presents . . .
Another approach to hidden HF antennas
Updated by K3MT October 2019
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 20 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
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.
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
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
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
Date GMT CALL his/my RST FREQ Power
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
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:
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
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.
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.
For more unusual antennas, look over this page.
And check out my Books and DVDs.