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Thread: 'Grasswire' - Anyone Modelled It?

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  1. #1

    Default 'Grasswire' - Anyone Modelled It?

    Having taken to portable operations recently, the vast expanses of open ground ahead of me yields thoughts of all sorts of simple antennae that could be thrown out, especially as 10m-high fishing poles are sometimes out of the question due to high wind, and are always something of a pain in the proverbial to mess about with!

    I wonder if anyone has - or could - model the radiation patterns of a grasswire? I'm curious as to whether there is any usable DX pattern from such a wire? I'm rather useless at modelling software, so better to ask those who are not! Given that most portable operations, especially up a rocky mountain, probably can't get anywhere near an ideal ground, the model would be most useful by incorporating a counterpoise (shown at right angles to the radiator in the web-based information.)

    Thanks!

  2. #2

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    I have a friend who lives on pretty poor ground and he has experimented a fair bit with dipoles just lying on the ground. They will work to some extent, of course. Without belaboring the obvious, with 160 and 80 meters, it's probably a decent NVIS antenna, and it's effectiveness on higher frequencies is going to depend on the real ground quality. Are there any condtions in which this would be superior to an elevated antenna? Doubtful unless your intention is only local NVIS comm, in which case you might be happy.
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  3. #3
    Join Date
    Aug 2001
    Location
    College Park, MD
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    429

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    Quote Originally Posted by MW1CFN View Post
    I wonder if anyone has - or could - model the radiation patterns of a grasswire? I'm curious as to whether there is any usable DX pattern from such a wire?
    I can't see K3MT's page right now so I don't know the dimensions of the original but I did a 200 foot wire with a couple of 10 foot counterpoise wires on 15m.

    The pattern is fine... here's a comparison of the pattern shape of a 200 foot wire an inch from ground (with conductivity 5mS/m and dielectric constant 13) on the 15m band compared to a 1/4 wave vertical:

    http://n3ox.net/files/grasswire_directivity.png

    But the problem is the gain. In order to make a good comparison of the pattern shape above, I had to add 370 ohms of loss resistance to the vertical to equalize the gain. The grasswire is predicted to be extremely inefficient (not a surprise perhaps ). Here's a comparison without the intentional huge added loss resistance in the vertical:

    http://n3ox.net/files/grasswire_gain.png

    These would be better done with NEC-4 validated with actual measurements, so who knows if it will be 8dB or 10dB or 20dB or whatever worse than a vertical but it's going to be pretty bad. The grasswire is a terrible transmitting antenna with probably about 10dB of loss. But it's completely invisible on flat ground and can handle very high power so I'm sure its a fine for many uses. Probably pretty nice to listen on because it's got a pretty good beam pattern with nice front to back and narrow beamwidth.... for reception, provided the natural noise floor can be heard, a long grasswire is like using a yagi. Actually it's more like using a self-terminating Beverage because that's what it is... a nearly pure traveling-wave antenna.

    But it's lossy enough that I think it's probably a no-go for QRP output. If you're in a vacation house running 500W and you can still radiate 25W or 50W in the desired direction, you'll make plenty of contacts. If you reduce your 5W to 0.25W or 0.5W that might not work as well.


    73,
    Dan

  4. #4
    Join Date
    Jan 2002
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    harms way
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    In some cases lrrp or other sf units bury their hf ants and feeds a few inches or less under soil rather than have them laying on top. This is due the need for concealment more often than sheer necessity. Some hf ants have been developed specifically for this use with reduced loss at the price of relatively huge elements to be buried. But a wire laying on the ground sure is quiet, reliable, cheap, and directional too if long enough.

  5. #5
    Join Date
    Jan 2008
    Location
    St. Mary's County Md since 2000
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    I read the the Canadian Forces SIGINT station at Alert in the high Arctic had an HF antenna failure once during the winter and lost communications with Ottawa. The hams on staff rigged a ground Beverage and established comms. There being no trees, and no digging of holes it was the only option!
    "RF gotta go somewhere!"

  6. #6

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    My club uses a hydrogen filled weather balloon for some of our Field Day activities to hoist a wire for 80 meters. One year, the balloon popped sometime after sunset and the crew on the ground didn't realize it was gone. The 80 meter wire was lying on the ground and the op at that station remarked that the noise on the band had become markedly quieter, but the contacts kept coming. It's likely that they were mostly NVIS, but there were still contacts to be made.

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