AMX TXC16 Plus Specifications download pdf (Page 51)

SG-2000 MANUAL 44

The SGC Building, 13737 S.E. 26th St. Bellevue, WA. 98005 USA

11.5 Antenna Couplers

If the transmitter and the feedline are both 50 ohms and the antenna is 100 ohms, a

mismatch will occur. A portion of the power coming from the transmitter which should

be radiated by the antenna will be "reflected" back toward the transmitter. As this

occurs, standing waves develop on the feedline. The ratio of the peak voltage going to

the antenna and the measured peak voltage reflected back to the transmitter by an

unmatched condition is called Voltage Standing Wave Ratio (VSWR).

When a VSWR (SWR for short) exceeds about 2 to 1, the HF transmitter will reduce

power to reduce stress on components. Reflected power has nowhere to go but to be

turned into heat. This heat can be dissipated by the coax feedline or it can be dissipated

by components in the output circuit of a transmitter. Either way, one of the rules of HF

equipment is “heat is bad!” So it is in SWR considerations.

You can reduce SWR either by changing the length of the antenna or by introducing a

matching network which will precisely match the conditions of the antenna to the

requirements of the feedline: an antenna coupler.

If the SWR presented by an antenna is less than 2— to —1, the addition of an antenna

coupler will make only a modest improvement in the transfer of energy. If the SWR

exceeds 2 to 1, however, the results can be dramatic from this one piece of equipment.

One of the problems with earlier coupler designs was that the coupler was either a

manual tuning device, such as a series of capacitors and roller inductors which had

knobs and cranks on them, or the job was handled by large servo-motors which read

voltages and made adjustments accordingly. But neither of these approaches was good

enough for the new technology which has become available and which includes the

next generation of adaptive HF radio controllers.

So SGC began to build computer controlled antenna couplers including the SG-230 and

SG-235 Smartuner™ couplers. SGC began building the servo-motor type automatic

couplers in 1975 so the transition to a computerized coupler with extremely fast retune

times was a logical development. The SGC Smartuner™ is widely acknowledged as the

finest antenna coupler available. We encourage you to use a Smartuner™ with the SG-

2000 because few antennas will provide an even load over the wide range of frequencies

covered by the SG-2000.

11.6 Mobile Antennas

The standard whip antenna used in the HF environment has a brief but interesting

history related to the Citizen's Band radio craze which swept the United States in the

1970's. The "generic" 108 inch long stainless steel whip, while it could be mounted in

such a way as to operate at 27 MHz without any coils, traps or matching network, it is

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AMX TXC16 Plus Specifications Page 51