Air-coil design information
Page 1, Cylindrical, Single-Layer, Air-Core Coils of Various Proportions
Author: R.J.Edwards G4FGQ © 13th May 2003
This
program assists with the design of single-layer coils having a wide range
of proportions, including single-turn loops, close-wound solenoidal forms,
coarse helices of thin wire, and ranging in size from miniature coils
in VHF receivers to large coils used in high-power radio transmitters,
antenna tuners, and RF traps.
Program Output data: Coil inductance, reactance,
self-capacitance, self-resonant frequency, loss resistance, Q, capacitance
needed to tune to a test frequency, resonant impedance at a test frequency,
and power dissipated for a given RF voltage across the coil.
The program has facilities to sweep these input data
values: Coil length, diameter, number of turns, wire diameter-to-pitch
ratio, and frequency over wide ranges using pairs of adjacent up/down
numerical keys. Refer to the menu near bottom of data screen.
"Ratio" refers to ratio of bare wire diameter
to the coil winding pitch. If turns touch, the ratio equals 1, but to allow
for the thickness of wire insulation the ratio is automatically restricted
to 0.97 of maximum. For close-wound turns just enter 1.
Important
- Coil Diameter = diameter of the former on which the wire is wound.
- Coil Length = distance between the wire centres at the ends of the
coil.
Coil Q is officially defined as the ratio (Inductive reactance)/(Series
loss resistance). That is the value computed by this program. However,
it is impossible to achieve that value of working Q in a practical circuit
due to other losses. Imperfections in associated components such as capacitors,
tubes and transistors increase the effective circuit resistance.
The resulting circuit Q of a coil with Q = Q1 in resonance
with a capacitor of Q = Q2 is given by Q1*Q2/(Q1+Q2). This causes
Q meters to underestimate the true Q of high Q coils at the higher radio
frequencies. In fact, all direct methods of measurement reduce the effective
Q of the item under test. Fortunately, high values of Q seldom need to
be known with an accuracy better than approximate.
Two values of Q are computed depending on the type of
circuit in which a coil may be used. The higher value applies when the
tuning capacitor is connected directly in parallel with the coil as in
a tank circuit or rejector circuit, such as an antenna trap. The coil's
self- and external-capacitances are then in parallel.
The lower value of Q applies when the tuning capacitor
is connected in series with the coil and another part of a series-resonant
circuit. The coil's self-capacitance in shunt with the coil causes both
the effective-inductance and loss-resistance to increase. As the operating
frequency nears the coil's self-resonant frequency the circuit Q rapidly
falls, for example, in frequency filters.
- Note: Ordinary Q meters measure Q with the coil in a series-resonant
circuit.
The computed self-resonant frequency is the value with
the coil isolated, i.e., not within several coil-diameters plus a coil-length
of any other material, either conducting or insulating. Coil wiring, components
and other materials in the immediate vicinity will reduce resonant frequency
by an indeterminate amount.
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