Practical_Antenna_Handbook_0071639586

C h a p t e r 2 4 : a n t e n n a T u n e r s ( A T U s ) 539
C 1 C 2
C 1A
C 2
L
R 2
R C 1 1B
R L
1 R 2
Figure 24.4 Split-Âcapacitor transmatch network.
Figure 24.5 Tee-Ânetwork transmatch.
The long-Âpopular E. F. Johnson Matchbox is a fundamentally balanced variant of this
circuit, designed primarily for use with balanced feedlines such as open-Âwire line, twin-Â
lead, et al. In the Matchbox, coupling to the transmitter or receiver is accomplished via
an inductive link (an isolated secondary winding on L), thus eliminating the connection
of R 1 across half of C 1 . C 2 becomes a split capacitor connected across the full length of
tapped inductor L, and the junctions of the two halves of C 1 and C 2 are tied together and
grounded to the chassis. Each half of the new C 2 is itself split again, and the two sides of
the balanced transmission line are connected at the midpoint of the split-Âcapacitor half
of C 2 on each side of ground. The center conductor of an SO-Â239 chassis-Âmounted coaxial
receptacle for feeding unbalanced transmission lines is hard-Âwired to the midpoint
of one side of C 2 .
Perhaps the most common form of transmatch circuit is the tee network shown in
Fig. 24.5. Like the reverse L-Ânetwork of Fig. 24.1B, it is basically a high-Âpass filter and thus
does nothing for transmitter harmonic attenuation.
An alternative network, called the SPC transmatch, is shown in Fig. 24.6. This version
of the circuit offers some harmonic attenuation.
Lumped-Âcomponent matching networks (or ATUs) for MF and HF are relatively
easy to build, although the cost of inductors and variable capacitors capable of withstanding
the voltages and currents associated with transmitter power levels is eye-Â
opening. (Even when purchased used at flea markets or via the Internet, good
high-Âpower components are not cheap!) Probably the most important thing to remember
when building your own ATU is to leave lots of space between any metallic enclosure
and the internal components. Failure to do so can
alter the circuit Q and reduce overall efficiency of the unit.
In the early days of radio, very few home-Âbuilt transmitters
or ATUs were shielded. The drive to shield RF-Â
C
C 2A
1
generating circuits gained strength in the 1940s with the
spread of television broadcasting and the concomitant potential
for interference to the neighbors’ TV reception.
Today, all commercially produced transmitters, transceivers,
and amplifiers are thoroughly shielded and R L
1
C 2B R 2
filtered
Figure 24.6 Improved transmatch offers harmonic attenuation.