Quarter Wave Antenna (λ/4)
A quarter wave antenna is a resonant antenna whose length is one quarter of the wavelength. Its theoretical length is 75 / f(MHz) metres, and on real wire, with the velocity factor, about 71.25 / f(MHz) metres: roughly 16.4 cm at 433.92 MHz, 8.2 cm at 868 MHz, 2.9 cm at 2.45 GHz. When the length is right, no additional matching circuit is needed.
This length is not arbitrary: at λ/4 the current distribution is maximum at the feed point and zero at the tip, and the antenna input impedance becomes largely resistive. That means a structure that can be connected directly to the transmitter output.
- Shortening is not forbidden but it has a price: reducing the length moves the antenna's resonance upwards, degrades SWR and lowers efficiency.
- Lengthening does the same thing in the opposite direction; extending the antenna cable and extending the antenna are different things.
- The velocity factor is typically taken as 0.95 on thin wire; that is why the real length is slightly below the theoretical value. For 433 MHz the free-space λ/4 is about 17 cm while the physical length of the wire comes out at 16.4 cm; when giving free-space dimensions such as ground plane radius, 17 cm is used, and when giving the wire length to be cut, 16.4 cm is used.
The most common mistake in the field is shortening the antenna or bending it to fit inside an enclosure. Both destroy the resonance and clearly reduce range. In AXI sets the receiver is supplied with its own antenna and cable gland; what is needed is not to shorten the antenna but to move it, at its given length, outside the enclosure and into an open position.
Context in which this term is used: 433 MHz or 2.4 GHz?