How much wire?
A general rule of
random wire antennas is to get as much wire in the air as you
can - longer is better. Does
this still hold for the Grasswire? The answer
is no. Measurements show
that anything over a wavelength does no appreciable good.
My first measurement
program parked a car on a dirt trail, with a spool of 18 gauge
insulated wire unwound, one end tied to the bumper and the rest
run on down the trail. The
dirt was average stuff, mostly clay and loam on top of granite.
At the car the wire was untied from the bumper, passed
through a small RF toroid, and connected to an antenna tuner,
the latter driven by a TR7 transceiver at approximately fifty
watts. The car itself served
as a counterpoise.
A ten-turn secondary
winding on the toroid drove a small diode and capacitor. RF current in the antenna developed a DC voltage
across the capacitor that I measured with a handheld DC voltmeter. As the toroid slid along the wire, the voltage
dropped and fell below 10 percent of the starting value a wavelength
along the wire.
There was a small
rise in voltage for a short bit farther along the wire, but at
a full wavelength it fell below one percent, and never showed
any further improvement.
This occurred on 80
40, 20, 15, and 10 meters.
This measurement indicated
that the current in the wire dropped almost exponentially along
the wire, and beyond a wavelength was more than 20 dB down, so
could produce little radiation.
The excess wire can simply be removed.
Thereafter my Grasswire deployments always used about one wavelength of
wire at the lowest operating frequency.
Continuing the measurement
at a later date, an assistant and I laid a center-fed wire dipole
on a grassy field, 396 feet of insulated, 12 gauge wire - all
that we happened to have handy.
Again a small toroid RF transformer and diode/capacitor,
similar to the earlier one, had one side of the dipole threaded
through it. A fiberglass
surveyor’s tape stretched from the center along the dipole to
one of its ends. The DC
voltage, measured as a function of distance along the wire is
a measure of the RF current. Figure 5 and Figure 6 show the falloff
of current along the wire at 7 and 29 MHz amateur Bands and its
attenuation by at least 20 dB at one wavelength (seventy, and
sixteen and a half feet, respectively.)