SWR & Power Meter

The circuit uses a current transformer in which the low resistance of the secondary is split into two equal parts. The centre connection is taken to the voltage sampling network so that the sum and difference voltages are available at the ends of the transformer secondary windings. The layout of the sampling circuit is important: the input and output sockets should be only a few inches apart and connected together with a short length of coaxial cable, as shown in the picture.  The coaxial outer must be earthed at one end only so that it acts as an electrostatic screen between the primary and secondary windings of the torroidal transformer.  The primary of the torroidal transformer is formed by simply threading a ferrite ring onto the coaxial cable.  12 turns of 24 swg enamelled copper wire equally spaced around the entire circumference of the ring form the secondary winding.  The ferrite material should maintain a high permeability throughout the frequency range up to 70Mhz; a suitable ferrite ring is the MullardFX1596.  Other components in the sampling circuit should have the shortest possible leads.  R1 and R2 should be non inductive types. For powers above about 100W   R1 should be several 2W carbon resistors in parallel.  R2 should be 150 ohms for 75 ohms systems and 220 ohms for 50 ohm systems.  RV1 should be a miniature skeleton preset soldered directly across R2 to keep any stray reactance to a minimum.  The detector diodes D1 and D2 should be matched point contact types with a PIV rating of about 50v, OA79 and OA91 are suitable.  The 27 ohm 2W current transformer resistors should be matched to 5%. The ratio of the sampling resistors R1 and R2 is determined by the sensitivity of the current sensing circuit. The two sampling voltages must be equal in magnitude under matched conditions, and RV1 provides a fine adjustment of the ratio.  The two meters are moving coil 50µA FSD and RV2 and RV3 are miniature skeleton presets soldered directly to their respective meters.

SWR/Power Meter Calibration

Accurate calibration requires an RF voltmeter and a tapped dummy load, but adequate calibration can be achieved with the transmitter's built-in power meter.  Connect the transmitter to the connector at the opposite end of the coaxial line from the voltage sampling network, and connect a dummy load of the correct impedance to the other connector.  Switch on the transmitter and adjust it's power output to 100 watts, (or until the voltage across the dummy load equates to 100 watts).  Adjust RV1 for minimum reflected power indication.  Calibrate the forward power meter first by adjusting RV2 to give full scale deflection on the forward power meter with the transmitter power output set to 100 watts.  Then reduce the transmitter power output in 20 watt steps, marking the forward power meter scale accordingly.  The forward power meter scale can then be marked permanently in watts.  To calibrate the reflected power meter for direct SWR reading, switch off the transmitter and reverse the transmitter and dummy load connections, ie, connect the transmitter to the sampling end and the dummy load to the other end. Switch the transmitter back on, set it's power output to 100 watts and adjust RV3 for full scale deflection on the reflected power meter.  SWRs of 1.5, 2 and 3 correspond to reflected powers of 4%, 11% and 25% respectively, which for a forward power of 100 watts correspond to 4 watts, 11 watts and 25 respectively. Therefore, to calibrate the reflected power meter directly in SWR simply reduce the transmitter power to 25 watts and mark the reflected power scale for a SWR of 3, reduce the transmitter power to 11 watts and mark the scale for a SWR of 2, reduce the power to 4 watts and mark the scale for a SWR of 1.5.  For completeness, the meter pointer can be marked for a SWR of 1, (ie no reflected power) at its rest position and }  at FSD for a SWR of disaster.  Calibration is now complete and the meter will read simultaneous direct readings of forward power and associated SWR.