Sunday, June 30, 2013

Antenna Topics: A Junk Box Vertical. Post #206

I've been a "radio junkie" since age 8 when my father and I built a crystal set in the garage.  I was amazed what a Quaker Oats cardboard cylinder, some enameled wire, a crude slider, a piece of pyrite, a "cat's whisker", and a pair of high impedance headphones could do.  With about a hundred feet (30.48 meters) of #14 AWG house wire and a good ground, I was able to capture many AM broadcast stations, many all at once.  The pyrite crystal wasn't too selective, but I did have fun.

From there I graduated to better receivers and some homebuilt Heathkit audio equipment.  Although my father was a "stereophile" and appreciated good jazz and classical music, he never did become an amateur radio operator.  I kept putting off getting my amateur radio license until I left the Air Force and became part of a commercial broadcast station.  One of the engineers at KHLO-AM (Hilo, Hawaii) finally persuaded me to take the novice examination.  Once I got the license, it wasn't long until I became a radio "packrat", collecting all manner of equipment, parts, wire, microphones, and radio books.  That affliction has followed me for over 36 years as a "ham."

When I completed my last inventory of radio equipment, I found a large collection of mobile antenna coils, baluns, various lengths of wire, a box full of RG-8 coaxial cable, some ladder line, connectors, and insulators.  Next to the back wall was my first mobile antenna--a Hustler system consisting of the 54 inch (134.16 cm) mast, a bumper mount, shock-absorbing spring. loading coils for 40, 20, and 15 meters, and a 20-foot (6.09 meters) piece of RG-58 coax with "pigtails."

I put these antenna pieces in the house and began researching ways to convert a mobile HF antenna to fixed station use.  On Friday (28 June 2013) I found an interesting article by R. "Andy" Wiedeman (WA0AW) in the February 2012 issue of QST.  Andy describes how he used pvc mast sections, a small metal plate, a connector with a 3/8 X 24 thread, several Hustler coils, a "stinger", and 4 radials to build a portable antenna that could be used on his recreational vehicle (RV) or at his home qth.

With a few modifications, I was able to adapt my junk box parts to build a compact vertical in my back yard.  With apologies to WA0AW, here's how I used my old mobile antenna for a fixed station antenna.

I have two metal clothes line poles in my back yard, separated by about 16 feet (4.87 meters).  I bolted the bumper mount onto the cross arm of the clothes line pole furthest from my house.  The cross arm is approximately 8-feet ( 2.43 meters) above ground.

I attached the RG-58 coax with the "pigtails" to the mast and the bumper mount, with the center wire of the coax going to the Hustler mast and the braided shield going to four evenly spread out radials which left the the clothesline cross arm at approximately 45-degrees.  Since I was interested in pursuing 20 meter contacts with this modified antenna, I screwed in the 20 meter Hustler coil and its matching wire "stinger."  Each radial was cut according to the general formula 234/f (MHz)=L (ft), giving me an element length of 16.47 feet (5.02 meters).  Many antenna authorities recommend cutting the radials about 5% longer than the normal 1/4 wavelength for each dipole element.  I decided to stick with my original measurements.  Once I spread out the radials, I found that I had to run a portion of the radial length on the ground.

I used a barrel connector (UHF to UHF) to attach the RG-58 coax to 25 feet (7.62 meters) of RG-8 coax.  The RG-8X ran through a panel in the radio room window case and was connected to the Drake MN-4 antenna transmatch.  Small coax patch cords connected the low pass filter, dummy load, and the old Swan 100-MX transceiver to the transmatch.  All antenna connections were soldered and covered with clear fingernail polish.

After a few tests and adjustments, I found the mobile antenna conversion worked about as well as my low home station dipole.  The modified ground plane formed by the slightly elevated radial system performed well.  According to a recent article by Rudy Severns (N6LF), four elevated 1/4 wave radials "will perform about the same as many radials mounted on the ground and far better than using the RV chassis as a counterpoise.  Elevating a horizontal radial system results in an antenna impedance of about 30 ohms, resulting in poor match to a 50 ohm coax.  Sloping radials sloped at 30 to 50 degrees provide an improved match." Since my antenna was lower to the ground than the one described by WA0AW, I had a small mismatch to handle--easily done with the trusty Drake MN-4.

To change bands, I just screw on another loading coil and its "stinger" and add four sloping radials cut for my band of choice.

Since the antenna is mounted at a relatively low height in my backyard, it remains invisible from the street and  the prying eyes of neighbors.

I enjoyed making this antenna.  Other than my own labor and supply of  extra parts, the cost of the antenna was zero.  When I tire of this antenna, I will break it down, clean all the parts, and recycle the components for other antenna projects.

REFERENCES:

Wiederman, R. "Andy" (WA0AW).  "Double Your Mobile Antenna Use." QST, February 2012, pp. 41 to 43.
Steverns, Rudy (N6LF).  "An Experimental Look at Ground Systems for HF Verticals."  QST, March 2010.  pp. 30-33.
Robeson, R (K4YZ).   "One Ham's Fix for Limited Space Antennas."  QST, March 2011.  pp. 37-39.

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Aloha from the beautiful Hamakua Coast of Hawaii Island,

Russ (KH6JRM.

BK29jx15.