Ten-Tec 238C manual
Part #74428
First release, June 2009. Printed in U.S.A.
CHAPTER 3
SPECIFICATIONS
Circuit Type: L network
RF Power Rating: 2000 watts
Frequency Range: 1.8-30 MHz continuous
Input Impedance: 50 ohms nominal
Output Matching Range: At least 10:1
SWR, any phase angle.
Input/Output Connectors: Input and four
antenna coax connectors are SO-239, UHF
type. Studs with wing nuts for single wire
and balanced feeders.
Capacitor Voltage Rating: 3500 volts
Inductor: 0.2-18 uH silver-plated roller
inductor.
Size: 5” H x 12.125” W x 13.5” D
(12.7 x 30.8 x 34.3 cm). Depth
measurement cabinet only, does not
account for knobs or connectors.
Weight: 12 lbs (5.45 kg).
TECHNICAL INFORMATION
The matching circuit used in the model 238C
is basically an “L” network. The “L” network
has several advantages over other circuit
configurations. It has only two adjustable
parts; one inductor and one capacitor; most
other networks use three. Because there
are no internal nodes in the network, the
maximum circuit voltages and currents
which occur are never more than those
present at the input or output terminals.
Because there are only two variable
components, there is only one setting of
each which will provide a perfect match to a
given load impedance, and this unique
setting automatically provides the lowest
network Q possible. Low Q means low
circulating currents, hence low loss, and it
also provides the widest frequency
bandwidth of operation before retuning is
necessary. Finally, since the inductor is
always series, the network always provides
a two-pole low pass response to provide
harmonic rejection.
There are, however, some disadvantages
which have prevented wider use of the “L”
network in the past. First, to match all
possible antenna loads, two configurations
are required. One, for impedances greater
than 50 ohms (i.e. the antenna has a fairly
low SWR already) the values of L and C in
the network required for a perfect match
become very small, smaller than the stray or
minimum values of the components used.
To circumvent this problem, a small fixed
compensating capacitor or inductor is placed
into the circuit depending upon whether the
network is configured for low or high
impedance respectively (HI Z or LO Z on the
center tuner switch). At low frequencies, the
value of network capacity needed to match
some loads is quite large, requiring a large
and expensive capacitor. To provide for
this, fixed capacitors are placed in parallel
with the variable capacitor to obtain the
value needed. This function is performed by
the center switch; further rotation from the
center position increases the value of
capacitance in the circuit.
There are 5 tuner configurations that are
possible depending on the position of the
center switch.
