since I
often set up beside motel parking lots, and often after a day's work, the longer
wire is black, and the shorter one is red. This helps me determine which way to
point the windom. Remember, though, that it fires off the long end. Of
course, it fires the other way, too, but usually works best off the long end.
I hope this has given some of you a good case of curiosity. Go out and try
one of these ground - mounted wires. They're easy to build. Even the balun is
easy to build.
If you must, contact us: we can supply a core, a whole balun, or a whole
grasswire windom setup.
For more unusual antennas, visit my web page.
And check out my Books for a dozen
topics about HF antennas.
that includes the grasswire and other beverage
antennas.
73
K3MT
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