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Saturday, October 12, 2013

The K3MT "Grasswire Antenna". Post #233

How would you like to use what I call the "ultimate stealth antenna" at your deed-restricted location?  While I was doing some research for antennas at my new home location in the Puna District of Hawaii Island, I came across a fascinating article by Mike Toia (K3MT).  Over 20 years ago, Mike designed, built, and used something called the "grasswire antenna" which would serve as a  nearly invisible, yet effective antenna while he traveled from his various jobs.  Although the antenna appears to be a lossy dummy load on steroids, the data revealed by Mike show that this unusual antenna does work--not as well a dipole or a yagi, but it does get contacts in difficult operating situations.

The antenna is ridiculously simple to build and use.  Mike says his design "is an end-fed, longwire antenna that is laid right in the grass."  This would be perfect for those amateurs that have restrictive HOAs and CC&Rs prohibiting outdoor antennas.

Mike's original design used 204-feet (62.19 meters) of #12-#26 AWG magnet wire, a simple trifilar balun, and a counterpoise (laid on the ground) or a ground rod.  He has since shortened his "grasswire"  to 85-feet (25.91 meters), still retaining the balun and the counterpoise wire.  Mike says the "grasswire" is highly resistive with most of the radiation coming from the long end of the wire.  The antenna is vertically polarized.

Although the wire is quite lossy, Mike's data indicate that that the vertically polorized radiation has a 15 to 20 degree takeoff angle, which can be useful for DX.  Mike has also published a part of his operating log showing that the  antenna does do well for DX.

With all of this in mind, I decided to build a copy of the "grasswire" antenna just to see if it works.

I would build and use the antenna at my future home site, along side of the vertical, loop, inverted vee, and flat top horizontal 1/2 wavelength dipole already in use.  One can't have too many antennas, especially when my lot is off the main highway and out of sight of most neighbors.

MATERIALS:

One homebrew trifilar balun to match the "grasswire" with the RG-8X coax feed line.  I used the illustrations in Mike's original article, including the T-200-2 Core material for the balun form.

Eighty-five feet (25.91 meters) of #22 AWG hookup wire I had in stock.  This would be the radiating "grasswire".

Eighty-five feet (25.91 meters) of #22 AWG hookup wire for the "counterpoise".

SO-239 Coaxial connectors to join the balun to the RG-8X coaxial cable and to connect the balun to the radiating element and counterpoise.



Drake MN-4 antenna transmatch.

Several 3-foot (0.91 meters) pieces of RG-8X to connect the antenna transmatch to the low-pass filter, dummy load, and the Swan 100 MX transceiver.

Fifty-feet (15.24 meters) of RG-8X coaxial cable with UHF connectors.  This would serve as the antenna feed line.

ASSEMBLY:

The hardest part was winding the trifilar balun.  I was a little "rusty" in this area.  Once the balun was made, I connected all the other pieces.  Mike has a good illustration in his article...follow this and you won't make mistakes.

I laid out the main antenna element and its counterpoise in straight lines from the balun.  The counterpoise ran at a 90 degree angle from the "grasswire" antenna element.  Both antenna elements were laid on the freshly mown grass in back of the house.  The "grasswire" antenna was oriented to the north-north-east, with a lobe hopefully falling along the west coast of the United States.

I ran the coaxial feed line to the Drake MN-4 and connected the rest of the equipment in the shack with short patch cables.

RESULTS:

The antenna works very well on 40 and 15 meters running approximately 25 watts from the old Swan MX-100.  With a little nudging from the Drake MN-4, 20 and 10 meters tuned up nicely.  I was able to get SWR readings from 1.3 to 1.9 to 1 on these bands.  Reception reports varied from 53 to 57 for ssb and 549 to 579 for cw.  At my location in the Puna District, I had the most satisfactory results on 40 meters (evenings/early mornings) and on 20 and 15 meters (late morning to late afternoon).  Local state contacts were weak--something predicted by Mike Toia when he designed the antenna.

All told, this isn't a bad antenna.  It won't bust pileups and outperform a four-element monobander on a tower.  But the "grasswire" does work, despite its limitations.  The "grasswire" would make a nice portable or emergency antenna.  Those living in restricted operating areas might want to consider this unusual antenna.  It's almost invisible and blends in well with your lawn and garden.  This may be another case of "out of sight, out of mind."  Good luck!

REFERENCES:

f5ad.free.fr/Liens_copes-Ant/G/K3MT%20Antenna%/20gazon.htm.

http://www.rsgbshop.org/acatalog/PDF/StealthAntennas_Sample.pdf.

http://www.radiosurvivalist.com/antennas/Stealth-hidden-camouflage.asp.

Thanks for joining us today!

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Aloha es 73 de Russ (KH6JRM).

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.


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