Amateur Television
Amateur television sends real-time video and audio over amateur radio frequencies. The signal carries luminance, chrominance and synchronisation components that occupy far more spectrum than a voice transmission. On the 70-centimetre band operators commonly use frequency modulation; on the 23-centimetre and 13-centimetre bands both FM and digital modes such as DVB-T appear. Propagation is almost always line-of-sight, with additional range possible through tropospheric scatter or knife-edge diffraction over ridges.
How ATV Differs from Voice Work
Voice contacts use narrowband FM or single-sideband modulation whose bandwidth fits inside a few kilohertz. An ATV signal must reproduce picture detail up to several megahertz, so the transmitter and any following amplifier must pass that entire video baseband without compression or distortion. Peak-to-average power ratio is higher than in voice, and the duty cycle approaches 100 percent during transmission. These factors change the requirements placed on every stage after the exciter.
RF Amplifiers for ATV
Linear amplifiers are inserted after a low-level ATV exciter to raise output power. The amplifier must remain linear across the full video bandwidth so that intermodulation products do not appear inside the channel or in adjacent spectrum. Solid-state LDMOS or GaN devices are common; thermionic valves are still used where higher power or robustness against mismatch is needed. Bias networks keep the device in class AB or class A; any shift toward saturation produces visible streaking, loss of colour or adjacent-channel interference.
Practical Difficulties
- Heat removal: continuous carrier and sync pulses generate steady dissipation in the final device and its heat sink.
- Impedance behaviour: video sidebands extend well beyond the carrier, so the output network and any harmonic filter must present a controlled impedance across several megahertz.
- Linearity verification: two-tone or multi-burst test signals reveal compression that a simple wattmeter misses; operators watch the resulting picture for smearing or sync instability.
- Protection circuits: fast-acting VSWR fold-back is essential because an antenna mismatch during a long transmission can destroy the device before thermal protection reacts.
Digital ATV adds its own constraints. Peak-to-average ratio rises further with OFDM waveforms, and any amplifier nonlinearity causes spectral regrowth that fails emission masks. Operators therefore trade output power against error-vector magnitude and often insert a digital pre-distortion stage before the final amplifier.
Points of Disagreement
Some builders favour wideband FM because it tolerates modest nonlinearity and works with simple receivers. Others prefer digital modes for better weak-signal performance and narrower occupied bandwidth once coding overhead is included. A recurring question is whether an amplifier optimised for SSB voice remains adequate once video sidebands are present; measurements show that third-order intercept often needs to be several decibels higher for clean ATV. Cooling method, device technology and the choice between broadband and narrowband matching networks also divide experimenters.
Where to go next
Further reading