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Author Topic: grass-level loop antenna for Field Day?  (Read 5200 times)
NX5MK
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« on: June 17, 2012, 07:20:47 PM »

K3MT'S Grasswire antenna idea transferred to a 80' (24.38m) horizontal loop?

I am thinking of trying out an 80 foot horizontal loop antenna at near grass-level on field day. Wish I had found some post on the Net of someone else trying it out, but found none.

Any suggestions/ feedback welcome!
vy 73 de Marcus KD0JKM
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W0BTU
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« Reply #1 on: June 17, 2012, 08:31:35 PM »

Where can we see K3MT'S "Grasswire antenna idea"?

I suggest that if you try this, you also have a dipole plugged into your antenna switch. But antennas on the ground have a reputation for hearing better (superior S/N ratio). You'll never know unless you try. :-)
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NX5MK
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« Reply #2 on: June 17, 2012, 09:04:56 PM »

Hi Mike,

I read about it here:
http://www.rsgbshop.org/acatalog/PDF/StealthAntennas_Sample.pdf

[Excerpt]
K3MT’S GRASSWIRE ANTENNA
When I first read Mike Toia’s, K3MT, piece on the Grasswire antenna I checked the calendar to see if it was an April fool joke – it wasn’t! I’ll let him take up the story. Mike says: “This antenna will not out-perform a Yagi, or a decent dipole up a half wavelength. Not in gain or signal strength, at least. But it will survive an ice storm, wind storm, and is practically immune to lightning. And it doesn’t need a large tower or tall support. I deploy one from my hip pocket at times - the balun to match it is larger than the antenna!
“Put simply, it is an end-fed, long-wire antenna that is laid right on the grass, hence the name. The original Grasswire used by me in the summer of 1988 was just 204ft of #18 AWG magnet wire laid along the property line, anywhere from 1in to 6in above the ground. Either a ground rod or optional
counterpoise wires are also needed - use one or the other as both are not needed.” See Fig 8.2.
These antennas are largely resistive, with values ranging from 150 to 500Ω or so on average ground. They have been used successfully on all sorts of soil. One was used with great success by K3MT/VP9 in Southampton, Bermuda - the object of nightly pile- ups on 30m CW for four nights.
Mike says that the sceptic in you will doubt that such low antennas can work. After all, its image in the ground radiates and cancels out all radiation. True - if the ground is perfect. But nothing is perfect! The Grasswire radiates vertically-polarised signals off the end of the wire. Extensive monitoring tests have demonstrated the end-fire nature of the antenna. Mike says that launching a ray at, say, 15 - 20° take-off angle can be useful and that’s what his Grasswire does. It is lossy in all directions, but least lossy when exciting the ionosphere for a long-haul DX contact. He says that signal reports are not fantastic, but contacts are made and ham radio is enjoyed! And the best part of this set-up is that his neighbours never knew that a ham station was on the air.
Mike feeds the antenna through a simple trifilar unun (an unbalanced to unbalanced impedance transformer), choosing the connections he wants for 2.5:1, 4:1 or 9:1 ratios (see Fig 8.3). 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
Fig 8.2: An antenna that literally lies on the ground? Mike, K3MT, says that it does radiate.
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.
Mike has also experimented with laying a Windom antenna on the ground. He says that it also becomes directional in the direction of the longest end. In both of these examples you will need to use an ATU.
[Excerpt End]

Thus I thought I'd put my 80' horizontal loop "at grass level" by "erecting" it on some "painter poles" just a foot off the ground with some "Caution" signs around it so that (hopefully!) nobody stumbles over it. Would give me the flexibility to not have to depend on a tree and be able to move to the side of everyone else. Don't know how well it will work, but the "Grasswire antenna" described on the 'net gave me the idea.

vy 73 de Marcus KD0JKM
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K4SAV
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« Reply #3 on: June 18, 2012, 08:11:10 AM »

What he built was a BOG (Beverage on the Ground).  Some people use these for receiving antennas because they are directional off the ends of the wire. The gain is very low, but if people can make contacts on a Heath Cantenna, they you sure should be able to make contacts on a BOG.

If you put your 80 meter loop 1 ft off the ground, you may need to shorten the loop.  It will be detuned low in frequency.  The gain should be 15 to 20 dB below a half wave dipole at 50 ft  (depending on elevation angle), but I'm sure you can make some contacts with it.

Jerry, K4SAV
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K4SAV
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« Reply #4 on: June 18, 2012, 08:31:13 AM »

Put your loop on the ground.  Then you will have a LOG.  You could even rewrite the POG song (pants on the ground). Cheesy
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W0BTU
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« Reply #5 on: June 18, 2012, 11:52:44 AM »

What he built was a BOG (Beverage on the Ground).  Some people use these for receiving antennas because they are directional off the ends of the wire.

Yup, that's a BOG. I've sent some matching transformers to a couple of hams who made BOGs 140' and shorter. They say it really improved their S/N ratio on some bands, even though it's hardly directional on the lower bands as short as it is. It also reduced RFI pickup from power line noise.
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NX5MK
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« Reply #6 on: June 22, 2012, 09:11:44 PM »

OK, so I tried it out today.

Report is here:
http://kd0jkm.blogspot.com/2012/06/k3mts-grasswire-antenna-idea.html

Wishing all a good Field Day!

vy 73 de Marcus KD0JKM
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W0BTU
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« Reply #7 on: June 23, 2012, 08:11:15 AM »

I've read your experience, Marcus. Thank you for sharing it.

Although it might be fun to try uncommon antennas and learn new things in the process, I think an antenna like that might be best tried where you can instantly compare it to higher antennas. Your transmitted signal is going to be significantly weaker on an antenna close to the earth compared to a much higher one. This is because a large portion of the energy radiated by that low loop is going to be absorbed by the earth and converted into heat, rather than being radiated into space where we need it.

However, I occasionally read reports where people use antennas like this for receiving in order to obtain an improved signal-to-noise ratio. They say they can often can hear signals on a low loop or a very short (in terms of wavelength) Beverage On the Ground. I believe them, but I can't fully explain why, since antennas like that shouldn't have much directivity or nulls in the pattern. Maybe their higher antennas were simply closer to sources of RFI.

At any rate, I think you'll make a lot more contacts with a center-fed inverted-vee (for example) 40' high. :-)
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K4SAV
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« Reply #8 on: June 24, 2012, 02:33:55 PM »

How did your antenna do at field day?
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WM9I
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« Reply #9 on: June 27, 2012, 11:39:04 AM »

Hi Marcus!

Tnx for QSO on 60 m.  How did the antenna do on FD?  I didn't get time to participate, unfortunately.

73,
Joe, WM9I
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NQ3X
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« Reply #10 on: June 27, 2012, 06:59:42 PM »

Seems to me it'd work fine, provided expectations were for NVIS operation.  Perfect for the low bands for Field Day.

http://www.athensarc.org/nvis.asp

One of the NVIS antennas I used in my Army days was a series of wires laid on the actual ground.  Worked like a champ, but not for DX.  Wink

73 de Bob NQ3X
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