A Most Unusual Antenna

It’s Common Knowledge
You have heard it many times before; an HF wire antenna must be installed as high as possible to be efficient – preferably a half-wavelength above ground. It’s common knowledge! “Common knowledge” means everybody believes it to be true – well maybe not everybody, but if the majority believe it to be true then it must be so. The majority often believe something simply because, well, everybody else does. There are a few of us “contrarian thinkers” who disdain common knowledge, preferring to experiment for ourselves to see what works and what doesn’t.

Some other common knowledge is that an antenna that isn’t 100% efficient is a “compromise antenna”. There is a lot of discussion in online forums about compromise antennas. I prefer the maxim that: “any antenna you put up works better than the antenna you don’t put up”. Let’s face it, almost every antenna is a compromise antenna. “What’s the best antenna?” is a question often raised by people who would prefer to study endless possibilities rather than get on the air.

On the Contrary
Ham radio has many aspects. Some like to rag chew, others like contesting; I like to experiment. Being a contrarian thinker is a bonus for radio experimenters. So I was naturally drawn to the idea of challenging the common knowledge that a wire antenna should be installed as high as possible. It is fairly well accepted that an antenna erected low to the ground uses ground reflection to propagate a signal straight up in the air. It is called a Near Vertical Incidence Skywave (NVIS) antenna and works really well – under certain conditions – for getting signals out of valleys to destinations within a few hundred kilometres of the source.

On the Ground
But what happens if the antenna is erected even closer to the ground – or even on the ground? An antenna that is erected about a metre above ground – if it is long enough – is called a Beverage antenna. Beverage antennas are excellent, low noise, receiving antennas. Common knowledge says you cannot transmit into a Beverage antenna. There is that “common knowledge” challenge again!

Semper Fidelis
The US Marines routinely used Beverage antennas for transmitting and receiving during the Vietnam War. They erected very long stretches of “commo wire” – about 4 or 5 wavelengths long, close to the ground. Those antennas were very inefficient and that was just perfect for the Marines! A Beverage antenna will radiate unidirectionally off the far end of the wire if it is terminated in a resistor of around 600 ohms. A resistor? More losses! The intent was to establish communications between a forward base and a command centre close by. If the range was too long the enemy might be able to intercept the signals. A Beverage antenna can be erected by soldiers crawling along the ground, protected from detection. Nowadays we might use VHF but, although they had that option in the 1970s the HF option was still implemented successfully.

Oh, Way Too Lossy!
About a decade after the Vietnam War ended, an amateur radio operator in the US started experimenting with the idea of using a long wire laid very close to, or directly on the ground. His name is Mike Toia and his callsign is K3MT. He invented an antenna he called the “K3MT Grasswire Antenna”. Adherents of common knowledge will immediately dismiss the idea. “It is too lossy”, “all your signal will be absorbed by the ground” they will say. Yes that is partially true. A Grasswire is inefficient and it is lossy but it really does work anyway.

In 1988 K3MT visited Bermuda and set up a 204ft wire on the ground and was the subject of nightly pile-ups on 30 metres for several nights. Inspired by his story, I have experimented extensively with my own grasswire antennas with mixed results. My best result was with a 148 feet long antenna laid on the ground. I operated with 5 watts, CW and received a 559 RST report from a station 25km away. On other occasions I have received reports from the Weak Signal Propagation Reporter (WSPR) from hundreds of kilometres away using a grasswire (see image below). K3MT’s operation in Bermuda benefited from the “salt water effect” so he was able to enjoy DX contacts.

Here’s How It Works
Laying a long wire – at least one wavelength long – directly on the ground has some very surprising results. First of all, yes it is lossy but it is least lossy for low angle propagation off its far end. I modelled the idea using EZNEC and came up with a 12dB loss for a grasswire. So let’s analyze that for a moment. If my radio puts out 100 watts, the effective radiated power will be just over 6 watts. My 6 watt signal will be radiated at a low angle with a fairly narrow beamwidth. Now compare that with a quarter wave vertical antenna. A vertical radiates equally in every direction so a 100 watt input signal results in an energy density that is spread all around the compass. Assuming unity gain for a vertical we can calculate the effective radiated power for a beamwidth of say 20 degrees is actually about the same as the grasswire!

Noise? What Noise?
An efficient vertical antenna needs a lot of radials or it too will be very lossy. So an efficient vertical antenna is not usually the kind of antenna that lends itself to rapid deployment. A grasswire, on the other hand, can be rolled out along a trail, or a field in a couple of minutes. Vertical antennas pick up noise very easily, since most man-made noise is vertically polarized. A grasswire is essentially noise-free.

A Scotsman’s Dazzling Revelation
If you stand at the side of a lake as dusk draws near you will begin to see a reflection of the Sun’s rays on the surface of the water. As the Sun begins to set, the reflection will become very strong. The angle of incidence at which this happens is called “Brewster’s angle” after the Scottish scientist who first documented the phenomenon. There is a strong parallel with the way in which a grasswire antenna works. An electromagnetic wave encountering the interface between two different media will be totally reflected at Brewster’s angle.

On Reflection
What causes the reflection? When a radio signal is sent along a wire on the ground a reflection in the ground is generated. The reflected wave in the ground travels more slowly than the incident signal so that it absorbs the signal at some points along the wire, but at other points the antenna acts as if the ground isn’t even there.

At Great Length
How long does a grasswire have to be? At least one wavelength is the rule. Even longer wires do not really help because the resistive losses outweigh the advantages. And forget about resonant length considerations; the wire is detuned by the ground and becomes a random wire. It should be matched with a 4:1 or 9:1 balun and a tuner to bring it into resonance at the radio.

Set your preconceived ideas aside and give it a try – it works and that is what you might call “uncommon knowledge”.

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