Quirks of the Fuchs B25 Antenna Tuning Unit

 As illustrated in this photo from the manual, the Fuchs B25 transceiver was designed to be operated manpack portable with a whip antenna.

To couple this whip antenna to the transceiver, an Antenna Tuning Unit (ATU) is employed. ATUs normally enable the use of a wider range of antennas, however the B25's ATU was designed for a specific purpose and is not so adaptable.

The ATU has two settings relevant to impedance matching:

• Whip antenna setting, for use with the whip antenna in manpack portable situations.
• Wire antenna setting, for use with a wire antenna in stationary situations.

Although I don't have this, web photos of the wire antenna for use with the B25 shows a frequency-length chart on the wire spool:

Digging into the schematics of the ATU shows:

• Only the variable inductor is in use during the "whip" setting.
• A 27pF capacitor is in series with the variable inductor during the "wire" setting.

This series LC arrangement isn't a matching network topology you find in a typical ATU.

Using the SimSmith software tool, this ATU transforming various impedances can be modelled.

I make the assumption for all of the models below: the transceiver's port impedance is 50 ohms resistive. This assumption is based on:

• 50 ohms being a common transceiver port impedance,
• The B25 ATU having a 50 ohm BNC connector for use when the ATU is bypassed.

Can it match a 50+j0 load? Yes

Can it match a 50+j100 load? Yes

Can it match a 50-j100 load? Yes

Can it match a 100+j0 load? No

Put simply: a series LC circuit cannot transform resistance. It can only transform reactance.

This means: the B25 ATU expects to see a load with 50 ohm resistance, and can only tune away reactance.