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The M50 Match Meter: The M50 is a "little black box" that can be inserted in the line between the transmitter and the AMU and used to determine the Z-plane region in which the impedance to be matched lies. If you already know roughly where to set the AMU controls for a particular frequency of operation, using the M50 will probably be slower than your normal procedure; but the M50 will give additional information about the antenna system and will quickly identify any bad settings that you might previously have adopted. The real usefulness of the M50 therefore, is in creating the reference table of AMU settings that ought to be produced every time a multi-frequency antenna system is installed or modified. This can be a very time-consuming job, and the M50 will provide an unambiguous set of solutions. The M50 will also tell you whether you should use a step-down or step-up balun or unun; and it will give subtle hints regarding the losses that may be occurring in any AMU inductors.. |
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Notes: • A suitable current-transformer core is Amidon FT50-43 (AL=523 nH/turn²) • A Faraday shield wouldn't go amiss. It should be earthed on the TX-side of the transformer core. • Transformer phasing is not shown. The correct sums are: |Vv-2Vi| = |Vv| for R=R0 and |Vi-2Vv|=|Vi| for G=G0 • The meter is 50-0-50 μA • The 4k7 resistor and 1 nF capacitor in each of the Mk1 detectors is redundant. These parts are omitted in the Mk2. • A metal box might not be a bad idea. • The phase bridge requires no calibration. (the amplitude of the quadrature component does not affect the null condition). |
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• The R and G bridges have no frequency compensation. Calibration (with a 50 Ω resistor connected across the antenna terminals) is by adjusting the upper voltage-sampling arm (3 resistors adding up to about 1130 Ω). A Cermet pot. giving about ±10% adjustment would facilitate matters. |
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