Since the opening of the 160-meter band to amateur use, the 1,8-band-selector position on my FT*102 has been gathering dust. Like a number of hams, I simply do nol have space to put up a 252-foot-plus wire antenna for 160 meters. Even if space was available I doubt if I would consider a wire antenna because of the difficulty I had in getting permission to put up my 5-band vertical at my apartment. It would be nice if there was a zero-space antenna for the 160-meter band!
Baltimore, Maryland, is approximately 500 miles from my QTH, and my contact with WB3GBF on 40 meters opened my eyes to the signal power of the Bilal Isotron antenna. My RST using a 5-band vertical was 5-9 while WB3GBFs RSTn using an Isotron 40-meter antenna, was5-20 dB over 9. And to top this, he was using it inside his apartment.
As an ARRL Assistant Technical coordinator for North Carolina, N4BTK asked me to help him with his 20-meter-antenna problems in a local Senior Citizen retirement apartment complex. John lives on the 12th floor and had been using a coil-loaded window antenna but for some reason, 20 meters, even at the altitude of a 12-story building, was very weak. ] suggested an Isotron 20-meter antenna as a better solution since the management would not permit a vertical or beam to be mounted on the roof. John's Jsotron 40 was mounted 3 feet from the apartment s win-dowsiil on an offset 1 v-mast sec-lion. By comparison, it out-performs my 5-band vertical on 20 meters.
Impressed with the two-band performance of the Isotron anten-nasr i contacted Ralph Bilai WDOEJA about his 160-meter an-lenna and proposed that I work wrth it to obtain details for an article. My thought was that perhaps only an Isotron could give me 160-meter operation in my apartment complex, and this was an opportunity for me to see what it could do.
The Isotron antennas come in a kit form thai can be fully assembled in less than an hour using only a screwdriver and 7/16" socket wrench (Photo A). They are definitely unique in that no frequency model occupies more than an area of 1.5' x 2' x 2.5 \ ana most frequencies even less Technically speaking, they defy the accepted antenna theorem of voltage and current distribution on a dipole antenna. One cannot help being impressed when seeing an Isotron in operation.
Assembly begins by attaching a formed aluminum píate to the insulated top support member, tn my assembly of the 160-meter antenna. I first installed the mast clamp, U botts in the end of the insulated bar and attached it to a mast section. lh\s method supports the insulated bar while you attach the formed metal plates,
I followed the same method to mount the square metal lower support section to the mast for attaching the lower formed metal plate. Since the top and bottom plates are aluminum and will quickly oxidize, I spread a thin film of No-Ox over the surfaces that would overlap when bolted to the support bar. No-Ox is a common non-oxidiz ing compound used to prevent aluminum oxidization of electrical wiring and is usually available at most electrical supply houses. After attaching the top and bottom formed plates, my antenna looked like bird wings, one above the other (Photo B).
The rather large antenna coil is attached between the top and bottom support bars by adjustable bolts inserted in the top and bottom sections of the coil. I inserted both bolts and loosely positioned their lock nuts so that final centering of the coil could be accomplished with minimum effort. I attached the top of the coil to the top insulated support bar first and ran the coil's bottom bolt through the bottom support without attaching the securing nut. This was to permit attachment of the two plastic side panels without first adjusting the position of the lower support bar attached to the assembly mast.
At this point of the antenna's assembly, only the 1/4-20 bolts holding the formed metal plates to their respective support bars had been tightened securely.
There are two transparent plastic side panels to be attached to the top and bottom formed plates. One plastic plate contains a terminal. After loosely attaching the top ends of the two side plastic plates to the top plates, loosen the bottom support bar mast clamp and gently raise or lower the bar until the side plate holes line up with the bottom plate's holes. Insert the 1/4-20 bolts. At this point, ail 1/4-20 bolts joining metal to metal may be tightened securely
Those 1/4-20 bolts connecting the side plastic plates to the metal of the top and bottom plates should be tightened with tess force than the metai-to*metal bolts to prevent shattering of the plastic. These botts may be finger-tightened if used in connection with locktite or other thread-locking compounds.
Centering the coi! between the top and bottom plates is not difficult. Merely measure the distance between the bottom and top ends of the coil to their respective metal plates—not the support bars, There should be some tension put on the top and bottom formed plates by the coil's bolts that will pull the formed plates in slightly. This increases the rigidity of the plates to eliminate wind flutter and subsequent detuning of the antenna. (See Photo C.)
Pre-sofdered wires with closed terminal ends connect the PL-259 antenna cable connector to the
coil's top-end winding and the top formed plates- A second closed terminal from the PL-259 connector connects to the coil's bottomend winding and to the bottom formed plates (see Photo D), The resulting electrical circuit is a simple LC series circuit with the top and bottom plates forming the plates of a capacitor connected in series to the coil.
The final assembly consists of two threaded L-shaped rods on which two small rectangular aluminum plates are attached (Photo
B), It would be advisable to round all edges of these plates with a file, including each corner, before attaching to the bolt. This will reduce rf-energy discharge into the atmosphere,
The actual antenna assembly is basically an easy push-in-a-boll-attach-a-nut operation At worst, it takes no more than an hour The hard part comes in the final tuning of the antenna. This is done by moving the tuning plates on the L-bolis until the lowesi swr raiio has been reached.
Photo C Side view, The plastic holds the upper and lower wings together under slight tension,
This will be a trial-and-error approach where you select your transmit frequency, and, ai the transmitters lowest power level, tune the finals to resonance and note the swr indication. Now turn off the transmitter and make a very slight change in the positions of both tuning plates. Turn on the transmitter and recheck your swr. This procedure is repeated until the swr lowers to between 1:1.5 and 1:2.0,
The Isotron 160-meter antenna produces such a strong rt field at 100-Watts input that it is unsafe for indoor use. It is best used outdoors at a height above ground where no one walking by could possibly reach it and suffer rf burns or shock,
The advantages of the Isotron antennas are small physical size, no ground-plane radials required, and minimal parts, For trouble-free operation. Its radiation patterns
are typical of ground-mounted vertical antennas when il is connected to ground by an Moot-cop-perclad ground rod. The only disadvantages are tuning It and a somewhat narrow bandwidth.
I believe that a remote tuning system, using plastic components and nylon or monofilament control lines, could be added to the tuning L-bolts that would enable the antenna to be remotely tuned under power to eliminate the present difficulties when frequencies outside of the antenna's bandwidth are wanted.
I visualize connecting the two L-bolts together with a small plastic rod which could be moved forward or backward by the movement of the line in the manner that a pointer on a radio-tuning dial is moved by the dial cord. Another approach wouid be to use two control lines so that each tuning plate's bolt could be moved independently by a dual-cord system.
I do not know if atmospherics or lack of 1.8 MHz activity in my area was the reason I made no t. & M Hz contacts in the period I had access to the antenna. I do know that it boosted the background noise level from zilch to S-9 on my FT-102.
James A. Grupton, Jr. KF4PW
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