Thursday, April 11, 2013

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Antenna Tuning Unit ATU For 27 MHz CB Radios

This antenna
tuning unit (ATU) enables half-wavelength or longer wire antennas to be
matched to the 50-? antenna input of 27-MHz Citizens’ Band (CB) rigs.
The ATU is useful in those cases where a wire antenna is less obtrusive
than a roof-mounted ‘vertical’ or ground-plane. It is also great for
‘improvised’ antennas used by active CB users on camping sites and the
like because it allows a length of wire to be used as a fairly
effective antenna hung between, say, a tree branch at one side and a
tent post, at the other. Obviously, the wire ends then have to be
isolated using, for example, short lengths of nylon wire. It is even
possible to use the ATU to tune a length of barbed wire to 27 MHz. The
coil in the circuit consists of 11 turns of silver-plated copper wire
with a diameter of about 1 mm (SWG20).

The internal diameter of
the coil is 15 mm, and it is stretched to a length of about 4 cm. The
tap for the antenna cable to the CB radio is made at about 2 turns from
the cold (ground) side. Two trimmer capacitors are available for tuning
the ATU. The smaller one, C1, for fine tuning, and the larger one, C2,
for coarse tuning. The trimmers are adjusted with the aid of an
in-line SWR (standing-wave ratio) meter which most CB enthusiasts will
have, or should be able to obtain on loan. Select channel 20 on the CB
rig and set C1 and C3 to mid-travel. Press the PTT button and adjust C2
for the best (that is, lowest) SWR reading. Next, alternately adjust
C3 and C2 until you get as close as possible to a 1:1 SWR reading.

Antenna Tuning Unit (ATU) For 27-MHz CB RadiosC1
may then be tweaked for an even better value. No need to re-adjust the
ATU until another antenna is used. In case the length of the wire
antenna is exactly 5.5 metres, then C3 is set to maximum capacitance.
Although the ATU is designed for half-wavelength or longer antennas, it
may also be used for physically shorter antennas. For example, if
antenna has a physical length of only 3 metres, then the remaining 2.5
metres has to be wound on a length of PVC tubing. This creates a
so-called BLC (base-loaded coil) electrically shortened antenna. In
practice, the added coil can be made somewhat shorter than the
theoretical value, so the actual length is best determined by trial and
error. Finally, the ATU has to be built in an all-metal case to prevent
unwanted radiation. The trimmers are than accessed through small
holes. The connection to the CB radio is best made using an SO239
(‘UHF’) or BNC style socket on the ATU box and a short 50-W coax cable
with matching plugs.

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