Section 1
;
Packet Radio “The Basics
”
A Packet Primer for the new Packeteer
__ Page 15
THE ANTENNA IS THE KEY "ELEMENT":
The antenna for digital as well as any other modes of communications is only as good as the transmission line that feeds the
antenna. Consult the handbooks and the catalogs for the latest and greatest coax or transmission feedline. Look at the
manufacturers printed specifications for a given type of feedline or coax. When the (VHF/UHF) coaxial cable runs are less than
100 feet, I spec for Belden 9913. When the VHF/UHF cable runs require MORE than 100 feet, I spec for Andrew LDF-4-50 or
larger “heliax.”™ hardline.
The main points of interest are the specs regarding the loss factor (expressed in DB) per hundred feet, the velocity factor, and the
frequency at which the measurement was taken. Over the long haul, the "hard- lines" or multiple shielded coax cables will prove to
be the better value.
The coaxial (coax) cable or the transmission line plays a major role in the antenna performance. The coax is a very vital part of the
overall antenna system, but the coax has a personality of its own and can reek havoc if it is not cut to or "tuned" for optimum
performance along with the antenna. It is even more important to say that antenna performance will depend on the behavior of the
transmission line at the time of antenna tuning or setup. In other words, if the coax is not prepared before the antenna is tuned,
then tuning of the antenna will not render optimum performance.
The coax is the "life-line" that delivers the energy to the antenna. since the energy is handled by the coax, this means that the coax
is either an external extension of the "tank circuit" or it is part of the antenna, but which is true.
This is not a trick question, but a way to make a statement that can be easily remembered. The antenna feed line is BOTH; Because
the complete antenna system is part of the tank circuit.
Wow! Now we are beginning to understand why the antenna should be tuned.
