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The SGC Building, 13737 S.E. 26th St. Bellevue, Washington 98005 USA
P.O. Box 3526, 98009 Tel: (425) 746-6310 Fax: (425) 746-6384
© 1997 SGC Inc.
common antennas that can be fed with coaxial cable (and not require any matching devices) are those in the
dipole family.
With coaxial cable, the grounded shield prevents interference from entering the "hot" center conductor and it
keeps the transmitted signal from leaking out through the line before it can be radiated by the antenna. Coaxial cable
is an unbalanced line; the signals traveling down each side are not the same.
The antennas in the longwire group (for example) all require either twin lead, open wire, or ladder line
feedline. These types of feedlines are all types of balanced lines. Balanced lines are merely two (or sometimes four)
parallel conductors that are separated by a particular distance (usually anywhere from one inch to a few inches).
Balanced lines don't have any shielding because they don't need any; having the same signal running down each line
keeps the lines balanced and prevents them from radiating the signal. No transceivers have outputs for balanced
lines; if you want to use this type of transmission line, the most common solution is to run a short piece of coaxial
cable to an SG-230 Smartuner™, which will solve the problem.
Another solution is to use a balun in line with the antenna. A balun is a contraction of balanced-unbalanced.
The system features a transformer that matches the impedance of the 50-ohm output of the transceiver to the 300 or
600 ohm antenna, depending on the value of the balun that you choose. Most commercial baluns are small cans that
screw into the coaxial cable at the connection between the antenna elements. Not only do these baluns improve
performance, but they are constructed so that it is easier to build the antennas.
The most important characteristic to look for when purchasing balanced feedline (or an antenna that uses
unbalanced feedline) is strength and flexibility. If it looks like it will last in the elements, use it. Coaxial cable is a
much trickier purchase. Every component within the cable is essential to good operation. First, the center conductor
should be of a heavy enough gauge to handle transmitting powers (usually about #16 or thicker). Next, you should
make sure that the insulating dielectric is strong and won't break down in the presence of heat or water. The most
important option is to make sure that the shield is made from a solid copper braid that covers 95% (or better) of the
cable. Without this high-percentage, low-loss braid, some of your power will drain out through the coaxial cable.
The last part of the cable is the outer sleeve, which must be strong, flexible, heat-resistant, and resistant to ultraviolet
rays. In this era of increased environmental awareness, coaxial cable is one product that can't be biodegradable!
The safest way to choose good-quality coaxial cable is to pick a type that is made according to industrial or
military specifications.
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