GENERAL DESCRIPTION

The Mallory UHF Inductuner operates on the principal that inductance of two lines, shorted at one end, is proportional to the area that these lines enclose. Two such lines are laid concentrically on a coil form and a shorting contactor, connected to the tuning shaft, is allowed to be moved along their length, changing the inductance.

This principal allows inductance changes upwards of 5 to 1, corresponding to changes of frequency on the order of 2 to 1. Typical inductance values run from ten nanohenries at one stop to fifty nanohenries at the other stop. Typical values of “Q” exceed 100.

The coil form material, a thermoplastic, is selected for its low dielectric constant and dissipation factor. In the coil form is secured a copper ribbon with an overlay of sterling silver. The contactor is a silver alloy, Elkonium® 18, which has excellent properties, both as a contact and as a spring.

The case and cover are brass plated for corrosion resistance, with component parts soldered together for maximum shielding efficiency.

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MECHANICAL CONSIDERATIONS

The outline drawings (right) detail the standard “700 series” Inductuner, which is available in from one to eight sections, any twelve shaft lengths, any of three line widths, and with the grounding lug either in any of these three locations or omitted. Where one of these 1152 standards will meet your requirements, it is to your advantage to specify a standard.

Mallory has anticipated, however, additional application possibilities and therefore the following options are available (at additional cost):

SHAFT ROTATION

Rotation of the shaft can be either clockwise (standard) or counterclockwise, this being the direction of decreasing inductance (increasing frequency).

The shaft rotation may be 320° (standard) or any angular quantity less than 320°.

SHAFT CONFIGURATION

Shaft lengths may be in increments of ¹/₈ inch up to 1 ½ inch measured from the end of the frame (standard), or of the other length, and the shaft extension may be round (standard) or flattened or knurled or otherwise modified under the 0.248/0.250 diameter.

Under Construction: Graphics to be added...

The shaft may extend from both ends of the Inductuner.

The ends of the shaft may be supported by self-lubricated brass bushings (standard) or by ball bearings.

INDUCTANCE

The coil forms may be mixed within the Inductuner so that some sections will have one width of line and other sections have another width of line. Omission of coil forms from sections is possible where spacing requirements dictate.

The coil forms may be mixed within the Inductuner so that those at one end decrease in inductance while those at the other end increase in inductance.

The lines in the coil form may be 0.073, 0.093 or 0.125 wide (standard) or they may be any fixed width or tapered to customer specification within the range of 0.073 to 0.130.

The internal terminations may be shorted together (standard), left open, or tied together through a resistor.

The end of the inner line may be shorted to the beginning, causing the inner ring to be circular in form.

The ends of either or both of the lines may be tied to the case through a resistor.

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MOUNTING

Clinch nuts, with a 4-40 class 3-B thread, may be installed in any or all of the four mounting holes.

The Inductuner terminals extend ¹³/₆₄ below the base (standard) or they may be extended to another length as required. The length of the terminations need not be the same for both lines in a section nor for all sections.

The terminals may have a single ¹/₁₆ diameter hole (standard) or they may be unpunched or punched to customer requirements.

The grounding lugs may be placed at none, any, or all three possible locations on each shield between sections and at the ends. The locations on each of the shields need not be the same.

MISCELLANEOUS

The case will bear a nameplate stamped with the Mallory number and EIA date code (standard) and with the customer part number (if requested). In addition, the case may be stamped with customer purchase order number, other identifying data, and/or any special circuit information.

Plating of parts may be specified to meet unusual environmental conditions or military specifications.

Measurements at ultra-high frequency are uniformly difficult because of the requirements of keeping leads down to a small fraction of the wavelength. Fixtures and instrument terminations add appreciable amounts of inductance and capacitance, neither of which can be effectively compensated for. In practice, readings are taken of Inductuner parameters by several methods, and the result is an approximation of the true characteristics at best. Final testing a UHF Inductuner is best performed by the customer in the circuit he intends to use.

The parameters which are most meaningful and which are most normally used for specification are these: Inductance (L), distributed capacitance (C_D), and “Q”. Of these, the first two are determined as follows: The resonant frequencies, w₁ and w₂ (radians per second), are determined for each of two values of external shunt capacitance, C₁ and C₂.

These two equations have two unknowns, L and C_D, and they may be solved as follows:

For the above equations, the inductance of both Inductuner and fixture and the distributed capacitance of both Inductuner and fixture, are lumped together. In order to determine the parameters for the Inductuner only, it will be necessary to subtract the appropriate parameters for the fixture.

It is also desirable to determine “Q”, the ratio of inductive reactance to equivalent series resistance. If the above measurements have been taken using a “Q” -meter, the reading of apparent Q(Q_A) may be taken with each reading of frequency and capacitance. Another approach to Q determination uses a generator tuned first to the resonant frequency, then to the 3 db points on either side. A sweep generator and oscilloscope may be set up to provide a quick, if approximate, means for taking such readings. The difference between frequencies at the 3 db points, the bandwidth, divided into the resonant frequency is equal to the apparent Q.

The Q determined by the Q-meter, however, is the Q of the net reactance, inductive reactance less capacitive reactance, and not that of the inductance alone. To determine the Q of the inductance alone, the true Q, we must multiply the apparent Q (Q_A) as follows:

where C is the capacitance value read from the Q-meter.

Since the multiplying factor must be greater than one, true Q is always greater than apparent Q.

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Inductuners are most often designed into three types of tuned circuits: oscillators, multipliers, and tuned amplifiers. Because the typical frequency range of an oscillator is different from that of the other sections of a tuner, it has proved advisable in some applications to choose a special oscillator section. Multipliers, however, take advantage of the uniformity from section to section, and they use identical sections with differing amounts of external capacitance.

In general, the arrangement of sections follows good engineering practice: the high power sections are separated from the low power sections to minimize interaction. The oscillator is best placed at the shaft end to minimize the effect of the torsional strain.

Inductuner circuits most generally have small but adjustable “end inductor” add externally. This end inductor and a tuning capacitor, adjustable over a small range, provides an adequate circuit for tracking the Inductuner at two points.

In unbalanced circuits, one of the lines, most often inside line, is grounded either by shorting it to the case either directly or through a low impedance. This markedly increases the distributed capacitance because the capacitance of the ungrounded line to the case must be added to the line-to-line capacitance.

In balanced circuits this capacitance appears as a series combination of two capacitors which shunt the distributed capacitance. The effect of these capacitors may be minimized by the addition of a low impedance path to ground from the internal termination. This can increase the self-resonant frequency of the Inductuner to over one gigahertz. Balanced circuits with the low impedance tie to ground become mandatory design practice at frequencies exceeding 600 Megahertz.

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