ANTENTOP- 01- 2017 # 021

The GRASSWIRE:
Another Approach to Hidden HF Antennas

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!

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!

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

27 OCTOBER

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 OCTOBER

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 OCTOBER

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.

Typically 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.

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