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A.,E. McGee. Jr., KSLLJ
2815 Ma rerhorn Drive
Dallas, Texas 75228
n-« To TUlle A
Circuit
In these days of intricate and relatively
inexpensive commercial radio equipment,
home building of ham gear is not so co rnmon
as it used to be. However, a great deal
of pleasure and satisfaction may still be had
from the designing and building of simple
receivers, co nverters, etc., even if you never
intend to actually use them on the air.
One fairly critical part of most simple
projects is the tuning circuit. At frequencies
through the VHF region, a tuning circuit
usually consists of an inductor (coil) and a
variable capacitor, which is adjustable over a
reasonably wide range. It is easy to find, by
the trial and error method, some combination
of inductance and capacitance that will
tune to the desired frequency. The trouble
usually begins when you try to band-spread
the circuit; that is, to make the entire tuning
range of the variable capacitor cover only
the desired frequency range. This frequency
range may be only several hundred kilohertz
wide, such as an amateur band or a shortwave
broadcast band,
This article will attempt to illustrate the
problems involved, and how to solve them
by the use of a grid-dip oscillator and some
simple charts and formulas. First, however, a
few words about circuit theory may be in
order.
An inductor or a capacito r will oppose
the flow of an alternating current. This
property is called reactance, and differs from
resistance in that the current through the
reactance is 90 degrees (or one-quarter
cycle) out of phase with the voltage. In an
inductor the current lags the voltage by 90
degrees, and in a capacitor the current leads
the voltage by 90 degrees. The amount of
reactance is determined by the value of
inductance or capacitance, and by the frequency
of the alternating current. Inductive
reactance increases with an increase in frequency,
while capacitive reactance decreases
with an increase in frequency.
When an inductor and a capaci tor are
connected, either in series or parallel, there
will be one frequency at which their reactunces
arc equal. Since the inductive reactance
causes a current lag of 90 degrees, and
the capacitive reactance causes a current lead
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Fig. 1. Resonant frequency c h a r t . By laying
a straight edge across two known quantities.
the third can be determined.
5
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of 90 degrees, the reactances cancel each
other, and the circuit is said to be in
resonance. The series resonant circuit offers
a very low resistance to the flow of alternating
current at the resonant frequency,
and the parallel resonant circuit offers a very
high resistance to the flow of alternating
current at the resonant frequency.
The frequency of resonance may be
found by using the formula :f=21Tk with fin
Hertz, L in Henrys, and C in Farads, A
simpler method is to use a chart like the o ne
in Fig. I, By laying a straight edge across
two known values, the other quantity may
easily be found. Charts covering a wide
frequency range may be found in Allied's
Electronic Data Handbook and other similar
publications. There is also a chart on page 70
of the August 1967 issue of 73 Magazine.
Let us say that you wish to build a circuit
that tunes from 7.0 MH z to 7.3 Mllz. You
have a variable capacito r from the junk box
that you wish to use, a few surplus coils with
unknown inductance values, and an assortment
of small fixed capacitors. You also
must have a calibrated grid-dip oscillator.
In order to find the capacitance of the
variable capacitor, you will first need a
known value of inductance. Pick a likelylooking
co il from the junk box, or wind one
by guess or by using a coil winding chart.
The chart found in Allied 's Electronic Data
MAY 196 9 123