The J-Pole is one of the most versatile antenna designs. They have been constructed out of everything
from wire to copper tubing. I have a beautiful J-pole built of 3/4 inch copper tubing.(Thanks to KG4SIX)
One thing that surprised me was that most hams don't recognize it's type.
The dimensions of a J-Pole can be calculated using the 468/Fmhz formula with the exceptions of the
separation between the main pole and stub and the 1/8 wave stub used between the upper and lower
sections of the Super J. These should be calculated using the free space formula of 492/Fmhz
.Maximum separation between the elements should not exceed 1/70 of a wavelength to prevent
unwanted radiation. The stub used between the upper and lower sections is a total of 1/4 wavelength.
(1/8 out and 1/8 back). The 1/4 wave stub used for the feed is normally 1/2 to 2/3 the diameter of the
main pole. The standard J-Pole antenna is a end fed 1/2 wavelength antenna. The 1/2 wavelength
section can be extended to 5/8 wavelength increasing the gain about 1db. The tuning procedure is the
same in either case. Starting at about 2 and 1/2 inches above the "U", simply slide the coax connection
up or down the stub until the lowest SWR is reached. You will get a more uniform radiation pattern if a
balun is used. BALUN
However, it is not a requirement with this antenna.
The Expanded Super J-Pole antenna provides about 4.5 dbd gain. It consist of an extended (5/8) lower
section and a 5/8 wavelength upper section. This provides a total of 1and1/4 wavelength, resulting in a
current feed antenna. To get back to a voltage feed we couple these 2 sections together using a 1/8 wave
stub.(1/8) out (1/8) back. this gives us 5/8 + 5/8 + 2/8 = 12/8 or 1 and 1/2 wavelength. (a voltage feed.)
The lower section was made from 3/4 and the upper section from 1/2 inch EMT .
These antennas can be built from EMT (electric conduit pipe) . The local "Home Improvement Store"
sells the 3/4 inch 10 feet long tubing for $2.60 and 1/2 inch 10 feet long $1.78. 3/4 inch PVC pipe was
used as the insulator between the upper and lower sections. Stainless Steel or Galvanized hardware is
recommended. PVC pipe deteriorates quickly when exposed to UV from the sun. A quick coat of
paint will extend it's life by years.
I've also built these antennas out of 10 gauge copper wire. Making a loop at the top and looping either
a nylon tie or fishing line to support it from the ceiling. Great emergency antenna.
MORE TO COME