Accessed on 25 July 2017, 20:00 hrs, UTC.
Author: Mike Higgins (K6AER).
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Most radio amateurs are acquainted with the J-Pole antenna, which finds a ready application for our shared VHF/UHF bands. The antenna is simple, easy to make with commonly available materials, and provides a modest amount of gain.
In this post, Mike Higgins (K6AER) describes the HF version of the J-Pole antenna scaled for both 20 and 40 meters.
He briefly describes how the antenna is designed and built:
"There is nothing special about a J pole, it is an end fed ½ wavelength vertical radiator that needs no radials. Just like its 2-meter brother but on 20 meters it is ten times larger. The bottom ¼ matching section is fed with 50-ohm coax. Bottom of the antenna is shorted and the coax attaches at the 50 ohm point in the matching transformer. The higher the attach point attachment, the higher the feed impedance. The antenna is shorted at DC and thus, P static is never an issue. I ran a nylon line over the top of the tree and the antenna is held vertically by the tree limbs. Leaves seem to have no effect on the radiation at HF. The long side is ¾ wavelength and the matching side is ¼ wavelengths
The 6 inch spreaders are made from ½ inch PVC and spaced about every 2 feet for the parallel matching section. Drill the spreader and feed the wire through the holes on each end. Spreaders are held in place on the wire with good old vinyl electrical tape. The top of the matching section is tied to the radiator with parachute cord to hold up the matching section. Fifty-ohm coax is attached about 8 inches from the bottom with the center conductor connected to the long side and the shield attached to the short matching side. 2:1 VSWR is about 4 % of the Frequency. Also, the antenna can be grounded at the center of the shorting section for a lightning ground.
To tune the antenna just move the attach point up and down for lowest VSWR. Match should be 1.0:1 with ease. I used THHN 14 gage wire to build the antenna. I don’t have $10 into the whole project. The antenna will handle 2KW easily. Mind you if you are running power the voltage at the top will be very high. Try to keep the last 10% in the clear and away from anything conductive."
Mike provides tuning instructions, dimensions, and a diagram for this antenna. I've built HF J-Pole antennas for 15 meters and 10 meters, and they work very well for DX contacts.
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https://paper.li/f-1476233615 (Hawaii Science Digest).
https://hawaiiintelligencedigest.com (trends in geopolitical intelligence and cybersecurity).
https://paper.li/f-1482109921 (My Daily Intelligence Briefing).
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Aloha es 73 de Russ (KH6JRM)