Atlas 210x, Installation, Operation And Maintenance Manual

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2-28. FIXED STATION INSTALLATIONS
In fixed station installations, the use of the 220-CS eliminates the necessity for making D.C. power connections.
The only requirement is that the Atlas Transceiver be firmly seated in the console. When installing the transceiver in
the console, always make sure that the unit is pushed all the way into the console. This will insure that all power,
Mic, and speaker connections are firmly made.
2-29. ANTENNAS
2-30. MOBILE ANTENNAS. The mobile antenna generally requires more critical adjustment than the home station
antenna. This is because it operates over a more narrow bandwidth, and must therefore be adjusted very accurately
for resonance. Also, the base impedance is seldom very close to 52 ohms. With the tube type transmitters the Pi
matching network will adjust to fairly low impedances, but with a broadband solid state transmitter, such as is used
in the Atlas transceivers, a close impedance match is necessary in order to operate at full power. Various claims
about impedance are made by manufacturers of mobile antennas, but unfortunately our tests on all the most popular
brands indicate that your chances of coming up with a close match are less than 1 to 10. Average base impedance is
18 to 23 ohms. Therefore, some method of transforming the antenna base impedance to 52 ohms is required. (See
Section 5-3 for Model MT-1 Broadband Transformer.)
2-31.CAPACITYMATCHINGMETHOD.This is one method for impedance matching to the mobile antenna
which works quite well. A capacitor is connected from the antenna base to ground. This capcitor is part of an L
network which transforms the base impedance from a low value up to 52 ohms. The small amount of "1" required is
actually "borrowed" from the lower part of the loading coil. The capacity value must be determined experimentally,
and will vary from band to band, as well as from installation to installation.
On 75 meters, the capacity will generally need to be in the 1000 to 1500 picofarad range. On 40 meters, 300 to 400
picofarads and on 20 meters about 200 picofarads. A variable capacitor can be useful to determine what value is
required or a collection of silver mica capacitors, some 100 pf's, 200's, 470's, and a 1000 pf can be paralleled in
various combinations until the SWR comes down to a low figure. Once you know how much capacity your antenna
needs, it is best to make up the permanent capacitor by paralleling two or more silver micas. This will divide the
R.F. current and reduce the chances of overheating a single capacitor with too much current. Follow the procedure
described in paragraph 2-14 when tuning the antenna.
2-32. NOISE SUPPRESSION. The subject of noise suppressing automotive ignition and alternator noise is
beyond the scope of this manual, so it will only be mentioned briefly. Many cars will create very little
interference in the IIF bands covered by the Atlas transceiver. Almost all cars now use resistance type
ignition wire, and will probably create very little ignition noise. More likely the high pitched whine from the
alternator will cause more trouble. Refer to the various amateur radio handbooks available from your dealer
for information on noise suppression. It will usually be found in the mobile sections. Estes Engineering Co.,
930 Marine Dr., Port Angeles, Wash. 98362, manufactures an excellent line of suppression kits which can
help cure the more stubborn cases. It is quite likely that your dealer sells the Estes Engineering line also.
IMPORTANT: Make sure that the transceiver mounting brackets are well grounded to the transmission hump or
bulkhead.
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