SG-2000 MANUAL
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The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA
©1995, SGC, Inc.
TEL: (206) 746-6310 FAX: (206) 746-6384
At closer distances, inverted “L” wire antennas with a suitable high efficiency coupler
such as an SG-230 work very well. Dipoles, folded dipoles, and any other efficient 50
ohm antenna will work well at distances up to 1,000 Km.
Loop antennas are best for receiving because they are quieted by presenting a short
circuit to local noise pulses from industrial and commercial sources. But loop antennas
need to be very large at low frequencies to radiate efficiently.
The basic design rule for all fixed antennas is “If you have real estate — use it.” The
bigger the antenna — within reason — the more effective the antenna will be. Also,
except for NVIS applications, the higher the antenna, the better.
11.4
Antenna Feed Lines
You have to get power from the transmitter or transceiver to an antenna. To do this
you need a transmission line. A lot of “old timers” in HF believe to this day that high
impedance feedlines are best, but science doesn't agree with that point of view.
High impedance feedlines worked so well in the “old days” because tube type
transmitters had a final amplifier impedance of between 5,000 and 10,000 ohms.
Matching 5,000 ohms to a 500 ohm feedline meant a transformation of 10 to 1 — a
simple task for a transformer — which is what the output section of a transmitter is.
Changing the match to 50 ohm lines meant the matching transformer had to match 100
to 1 — a more difficult chore with the opportunity for a lot more loss if everything
wasn't properly adjusted.
With the advent of solid state radio equipment, the rules turned upside down. Now, a
final transistor LPA (Linear Power Amplifier) impedance runs in the neighborhood of 1
to 2 ohms. This means that a 500 ohm line would be a 1- to -500 or 1- to -250 match —
and not very good odds of getting things just right. The 50 ohm feedline represents a 1-
to -25 or a 1- to -50 match — which is easily and safely designed.
Given this little bit of history, if you do not have the luxury of having a transmitter right
at the antenna site — and this usually only happens in the case of AM broadcast band
Medium Wave transmitters — then the best way to get RF from point "A" to point "B" is
via 50 ohm impedance coaxial cable.