Microwave circuits and their types
At very high frequencies an ordinary wire and coil lose energy and act like antennas themselves. So microwave engineers use shaped metal parts, where the size of the part matches the wavelength.
- Waveguide: hollow rectangular metal pipe. Waves travel inside. Very low loss. It has a cut-off: waves longer than twice the wide side (λc = 2a) cannot pass.
- Stripline and microstrip: a thin copper strip on a circuit board. Cheap, small, used inside phones, radar and Wi-Fi.
- Cavity resonator: a closed metal box that stores waves of one frequency, like a bell. Used in filters and in the magnetron.
- Directional coupler: takes a small fixed share of the power so we can measure it.
- Circulator and isolator: let power go one way only. A circulator sends the transmitter's power to the antenna and the received echo to the receiver, so one antenna can do both (used in radar).
- Microwave sources: magnetron (radar, ovens), klystron and travelling-wave tube, and solid-state diodes and transistors.
Types and characteristics of antennas
An antenna turns an electric signal into a radio wave when sending, and the wave back into a signal when receiving. The same antenna works for both. Its size depends on wavelength: a half-wave dipole has length λ/2 and a quarter-wave whip has length λ/4 (against a metal plane).
- Dipole / whip: sends around in a doughnut shape (omnidirectional in the horizontal plane). Low gain.
- Yagi-Uda: one driven rod plus a longer reflector behind and shorter directors in front. Directional, medium gain. TV and VHF links.
- Parabolic dish: a feed at the focus of a parabolic mirror. Very high gain and narrow beam. Satellite and microwave links.
- Horn, loop, array: horn feeds a dish or measures; loop receives well and is small; an array adds many elements for steering the beam (radar, phone towers).
Key characteristics: radiation pattern (shape of the signal), gain in dB (how much better than a reference in the favourite direction), beamwidth (angle of the main beam), polarisation (direction of the electric field: vertical or horizontal) and bandwidth. Gain of 3 dB means twice the power in that direction; 10 dB means ten times.
Types and features of feeder lines
A feeder line carries power from the transmitter to the antenna and the received signal back. It must lose little power and must match the antenna (same impedance), otherwise power reflects back and stands as waves on the line. This is measured by the standing wave ratio, SWR; 1:1 is perfect.
- Coaxial cable: a centre wire inside a metal shield. Shielded, easy to run, impedance 50 Ω or 75 Ω. Loss grows with frequency, so it is used up to a few GHz.
- Twin-lead (parallel wire): two wires side by side, 300 Ω. Low loss at low frequencies, but picks up noise and must be kept clear of metal.
- Waveguide: hollow pipe, lowest loss at microwave, used for radar and dishes, stiff and expensive.
Key formulas and definitions
- Wavelength: λ = c / f, c = 3 × 10⁸ m/s
- Half-wave dipole length ≈ λ / 2; quarter-wave whip ≈ λ / 4
- Gain in decibels: G(dB) = 10 log₁₀(P₂ / P₁)
- +3 dB is about 2 times the power; +10 dB is 10 times
- Waveguide cut-off wavelength: λc = 2a (a = wide side, rectangular guide, lowest mode)
- Power after feeder loss L dB: P_out = P_in / 10^(L/10)
Worked examples
1. Find the length of a half-wave dipole for 150 MHz.
λ = c / f = 3 × 10⁸ / 150 × 10⁶ = 2 m. Half-wave length = 2 / 2 = 1 m.
2. A quarter-wave whip is used on a VHF set at 100 MHz. How long is it?
λ = 3 m, so λ/4 = 0.75 m.
3. An antenna gives 20 dB gain. By what factor is the power in its best direction raised?
G = 10 log₁₀(ratio) = 20, so log₁₀(ratio) = 2 and ratio = 100 times.
4. A rectangular waveguide has a wide side of 2.3 cm. Find its cut-off frequency.
λc = 2a = 4.6 cm = 0.046 m. fc = c / λc = 3 × 10⁸ / 0.046 ≈ 6.52 × 10⁹ Hz = 6.5 GHz. Waves below this frequency cannot travel along it.
5. A transmitter gives 20 W into a cable with 3 dB loss. How much power reaches the antenna?
3 dB loss is about half the power, so 20 / 2 = 10 W reaches the antenna.
Common mistakes
- Thinking gain means the antenna makes more power. It only squeezes the same power into one direction.
- Using the same antenna length for every frequency. The length follows the wavelength, so a higher frequency needs a shorter rod.
- Using a waveguide below its cut-off frequency. Longer waves cannot go through.
- Forgetting that a dish must be aimed. A thin beam misses the target if the dish is a little off.