Austria Theory AM cheat sheet

ITU Radio Regulations divide the world into three Regions; Austria lies in Region 1, where the 144–146 MHz band is allocated to the amateur service on a secondary basis.

An amateur licence is a personal, non-transferable authorisation: the holder is responsible for every transmission made from the licensed station, including guest operators.

Callsign structure: the prefix OE identifies Austria; the digit and suffix letters identify the individual station, and the full callsign must be sent at the start and end of each contact and at least every 10 minutes.

A CQ call contains the words CQ followed by the callsign, sent no more than three times per call; the reply uses the other station's callsign followed by the caller's own callsign.

Amateur transmissions must not be encoded to obscure meaning; only control and identification signals (e.g. packet headers, telemetry) may use codes, and only where the licence expressly permits.

The station log must record date, time (UTC), frequency or band, mode, and the callsign of the correspondent; logs are retained for the period required by the national authority.

Harmful interference must be eliminated immediately, even if the transmission otherwise complies; the operator must change frequency or cease transmitting on request of an authorised inspector.

Third-party traffic is permitted only if the licence class and the destination country both allow it; messages of a commercial nature are generally excluded from amateur bands.

Ohm's law: U = I·R; power: P = U·I = I²·R = U²/R. In a series circuit the current is common; in a parallel circuit the voltage is common.

Resistors in series add (R = R1 + R2); in parallel the reciprocal adds (1/R = 1/R1 + 1/R2). Two equal resistors in parallel give half the value.

Capacitive reactance XC = 1/(2πfC) falls with frequency; inductive reactance XL = 2πfL rises with frequency. At resonance XL = XC and the circuit is purely resistive.

Resonant frequency of an LC circuit: f = 1/(2π√(LC)). The quality factor Q = f/B defines the bandwidth B of the tuned circuit.

Decibel definitions: power ratio dB = 10·log10(P2/P1); voltage ratio dB = 20·log10(U2/U1). +3 dB is a doubling of power, +6 dB a doubling of voltage, +10 dB a tenfold power increase.

A half-wave dipole in free space has a feed-point impedance of about 73 Ω and a gain of 2.15 dBi; a quarter-wave vertical over a good ground plane has about 36 Ω and the same 2.15 dBi gain.

Semiconductors: a silicon diode conducts when the anode is about 0.6–0.7 V positive of the cathode; a germanium diode at about 0.3 V. A bipolar transistor needs base-emitter forward bias (≈0.7 V for Si) to conduct.

FM deviation and bandwidth: Carson's rule gives bandwidth ≈ 2(Δf + fm). For 2.5 kHz deviation and 3 kHz audio, the occupied bandwidth is about 11 kHz.

A half-wave dipole length in metres is approximately 143/f(MHz) (the 0.95 velocity factor correction); a quarter-wave vertical is about 71.5/f(MHz).

Feed-point impedance of a centre-fed dipole is about 73 Ω in free space; it falls when the antenna is lowered toward ground and rises when fed off-centre.

A 1:1 balun is used at the feed point of a balanced dipole fed with coaxial cable to prevent common-mode current on the shield; a 4:1 balun matches a 200 Ω load to 50 Ω coax.

Characteristic impedance of coaxial cable depends on the ratio of inner to outer conductor diameter and the dielectric constant; typical values are 50 Ω (radio) and 75 Ω (TV).

SWR = (1 + |Γ|)/(1 − |Γ|), where Γ = (ZL − Z0)/(ZL + Z0). An SWR of 2:1 corresponds to about 11 % reflected power; 3:1 to 25 %.

Velocity factor: the physical length of a cable or antenna element equals the free-space length multiplied by the velocity factor (about 0.66 for solid polyethylene coax, 0.8–0.95 for antenna wire).

Ground-wave propagation dominates below about 2 MHz; sky-wave via the ionosphere (F layer) supports HF worldwide communication, with the maximum usable frequency rising with sunspot number.

VHF/UHF propagation is essentially line-of-sight; the radio horizon in km is approximately 4.12·√(h in metres). Tropospheric ducting and sporadic-E can extend the range considerably.

A transmitter must not emit spurious radiation exceeding the limits set for the licence class; harmonics are reduced by low-pass filters after the final amplifier.

Class C amplifiers are highly efficient but generate harmonics and are suitable only for constant-envelope modes (CW, FM); SSB requires linear Class AB amplification.

Receiver sensitivity is the minimum input signal producing a specified signal-to-noise ratio; selectivity is the ability to reject signals outside the passband, set by the IF filter.

The mixer converts RF to the intermediate frequency; image rejection depends on the RF preselector and the choice of IF. A high first IF improves image rejection.

A dummy load of 50 Ω dissipates transmitter power without radiation; it must be rated for the full PEP of the transmitter and have low reactance across the operating band.

An SWR meter inserted between transmitter and antenna reads the ratio of forward to reflected power; a reading of 1:1 indicates a matched load, infinity indicates an open or short.

Common-mode interference on feed lines is suppressed with ferrite beads or a choke balun; differential-mode noise from switching supplies is reduced with LC filters at the source.

Measurement accuracy: a power meter calibrated for one frequency band may read low on another; the oscilloscope probe must be compensated and the measurement made within its bandwidth.