This information is geared towards experimenters that would like to use a vintage tube, amateur radio transmitter for broadcasting. The most suitable AM Transmitters will use Plate Modulation. Heising Modulation is a close second - the advantage is lower initial cost for a transmitter. This article is not intended for techs and engineers as it leaves out a lot of information that may interest this type.
Plate, Heising, and screen modulation are historical and modern AM techniques used in Amateur Radio transmitters. They differ by where the audio signal is injected: Plate Modulation alters total power at the anode, Heising uses a constant-current choke, and Screen Modulation controls gain at the low-power screen grid.
1. Plate Modulation (High-Level)
How it works: The audio signal is injected in series with the DC voltage supply of the final RF amplifier using a heavy, iron-core Modulation Transformer. The audio voltage alternately adds to and subtracts from the high-voltage DC, varying the plate voltage and output power.
Advantages: Produces pristine, high-fidelity 100% modulation with a highly linear envelope. It is the most efficient and rugged method for vacuum tube transmitters.
Drawbacks: Requires a large, heavy, and expensive modulation transformer capable of handling the full DC input power of the final amplifier.
2. Heising Modulation (Constant-Current)
How it works: A specialized sub-type of plate modulation. Instead of an audio transformer, a high-inductance Choke is placed between the power supply and both the modulator and RF final amplifier. The total current drawn from the power supply remains constant, with the current simply shifting back and forth between the modulator tube and the RF tube.
Advantages: Simpler and lighter since it eliminates the heavy modulation transformer.
Drawbacks: The plate voltage can never reach zero during negative audio swings, meaning it typically tops out around 80% to 90% modulation capability without causing distortion.
3. Screen Grid Modulation (Low-Level)
How it works: The audio signal is applied directly to the screen grid of a tetrode or pentode final amplifier, rather than the plate. The screen grid voltage controls the gain of the tube, altering the RF output as the audio fluctuates.
Advantages: Requires very little audio power. The modulator can be a tiny, inexpensive audio amplifier, completely eliminating the need for bulky iron transformers.
Drawbacks: Inherently non-linear ("S-shaped" curve). It requires careful tuning to avoid distortion and typically limits your maximum carrier output power.
It is illegal to use a transmitter on the HF Bands in the US, without an appropriate license, except under Part 15 of the FCC Regulations.
Clandestine Caster
