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[coil measurements with a VNA antenna analyzer]
A low-loss coil of the transmitter station at Rugby/UK, early 1920s
Antenna tuning coil of the 1 megawatt Goliath transmitter
(these enormous variometers comprised a fixed coil (3.5 m diameter, ≈11½ft), into which a slightly smaller coil (3.2m diameter) could be inserted hydraulically with a precision of 0.1 mm! The coils were 5 m tall (16 ft) and weighed about 5000 kg (11k lbs)
Load short antennas
Separate antenna sections LC Trap
L in circuitry
Note that, as always, "Harry's Law of
Coils" applies. As Harry (SMØVPO) says:
1. You
cannot wind coils like I, and
I cannot wind coils like you.
2. Coil-winding data
is a constant that varies from person to person.
By definition, a tuned electrical circuit is at resonance for a frequency at which the reactive component X of the circuit's impedance is zero (capacitive reactance = inductive reactance; XC = -XL). Hence, antenna matching coils should be tuned for minimum reactance at the desired frequency, rather than lowest SWR. If the antenna impedance is not 50 ohms at resonance, then lowest SWR will not occur at the resonance frequency!
Optimal coil-Q appears to occur when the space between adjacent windings is equal the diameter of the bare wire. Optimal length-to-diameter ratio of a coil typically ranges from 0.5 to 2, and depends on the gauge of the windings, how far the operating frequency is below the self-resonance frequency of the coil, and conductive objects near the coil.
Articles:
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"How Does an Inductor or Loading Coil Work?" by Tom, W8JI |
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"Q measurement of coils and tuned circuits with the AIM4170" by W5BIG |
On-line coil and LC-filter calculators (must have java enabled in your browser):
Miscellaneous:
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"Get/Make a "Coilmaster"" (K5BCQ) [coil winding device] |
“A fool with a tool is still a fool”
When I got my miniVNA antenna analyzer, I wrote the following short paper on how to use the miniVNA or a dip meter to measure LC filters:
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"Measurements of an antenna loading coil" by Frank Dörenberg (N4SPP), 13 Aug 2008 |
If you are sure that coils and coax cable are not polarized components, you may want to try and explain the observations documented in this paper:
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"Measurement of an LC filter with coax as parallel capacitor" by Frank Dörenberg (N4SPP), 7 Sept 2008 |
©2010 F. Dörenberg N4SPP
