September 1947 QST
Table of Contents
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
Although single capacitor, single inductor resonant
tank circuits are not widely used, one place you do find them is in transistor amplifier stages in
order to peak gain in a particular frequency band. Author Jack Najork offers here a few pointers
regarding preforming quick mental calculations for how ranges of capacitance values affect the
resonant frequency. For instance the ratio of capacitance values needed to cover a certain range of
resonant frequencies is equal (ideally) to the square root of the frequency ratio. This comes into
play both when determining values during the design phase and the selecting a variable capacitor for
providing tuning over a continuous range of frequencies. The same is true for inductor selection, of
Simple L and C Calculations
A Useful Formula for Bandspread and Padder Problems
By Jack Najork, W2HNH
This article discusses a well-known but seldom-appreciated simple relationship between frequency
and circuit capacitance. It has the advantage over many radio calculations in that it takes less time
to work out the formula than it does to arrive at the answer by the "cut-and-try" method.
One of the most useful formulas, and at the same time one most neglected by hams who build or alter
their own gear, is the simple mathematical relationship between a change in the frequency of a tuned
circuit, such as indicated in Fig. 1, and the change in capacitance which causes it. Its most common
application is in the determination of capacitance values in bandspread or other tuning-range problems
in receivers or VFOs. It shows the frequency range that any variable condenser will cover or, conversely,
the variable capacitance necessary to cover a desired frequency range.
The relationship is expressed by
In other words, it always takes a capacitance ratio of the square of the frequency ratio to cover
the desired frequency range. If the frequency ratio, maximum to minimum, is 2 to 1, the required capacitance
range to cover it is 4 to 1; if the frequency range is 3 to 1, the capacitance range must be 9 to 1,
etc. And, of course, the converse is true - the frequency range that a variable condenser will cover
is the square root of the capacitance ratio, i.e., a capacitance range of 4 to 1 produces a frequency
change of 2 to 1. Example:
Desired frequency range - 3500 kc. to 4000 kc.
Minimum circuit capacitance - 30 μμfd.
Frequency ratio = 4000/3500 = 1.143
1.1432 = 1.3
Therefore the capacitance change to tune over this band must be in the ratio of 1.3 to 1. Since
the minimum circuit capacitance with the condenser set at zero is given as 30 μμfd., the maximum
must be (1.3) (30) = 39 μμfd. The difference between maximum and minimum, 9 μμfd., is the
The formula can be twisted around, of course, to give any one of the desired values by substituting
known values for the other three factors.
In all of the foregoing, Cmin is the minimum circuit capacitance with the variable condenser
set at zero, Cmax the total circuit capacitance with the variable in full, fmax
and fmin respectively the highest and lowest frequencies to which the circuit tunes
and Cvar the variation in capacitance in the tuning condenser. Cmin, of course,
includes any fixed capacitance that may be used in the circuit either for bandspread or to obtain a
high-Q circuit such as in VFOs. Of course fmin and fmax, and
Cmin, Cmax and Cvar, should be in the same units, i.e., kc. or Mc.
for f and μd. or μμfd. for C.
Fig. 1 - A - Circuit for fmax. B - Circuit for fmin.
Cmin is the total minimum circuit capacitance which includes tube capacitances, minimum capacitance
of the tuning condenser and capacitance introduced by sockets and wiring as well as the distributed
capacitance of the coil. Cvar is the variable part of the tuning-condenser capacitance (maximum
In practical application it should be recognized that the variation in capacitance that may be expected
from a variable condenser is not the rated maximum capacitance shown in the catalogues; it is the difference
between the condenser's maximum and minimum capacitance. A typical small 100-μμfd. variable may
have a minimum capacitance of 10 μμfd. making the variation only 90 μμfd. This minimum as
well as any capacitance introduced by mounting the condenser close to grounded metal must be included
in the minimum circuit capacitance, Cmin. This minimum circuit capacitance usually is the
only factor that is not readily determined and since it may run as high as 50 to 75 μμfd. when
capacitance coupling is used between tetrode or pentode stages, especially when the condenser is mounted
close to the chassis, it isn't something that can be neglected entirely without introducing considerable
error in the calculations. In many cases, however, it is possible to estimate the total minimum capacitance
with an accuracy sufficient for the purpose. In the case of capacitance coupling between stages, the
output capacitance of the driving tube and the input capacitance of the driven tube are in parallel
with the tank coil and therefore form part of the minimum circuit capacitance. The values for any particular
combination of tubes may be made up from the information given in tube manuals.1 Variable-condenser
minimum capacitances also are usually shown in the manufacturer's catalogue.
Mounting a midget-type condenser on metal in the usual manner introduces a stray capacitance between
stator and ground equivalent to from 2 to 4 times the condenser's rated minimum capacitance. This must
be added to the circuit minimum although it does not alter the amount of capacitance variation to any
appreciable degree. Socket, wiring, and coil capacitances may add another 10 to 20 μμfd. If the
circuit minimum is to be held down, the condenser in particular must be well spaced from grounded metal
chassis and shields.
The inductance required to go with the condenser may be determined from the following formula:
where the frequency is in Mc., the capacitance in μμfd. and the inductance in μh. Coil dimensions
to give the calculated inductance may be selected from published tables or the ARRL Lightning Calculator.
1 See p. 22, February, 1947. QST, for popular .audio-receiving-tube
Posted July 11, 2016