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This Antenna Tuning Unit also incorporates a 50-ohm QRP dummy load, power
meter (1 or 10 Watts full scale), and SWR meter! I have used this ATU for a
variety of longwire antennas, including outdoor antennas of various lengths and
much shorter indoor (attic) antennas. I have used it on 80, 40, 30 and 20m. It
has always had no difficulty providing a good match to the 50-ohm transmitter
output.
1. R1, R2, and R3 are 50 ohm resistances, each one is made by connecting five
2-watt carbon film 10 ohm resistors in series.
2. R4, the dummy load is
a 50 ohm resistance made by connecting in series two 25 ohm resistances, each of
which is constructed out of four 2-watt carbon film 100 ohm resistors connected
in parallel. Therefore the dummy load consists of a total of eight 100 ohm
resistors capable of safely dissipating 16W.
3. 0.1
4.
The inductor L is wound from 0.6mm insulated hookup wire on a piece of plastic
pipe 35mm in diamater and approximately 90mm long. The Antenna side of the coil
consists of 48 turns of wire with taps every 4 turns, one of which is selected
by the switch S3. The TX winding consists of 16 turns of wire wound on top of
the "cold" end of this coil. Holes were drilled in the plastic pipe to secure
the ends of the windings, additionally a blob of epoxy glue was added. The coil
former is mounted on the base of the box using a nut and bolt at each end.
5. The meter used was a result of the dismantling of some piece of home
audio equipment.
6. S1 is a 2-way 2-pole switch, to select between
displaying forward (transmitted) power and reflected power.
7. S2
selects between the Antenna and Dummy loads.
8. S3 is a 12-way 1-pole
switch selecting the size of the inductance.
9. S4 selects the full
scale deflection range of the meter, between 1W and 10W. The resistors R5 and R6
have values which suit my meter, other meters could have a different full scale
deflection current which would require different values.
10. S5 is a
2-way 2-pole switch allowing the variable capacitor to be connected in series
with the inductance, or in parallel. With different antennas, one configuration
or the other can sometimes give better results.
11. The variable
capacitor is the result of the dismantling of some old radio. It has two sets of
vanes, which I connected in parallel. I don't know the exact maximum capacitance
but would estimate 350 pF. Because the capacitor can be connected in series or
parallel, its chassis cannot be connected to the metal box. Insulating washers
were used to mount the variable capacitor in the front panel.
12. On the
rear of the ATU, two TV-aerial type coaxial sockets are used to connect to the
transmitter and antenna.
13. The unit is enclosed in a metal box, which
is connected to the transmitter earth. The antenna earth is insulated from the
metal box. 