Z BOX |
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Bernie's Bench |
Z Box - A companion accessory for the micro watt meter. It transforms the 50 ohm load of the watt meter to 200 or 800 ohms. It can also be used in reverse to provide low impedance loads of 12.5 ohm and 3.12 ohm. This is handy when checking oscillator, driver, or amplifier stages in a transmitter that is not 50 ohms output. Oscillators and low power drivers can be measured with an 800 ohm load and drivers in the 100 mw can use the 200 ohm load. Or an output from an amplifier could be in the 12.5 range. |
| CONSTRUCTION: This device is made from 2 each broad band, 3-30 Mhz, 4 to 1, toroid transformers, wired in cascade. (see diagram) | |
| I used FT 50-43. (Ferrite, toroid, .50 inch diameter, #43 mix.) Winding the transformer core, trifiler, that is three strands of wire wound like a ribbon, twelve turns each. I used #26 enameled wire, each approximately 18 inches long. (NOTE: In the schematic, the dot signifies the beginning of a winding. Polarity must be observed in connecting windings in series) Be careful and try not to twist the wires while winding. Also be careful not to scrape the insulation on the sharp edges of the core. A good tip is to sand the sharp edges with 100 grit paper prior to winding. Then dip the finished transformer in polyurethane enamel. This will help prevent windings from shorting out. I learned this the hard way after two catastrophic failures. | ||
| The two transformers are now wired in cascade. One winding equal one unit of load resistance, two windings in series equal four units, one each a 4-1 ratio. The output of one transformer is connected to the input of the second transformer. When terminated with 50 ohms on the low end, it will be 200 ohms at the intermediate tap and 800 ohms on the high end. When terminated with 50 ohms on the high end, the intermediate tap will be 12.5 and the low end 3.12. Mount in an box with an insulated panel and use your favorite connectors. | ||
| NOTE: When using the 200 ohm tap, the extra transformer has negligible effect like on a tap on a transformer winding. (See diagram.) |
| Reference: "Radio Amateur Handbook," AC Theory, Broadband Ferrite Transformers.
Chapter 17; Reciever-Transmitter Projects, Transmitter Design (4 to 1 cascade broadband transformers). -'73 Bernie W4ZBA
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last updated by Petrina,
August 9, 1999
to sig. gen.
to rec. mod.