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+---- Thread: Grasswire Antenna (/showthread.php?tid=153)
Grasswire Antenna - VA3KOT John - 2020-08-03
The conventional wisdom is: "get your antenna up as high as you can". But, as Mike Toia K3MT has demonstrated and published, a wire antenna laid directly on the ground also works. Mike calls his antenna a "Grasswire". Critics dismissed the idea as ridiculous. Ground losses will completely wipe out your signal, they say. But anyone who has tried a Grasswire (many, including myself) knows, it does work, with some limitations. Here is how it works.
The signal in the antenna will induce a reflection in the ground. The reflected wave in the ground exactly cancels the signal in the antenna - up to about a half-wavelength. Because the reflected wave travels more slowly than the above ground signal, the phase relationship between the two changes with antenna length. If the antenna is at least a full wavelength long, less of the signal is cancelled by the reflected wave. At some point, the phase relationship results in zero cancellation and the signal is radiated as though no ground is present (see attached diagram).
K3MT measured the current in the antenna wire along its length and determined that the current (and therefore the radiated power; remember P=i^2R) diminishes rapidly and that any antenna over one wavelength long is unnecessary. In contrast, the US Marine Corps used very long wires (5 wavelengths) for field expedient antennas in Vietnam, but they kept the wire at least 1 metre above ground, thereby reducing ground losses.
A very long wire (aka Wave Antenna) radiates a vertically polarized signal off its ends. At a certain angle between the incident wave and the ground (known as Brewster's Angle) the signal is reflected instead of being refracted into the ground. You can observe Brewster's Angle by going to the Huron coast in the evening and watching the sun get lower in the sky. At a certain height above the horizon the sunlight will reflect brightly off the surface of the lake. This is the effect that takes place with a Grasswire Antenna.
I have experimented with Grasswires for several years with limited success. My devotion to QRP ended with the poor propagation conditions accompanying the latest solar minimum. According to EZNEC, a Grasswire will result in a gain of about -12dB. A 5 watt signal will be attenuated down to milliwatt level but a 100 watt signal into a Grasswire will still radiate 12.5 watts.
So why use a lossy antenna? Stealth! A Grasswire is invisible. Lay it down anywhere and operate. It can be deployed in seconds and you're on the air. And fun. I enjoy the experimental side of ham radio.
Here are my results from today's test (interrupted by rain before I could get a QSO so I tried WSPR):