M73 Capacitors

m73The M73 is a high-voltage, heavy wattage, continuous duty capacitor. Insulation between the rotor and stator is achieved through " thick mil spec ceramics riveted to the end plates. The rotor rides on ball bearings housed between the front end plate and the stainless steel shaft, and the blades are .032" thick aluminum.

Several options are available, including nine standard air gaps. The shaft’s length can vary, and it can be insulated, slotted, or flatted; mounting holes can be tapped 6-32. There are several double-stator configurations available: In some, the two sections are adjacent to one another, and their capacitances increase together, enabling the user to eliminate the sliding electrical contact from the current path. In others, the two banks of stator blades lie opposite each other, and the capacitance in one section increases as the capacitance in the other decreases.


The M73A is essentially an improved version of the M73, tailored more toward controlled-plasma manufacturing equipment. The M73A is completely non-magnetic, eliminating loss and heat generation from RF-induced eddy currents. The improved high-amperage rotor contact can carry heavy loads while creating very little electrical noise at high frequencies. The rotor rides on two ABEC 5 sealed stainless steel instrument bearings, and final assembly takes place in a Class 10,000 clean room. This redesign addresses every possible mode of failure of the capacitor, while at the same time improving its high frequency, high amperage performance.

In addition to all of the options available in the standard M73, the M73A can be customized to fit other manufacturers’ mounting dimensions, including both metric and English threads. Another option is the use of massive .064 thick silver-plated copper blades, buffed to a mirror finish.

In the chart below minimum and maximum capacitance and mounting distance are given as a function of Z, the number of rotor blades, where there are (Z-1) stator blades and thus (2Z-1) blades total.

distance, L
.030 950 .537z + 5.0 37.36z - 8.1 .113z + 1.5
.032 1100 .616z + 5.4 30.735z - 7.8 .125z + 1.5
.037* 1325 .564z + 5.8 25.88z - 5.4 .112z + 1.5
.045 1550 .637z + 7.3 22.015z - 1.7 .152z + 1.4
.075 2500 .854z + 6.2 13.80z - 1.6 .211z + 1.3
.080 2700 .888z + 6.0 12.92z - 2.2 .222z + 1.3
.100 3100 .977z + 5.7 10.77z - 6.3 .264z + 1.2
.120 3500 1.096z + 4.5 9.455z - 6.8 .302x + 1.1
.140 3700 1.262z + 3.7 8.41z - 7.1 .342z + 1.1

*.037 spacing uses thin blades

Suppose, for example, that you need a deltaC of approximately 200pF and your maximum voltage is 2000Vrms. Set 200pF equal to deltaC for a .075 air-gap M73:

200 = deltaC = Cmax – Cmin = (13.80Z – 1.6) – (.854Z + 6.2)

200 = 12.946Z – 7.8

Z = 16.05

Let Z = 16; so L = 4.68. Thus, if you have enough room, a single-section, .075" – air-gap, 31 blade M73 (i.e., part #73-1-75-31) should suffice.


Standing 4.5" high by 4.5" wide, with typical lengths of over one foot, the M90 is our largest standard configuration of air-dielectric variable capacitor in terms of physical size, and can withstand voltages of up to 11000Vrms.

The latest revision of the M90 incorporates many of the improvements included in the M73A: Non-magnetic construction, ABEC-5 instrument bearings, clean-room assembly, a high-amperage rotor contact, and so on. This is a high-quality unit meant to give design engineers an alternative to vacuum variables in applications that involve high voltage (on the order of 10kV) and relatively small maximum capacitance.

Limitations of the design (and more often limited space in the customer’s device) limit the length, and thus the number of blades and the maximum capacitance, that can be achieved with a given air gap. For an air gap capable of withstanding 10kV, for example, the highest maximum capacitance that would be practical would be around 100pF.


The M97 was designed to replace vacuum variables in applications requiring large capacitances (up to 3250pF) and moderate voltages (from 800 to 4500Vrms). At 3.85" high by 3.85" wide, the M97 is more compact than the M90. The M97’s design includes every improvement incorporated into that of the M73A (non-magnetic construction, etc.), plus several other features meant to enhance its high-frequency current-carrying and voltage-withstanding characteristics.

This is the most robust, highest quality capacitor that we offer. Although the M97 is also the most expensive standard configuration that we offer, its price is still typically around 10% that of the equivalent vacuum variable capacitor, while it also offers advantages of near-instantaneous tuning (minimum to maximum capacitance in just one-half turn, as opposed to 40 turns*), efficient heat dissipation (an almost accidental feature of an air variable’s geometry is the fact that it’s an excellent radiator, as opposed to vacuum variables, which frequently must be water-cooled), and simple durability (as opposed to a vacuum variable, in which such tenuous devices as flexible copper baffles and glass-to-metal seals are relied upon to contain a hard vacuum, within which nesting concentric copper cylinders of opposite charge mesh together separated by only a few thousandths of an inch).

*Our capacitors can be fitted
with planetary reduction drives
if such a high ratio is required
(to make very small changes
in capacitance possible, or to make a drop-in replacement
for a vacuum variable).

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© WVS-Technology, 2004