Converting ZetaGi HP201 for ham bands
This modification replaces the original CB SWR sensing circuit
Modification for the CB ZETAGI HP201 SWR Wattmeter (0-1000watts)
This modification replaces the original CB SWR sensing circuit with the Tandem Coupler based on the Sontheimer & Fredrick Directional Coupler – 1969 Patent The meter wiring is left connected to the FWD/REV switch and new forward and reverse voltage feeds are taken from the S&F coupler. The negative of the meter is disconnected from the earth connection and is reconnected via a new variable scaling resistor to earth.
This is because the S&F circuit is more sensitive and produces a greater output than the original CB SWR sensing circuit. The circuit uses two Murata cores 18mm OD11mm and ID 8 mm thick. These are wound with 15 turns of 0.5mm PVC insulated wire, but enamelled wire can be used if the cores don’t have any sharp edges. The cores were supplied by UK Surplus Components Supplier www.surplectronics.com.
N.B. Not all types of toroids work with this circuit. Some provide accurate results at 80m but produce ever increasing errors at 20m and above. The accuracy also depends on the 50ohm line being correctly terminated R1 // R2 and R3//R4 =50ohms if available a single carbon 50Ohm resitor can be used instead.
Using the correct iron core toroids and calibrating the meter on 20m, linear results of +/- 2% are obtained between 160-10m even with 5 watts of RF. The modified Zetagi meter unit has been tested with 500 watts from a linear amplifer into a dummy load.