WATT ROLLER INDUCTOR ANTENNA TUNER
Introducing the new model 238A 2000 watt antenna
tuner. Featuring the
same tried-and-true "L" network circuitry as the original
238 antenna tuner.
The new 238A features a rugged steel and aluminum enclosure and styling
designed to compliment any ham shack.
238A Antenna Tuner $445.00
Metering of SWR or forward power in switchable
200/2000 watt switchable scales. Four position antenna switch. All four
positions have SO-239 output connectors for 50 ohm feedlines or position
#4 can be used to feed balanced ladder line with standard 4:1 matching
balun internal to tuner. Heavy duty hand built 3.5 kv variable capacitor,
18 uh silver plated hand built roller inductor. 11 position center switch
to set tuner configuration (see circuit description). BYPASS mode operable
in all four antenna positions. Provision on rear panel for additional
fixed external capacitance if desired (capacitor included with tuner).
The matching circuit used in the model 238A 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. This unique setting automatically provides
the lowest Q network possible. Low Q means low circulating currents,
hence low loss, and it also provides the widest frequency bandwidth
of operation before retuning is necessary (very useful if you're moving
up and down the SSB portion of the band, for example). Finally, since
the inductor is always series, the network always provides a two-pole
lowpass response to provide harmonic rejection. There are reasons that
the "L" network has not enjoyed wider use in the past. To
match all possible antenna loads, two configurations are required.
The first, for
impedances greater than 50 ohms, requires the capacitor to be across
the antenna. The second requires the capacitor to be placed across the
transmitter when the antenna impedance is less than 50 ohms. In the
238A tuner, this function is performed by the 11 position switch in
place between the roller inductor and variable capacitor on the front
panel of the unit. At HF, as the load impedance approaches 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 on whether the network is configured for
high or low impedance respectively (HI or LO positions on the 11 position
switch). At low frequencies, the value of the 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. Further rotation
of the main switch from the center position increases the value of capacitance
in the circuit. There are five possible tuner configurations for L and
C that are switched in and out by the main switch.
Modified "L" Network
RF POWER RATING: 2000 watts
FREQUENCY RANGE: 1.8 to 30 MHz
OUTPUT MATCHING RANGE: At least 10:1 SWR,
any phase angle,
1.8 to 30 MHz. 3000 ohms maximum at full power
INPUT IMPEDANCE: 50 ohms, nominal
CAPACITOR VOLTAGE RATING: 3500 volts
INDUCTOR: 18 uh silver plated roller inductor
SIZE: HWD (in inches) 5.5 x 13 x 11 (14x33x28
WEIGHT: 10 lbs (4.53 kg.)
200 WATT "T"
NETWORK ANTENNA TUNER
The Ten-Tec model 291 is a "T" network tuner rated for 200
watts. Two series capacitors and a shunt inductor are used to transform
the complex impedance at the feedline of the antenna to 50 ohms for
the transmitter. No SWR or power metering is provided.
The two capacitors used in the 291 are 1000 volt 50-250 pF variables
connected in series with a 47 tap rotary inductor (0.5-24 uh) shunted
to ground. This tuner can be used with a single 50 ohm coax input and
output, or has a wing-nut lug for use with end-fed longwire antennas.
Extra capacitance can be added externally to the back panel for increasing
the matching range if desired.
RF POWER: 200 watts
INPUT IMPEDANCE: 50 to 75 ohms
OUTPUT IMPEDANCE: 10 to 350 ohms
REAR PANEL CONNECTIONS: Input, SO-239 Output,
SO-239 and long wire
terminal Grounding bolt
INDUCTOR: 47 tap, 18 gauge silver plated
wire on 2" diameter powdered iron toroid
CAPACITOR VOLTAGE RATING: 1 kv.
FREQUENCY RANGE: 1.8 to 30 MHz
SIZE: HWD (in inches) 3 x 7 x 5.5 (7.6x17.8x13.8
WEIGHT: 3.5 lbs (1.6 kg)
291 200 Watt Antenna Tuner $89.00