Antenna Tuner Model 238B

Note: Model 238B is the same as Model 238A but with a black case.



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 238B 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 238B 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.

Here are the tuner configurations possible:


  • CIRCUIT TYPE: 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
  • INDUCTOR: 18 uh silver plated roller inductor
  • SIZE: HWD (in inches) 5.5 x 13 x 11 (14x33x28 cm)
  • COLOR: Black
  • WEIGHT: 10 lbs (4.53 kg.)
  • PRICE: $ 595.00


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Additional Information

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