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Thursday, June 12, 2014

Simple Ham Radio Antennas. On the ground antennas. Post #280

Over the past few weeks, I've become fascinated with antennas mounted on or near the ground. I've even built a few, such as Mike Toia's (K3MT) "grasswire" antenna and a similar "earth" antenna from Australia.  A few months ago, I built a simple beverage antenna for my old Hallicrafters SX-62A receiver.  All or these wire antennas worked very well, especially for the AM broadcast band.

With a few modifications, these wires can be used as a separate receive antenna for amateur radio stations located in noisy or desert areas.  I've used vertical antennas for transmitting and low-mounted antennas for receiving.  

So, what are these ground hugging "long wires?"

According to articles published in the "NASWA Journal" for December 1992 and January 1993 by author Joseph Buch, these antennas mounted on, near, or slightly below ground level are called "wave" antennas because "they extract energy from the wave as it travels down the length of the wire and because its low height minimizes static induced by charged particles blowing in the air."  Buch adds that "Anyone who has tried to listen on a normal dipole antenna during a dry desert windstorm will appreciate this characteristic."  Experience gained from communications operations in Iraq and Afghanistan bears this out.

These antennas include one of the earliest and simplest of receiving antennas--the beverage--invented by a team led by Harold H. Beverage in 1922.  His experiments were republished in the January 1982 issue of "QST."  Basically, " The beverage antenna is a wire near the ground running in the direction of the desired station and terminated at the far end in a resistance to ground equal to the characteristic impedance of the line, about 500 ohms."  

In a letter to the December 1981 issue of "QST", Beverage said the antenna "should be no longer than one wavelength at the frequency of interest."  Experts can't agree on what length is best, because the length depends on local ground conditions.  According to Buch, "The bottom line is that one or two wavelengths is probably sufficient for good results."

In my experiments with beverage antennas, I was constrained by the size of my lot to a one wavelength piece of wire.

Is there a simpler approach to a "wave" antenna that doesn't involve very long lengths of wire?  Yes, there is.  It's something amateur radio operators call a "snake" antenna, a design that's been around since the late 1980s.  According to Joseph Buch, the snake consists of 50 ohm coaxial cable, approximately 100 ft/30.48 meters to 200 ft/60.97 meters long, which is laid along the ground or slightly above ground.  The coax cable center conductor is connected to the receiver antenna terminal.  The shield is left floating at the receiver.  The shield and center connector are shorted at the far end.

Last Sunday, 08 June 2014, I built one of these "snake" antennas and was suitably impressed with its receive capabilities, especially on the broadcast band, 160 meters, 80 meters, and 40 meters.  I had 100 ft/30.48 meters of RG-58 coaxial cable in the garage and used that cable to make the antenna.  Once I shorted the shield and center connector at the far end (soldered, of course, and protected from the elements with a tight fitting plastic bag) and connected the center connector to the receiver antenna terminal (leaving the shield to float) of the old Hallicrafters SX-62A, I was ready to go. The snake was rolled out through the garden and pointed towards Hilo (NE).  Stations that were once received with a strength of 54 to 55 were now 59+.  My new house is approximately 15 miles from Hilo, and AM reception is barely tolerable because of line noise and geography issues.

According to W0BTU, beverage and snake antennas are "broadbanded and do not require retuning to QSY because they are non-resonant."  I hooked up the snake to my Ten-Tec Argosy II transceiver and was pleasantly surprised by just how quiet the antenna was.  I did a few experiments using the Argosy II to transmit on my 40 meter vertical, while using an old Kenwood TS-520 to receive with the snake.  This was a great combination.  The snake works quite well on 80 and 40 meters and is often usable on 20 meters.

This combination will not outplay a tribander on a 50 foot/15.24 tower or a dipole supported atop a tall tree, but it does offer acceptable performance in areas of high noise or snoopy neighbors. My snake antenna can't be seen by neighbors, since it runs through the backyard garden into a grove of Norfolk Pine trees.  The vertical is suspended from a tree branch approximately 35 feet/10.67 meters above ground.  Four slightly sloping radial wires are attached to the base of the vertical.  The antenna is fed by 450 ohm ladder line to a 4:1 current balun, which in turn is connected to a Drake MN-4 antenna transmatch.  The antenna can be used from 40 through 10 meters.  If I need 80 meter capability, I switch to my 135 foot/41.15 meter doublet, also fed with 450 ohm ladder line.

If you want a general coverage receive antenna that can be used with a vertical transmitting antenna, you can't beat the "snake".  It's cheap, nearly invisible, and quiet.  This would make an excellent weekend project.

RESOURCES:

f5ad.free.fr/Liens_coupes_ANT/G/K3MT/Antennegazon.htm.

http://www.hard-core-dx.com/nordicdx/antenna/special/swlant.html.(this is a reprint of Joseph Buch's articles in the NASWA Journal, December 1992 and January 1993).

http://www.w0btu.com/Beverage_antenns.html.

http://www.youtube.com/watch?=GAxZs80Wyuw.

http://sa555.blogspot.com.

QST.  January 1982.

QST.  December 1981, letters section.

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Aloha es 73 de Russ (KH6JRM).

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