Transmitters & Receivers

3A0: Electromagnetic Radiation

Radio waves are a form of electromagnetic radiation. When an alternating current flows in an antenna, it creates oscillating electric and magnetic fields that travel outward at the speed of light.

• Electromagnetic waves include radio, infrared, visible light, X-rays, and gamma rays
• They all travel at the same speed — the speed of light (3 × 10⁸ m/s)
• Radio waves are the lowest frequency end of the electromagnetic spectrum

3A1: What is Modulation?

Modulation is the process of adding information (audio, data, video) to a radio carrier wave so it can be transmitted over the air.

A carrier wave on its own carries no information — it's just a steady radio signal. By varying (modulating) the carrier, we can encode information onto it.

3A2: Types of Modulation

3A3: Bandwidth Requirements

Different modes require different amounts of bandwidth:

3A4: Single Sideband (SSB)

SSB is a refinement of AM that transmits only one sideband and suppresses the carrier. This makes it much more power-efficient.

• Upper Sideband (USB) — the upper frequency component
• Lower Sideband (LSB) — the lower frequency component
• Convention: USB on frequencies above 10 MHz, LSB below 10 MHz
• Requires a stable frequency source and a good carrier suppression

3B1: Basic Transmitter Block Diagram

A typical transmitter follows this signal path:

• Oscillator — generates the radio frequency signal
• Mixer — combines the signal with the audio/data to create modulation
• Amplifier — boosts the signal power to the required level
• Antenna — radiates the signal into space

3C1: Types of Oscillators

3D1: Superheterodyne Receiver

The superheterodyne is the most common receiver design. It converts the incoming signal to a fixed Intermediate Frequency (IF) for easier processing.

• The incoming signal is mixed with a local oscillator to produce the IF
• The IF is fixed (e.g., 455 kHz for AM, 10.7 MHz for FM)
• Filters at the IF provide selectivity

3D2: Direct Conversion Receiver

A direct conversion receiver converts the signal directly to audio without an intermediate frequency. It's simpler and cheaper but has less selectivity than a superhet.

• Common in simple homebrew receivers
• Good for CW and digital modes
• Local oscillator is set to the same frequency as the incoming signal

3E1: Microphone Amplifier Bandwidth

Human speech has a frequency range of roughly 80 Hz to 8 kHz, but for radio communication, only the range 300 Hz to 3 kHz is needed. This is enough to make speech intelligible while keeping bandwidth narrow.

• SSB and FM use this 300 Hz – 3 kHz bandwidth
• Higher frequencies improve intelligibility but use more bandwidth
• Audio filtering can remove unwanted noise outside this range

3F1: Sensitivity

Sensitivity is the ability of a receiver to pick up weak signals. A more sensitive receiver can decode signals that are weaker.

• Measured in microvolts (µV) or dBm
• Lower values = better sensitivity
• Depends on the receiver design and noise figure

3F3: Selectivity

Selectivity is the ability of a receiver to separate wanted signals from unwanted ones. Good selectivity means you can tune into one station while rejecting others nearby.

• Determined by the IF filter bandwidth
• Narrower filters = better selectivity but may affect audio quality
• Critical in crowded band conditions

3G1: IF Filters and Crystal Filters

Filters in the IF stage determine the receiver's selectivity. Different types are used depending on the mode:

• Ceramic filters — used in FM receivers, moderate selectivity
• Crystal filters — very narrow, used in SSB receivers for excellent selectivity
• DSP filters — digital signal processing in modern radios, highly configurable

3H1: Antenna as Transducer

An antenna is a transducer that converts electrical energy into electromagnetic waves (when transmitting) and vice versa (when receiving).

3H2: Antenna Gain

Gain describes how well an antenna concentrates radio energy in a particular direction compared to an isotropic reference antenna (which radiates equally in all directions).

• Measured in dBi (decibels relative to isotropic)
• Higher gain = more focused signal in one direction
• A dipole has a gain of about 2.15 dBi

3K1: PSU Requirements

Amateur radio equipment requires a reliable DC power supply. Most modern transceivers operate from 13.8V DC (the nominal voltage of a car battery with the engine running).

• Must provide stable voltage under varying load
• Should have adequate current capacity for your transmitter
• Fuse protection is essential
• Batteries can be used for portable operation

3H1: Staying Within Your Allocations

It is essential to stay within the frequencies allocated to your licence level. Different licence levels have access to different bands and power limits.

• Check the band plan before transmitting — not all frequencies within a band are available for all modes
• Some segments are reserved for narrow-band modes (CW, digital) and others for wide-band modes (FM, SSB)
• Beacon frequencies should be avoided for other transmissions
• Foundation licensees have restricted power and band access — always check your licence conditions

3M1: SDR Concepts

Software Defined Radio (SDR) uses software to perform signal processing tasks that are traditionally done in hardware.

• The signal is digitised early in the receiver chain
• Filtering, demodulation, and processing are done in software
• Very flexible — can change modes by loading different software
• SDR dongles (like RTL-SDR) make an excellent and cheap receiver