F4IOZOlivier · radio amateur · JN18FS

Tune your antenna the L network

A coil, a capacitor, and the SWR coming down: match the antenna's impedance to the transmitter's 50 Ω, or know when it cannot be done. All games

The SWR is too high!

Five antennas to tune. Set the coil and the capacitor, choose where the capacitor goes, then lock in. The lower the SWR, quickly and in few moves, the more you score.

Badges

All-rounderFinish a game of each game.
SWR 1.5Tune an antenna to an SWR of 1.5 or less.
SWR 1.2Tune an antenna to an SWR of 1.2 or less.
Perfect matchAn SWR of 1.05 or less.
Clear-sightedRecognise a match that cannot be made.
Not a wrong noteEvery antenna of a game tuned (SWR 1.2, or a match that cannot be made recognised).
All bandsThe “One antenna, five bands” level without a mistake.
No help4,000 points or more at the No help level.
Regular tunerPlay ten games.

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Why tune?

A transmitter is made to work into 50 Ω. An antenna off its resonance shows another impedance, R + jX: part of the power comes back towards the transmitter. The reflection coefficient Γ = (Z − 50) / (Z + 50) measures it; the SWR is (1 + |Γ|) / (1 − |Γ|) and the reflected power |Γ|² × P: at SWR 2, 11% comes back; at SWR 3, 25%.

The L network

Two parts are enough: a coil in series and a capacitor across. The capacitor on the antenna's side brings a high impedance down; on the transmitter's side, it raises a low one. The coil then cancels the reactance left. An automatic tuner does exactly this, with relays.

The Smith chart: every impedance is a point on it; the centre is 50 Ω, SWR 1. A series coil moves the point along a circle of constant resistance, a shunt capacitor along a circle of constant conductance.
No match possible: a very short antenna on a low band needs a bigger coil than the tuner has; a very high-impedance antenna sometimes needs too small a capacitor. Better then to change the antenna or how it is fed.
Matched is not radiated: the tuner protects the transmitter, but the losses in the cable and the tuner remain. An SWR of 1.5 costs 4% of the power: no need to chase 1.0.

The game's numbers Coil from 0.05 to 30 µH, capacitor from 5 to 3,000 pF, lossless; 100 W forward. The antennas' impedances are orders of magnitude at the feed point, without cable. The calculation itself is exact.