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
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!
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.
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!
During our Bermuda
excursion, I took a TR-7, small antenna tuner, a power supply,
and a Grasswire. We were
guests of a small family-run group of rental cottages in Southampton
for four days. On the third
day, one of the elder family members chatted with me a bit, and
asked if I was perhaps a radio amateur.
I said, “yes.” In fact, I had been on the air for the previous
two days, using a Grasswire. It was only that morning that he, in cleaning
up around the area, came across the antenna! That’s
a low-profile antenna.
GMT CALL his/my RST FREQ Power
1554 SM6DYK 579 / 559 28004
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,
0227 W8AO 589 / 569 3547
15 BOB - SILVER
1720 G3RFE 579 / 559 21016 100
TOM - BARROW
1932 G0CBW 569 / 559 14029
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
2105 WA200JXT 599 / 599
28015 80 ND
Feeding the Grasswire
Since this antenna
is largely resistive, a simple trifilar
balun is all that I have ever had to use. Figure 4
shows how to make a Balun that works.
I pull the insulated jacket off 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 a T-200-2 core (available from Amidon and others), 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.