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The Match Meter
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The "Match Meter" was invented
by Nigel Williams, G3GFC
in about 1980. It is a three-function RF bridge, giving null
indications of resistance, conductance and phase error, which
can be inserted into the line between a radio transmitter and
an antenna matching network (ATU). The device is intended to
provide a means by which HF amateur radio operators can use an
impedance matching strategy which was first outlined in a paper
by Underhill and Lewis in 1973 [1].
The method involves assigning the impedance to be matched to
a region of the impedance plane or 'Z-plane', and thereby determining
the necessary ATU adjustments. The method is vastly superior
to the usual SWR-monitoring approach because it indicates the
order in which ATU adjustments should be made, and the direction
in which the impedance matching elements should be adjusted.
Adjustments made according to this scheme are also naturally
optimal in terns of efficiency; 'false' solutions which couple
an unnecessarily large proportion of the transmitter output power
into the matching network losses being precluded. The M50 Match Meter (for 50W systems) was originally conceived as a commercial product, hence the patent application which is reproduced below. The instrument however, like so many good ideas, was never marketed, and existed only as a handful of prototype units. |
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The M50 Match Meter: The M50 is a "little black box" which can be inserted in the line between the transmitter and the ATU 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 ATU controls for a particular frequency of operation, using the M50 will probably be slower than your normal procedure; but the M50 will give real information about the antenna system and will quickly identify any bad settings which you may previously have adopted. The real usefulness of the M50 therefore, is in creating the reference table of ATU settings which 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 which may be occurring in any ATU inductors.. |
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The main control of the M50 is the Function Switch, which
is used to select which of the three possible measurements is
to be made. "jX" denotes reactance, with the meter indicating zero when the reactance term is zero. "R" denotes resistance, with the meter indicating zero when the resistive term is 50W. "G" denotes conductance, with the meter indicating zero when the conductance term is 20mS (20 milli Siemens). The MHz Band Switch is used to select a network giving 90° phase shift for the jX measurement. Using different networks for different bands improves the accuracy of the measurement. The Gain Control adjusts the meter sensitivity and is used to prevent excessive meter deflection. Note that if your target impedance is not 50+j0, but say 75W; you can easily modify the M50 circuit to balance at R=75W and G=13.3mS and so produce an "M75". |
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General Description This instrument has been designed for use over the frequency range 1.5 to 30MHz, and is intended primarily for use between the output of a transmitter and the input to an aerial or aerial tuning unit (ATU). It will indicate what type of impedance is presented to it, and from that one can deduce what adjustments must be made either to the antenna or to the ATU to convert the effective load presented to the transmitter to 50W resistive. The instrument does not itself include any form of ATU and hence cannot perform the actual matching, but permits rapid and unambiguous adjustments to be made to an ATU, resulting in a true match. It is designed for connection to unbalanced impedances, such as a random length wire aerial and earth, but may also be used to measure balanced impedances, such as a balanced aerial feeder, provided some form of balanced to unbalanced (BALUN) impedance converter is used between the balanced impedance and the output of the M50. The M50 requires only a few watts of RF power for satisfactory operation, hence while it is being used the transmitter should be set to low power output either by internal adjustment or by the use of an external attenuator. When the ATU settings have been determined the M50 should be removed from circuit. |
Prototype Match Meters
© D W Knight and N Williams 2005.
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Radio Frequency Impedance Indicator |
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This invention relates to instruments used to facilitate the
conversion of an unknown impedance to a desired value of resistance,
such as may be required when a radio aerial is supplied with
electrical energy from a radio transmitter. The impedance of a radio aerial varies according to its design and the radio frequency at which it is used. In order to effect a maximum transfer of power from a radio transmitter to such an aerial it is common practice to convert the impedance of the aerial, which in general will contain both resistive and reactive components, into a pure resistance of some convenient value. Such values may be typically 50 or 75W. This conversion is commonly carried out by the use of a network of two or more reactances, either of which may be capacitive or inductive, and the process is known as matching the aerial to the transmitter. In order to indicate what values these reactances should have to carry out the matching process it is well known to employ three types of detector, which compare voltage phasors derived from the radio frequency voltages and currents flowing in the aerial. Such detectors are commonly located at the interconnection points between the various reactances of the matching network. and between the network and the aerial. Frequently the outputs of these detectors are used to indicate to a servo control system the way in which the reactances must be changed in order to bring about a match, and the servo system will then cause the required changes to take place, automatically matching the aerial to the transmitter. According to the present invention there is provided a simple instrument which may be manually switched to provide any one of the three detector functions previously referred to, and which will visually indicate whether the criterion for the function selected has been reached, or whether the figure attained is less than or greater than the criterion. |
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The three detector functions are listed below. 1) To indicate whether the impedance to which the instrument is connected contains a reactive term or not, and if so, whether it is positive or negative. 2) To indicate whether the impedance to which the instrument is connected contains a resistive term less than, equal to, or greater than the value of resistance chosen for matching purposes, for example, 50W. 3) To indicate whether the admittance to which the instrument is connected contains a conductance term less than, equal to, or greater than the value of conductance chosen for matching purposes, for example 20 milli Siemens in the case of a matching resistance of 50W. By noting the indications given by these three detector functions and comparing them with a given set of instructions an operator may determine how to vary the matching impedances so as to convert the impedance of the aerial to the required value of resistance and zero reactance. |
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Although the conversion of the impedance presented by a radio
aerial has been described by way of example, the instrument may
equally well be used to facilitate the conversion of any impedance
from any source to a pure resistance of some desired value. A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which Figure 1 is the circuit diagram of such an instrument. |
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Switch S1 is used to select the required one of the three aforementioned
functions, and also has an off position in which the secondary
windings of transformer T are short circuited. The meter A carries
a scale having its zero indication at the centre, and zero is
indicated when the criterion has been met for the function selected.
If the criterion has been exceeded the meter indicates to one
side of zero, or if the criterion has not been reached, the meter
indicates to the other side of zero. Variable resistor R4 is
used to vary the sensitivity of the meter A, and the ratios of
the resistors R1, R2 and R3 are determined by the required values
of matching resistance. The switch S2 selects capacitors C1 to
C4 according to the frequency range in use. The transformer T
is a current transformer having two secondary windings. Nigel Williams. |
© Nigel Williams 1983.
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