Principle and calculation of quarter wavelength antennas

Estimated 5 minutes to finish reading

Quarter wavelength antennas are more common
in areas such as walkie-talkies, and their name is more technical than that of
antennas that are distinguished by their shape. In this article, we will
further discuss the characteristics of the quarter wavelength antenna by
looking at the principles and applications of this antenna.

First, the antenna's
transmission principle

Regarding the quarter wavelength antenna,
we need to understand the basic principle of the antenna's emission first.
Electricity travels through a conductor at a speed close to that of light, and
when it encounters a discontinuity in the conductor, it is reflected back to
the signal source. If the current is alternating and the reflected current
returns to the origin or feed point at the right moment, then the current is
reinforced by the subsequent cycles so that only a small amount of energy is
required to maintain a standing wave within the antenna. That is, due to the
presence of standing waves so that the antenna is in a resonant state. Thus
transmitting electric waves into space.

In the resonant state, the voltage in the
current for the maximum of the midpoint (oscillator of the midpoint) is very
little, but at both ends there is a great value, Ohm's law applies to the
antenna, in the midpoint due to the current, the voltage is low, so the
resistance is small, at both ends of the situation is exactly the opposite,
thus the impedance is high.

Second, the principle and application of
quarter wavelength antenna

According to the theory of transmission
line, 1/4 wavelength open line is equivalent to a series resonant circuit, so
its entire load is purely resistive, in some occasions, due to environmental
factors, the length of the antenna is often limited, so there is a loaded
antenna. According to the theory of transmission line, the length of the antenna is less than 1/4 wavelength multiples of its impedance is capacitive, when the
antenna does not resonate, for this reason we can add an inductor on the antenna to
balance with the antenna, so that the antenna resonance, we call this antenna
for loading antenna.

Theoretical and practical evidence, when
the length of the antenna for the radio signal wavelength of 1/4, the antenna
transmission and reception conversion efficiency is high. Compared to
one-half wavelength, full wavelength, antenna size is much smaller, compared to
the antenna transmitting and receiving efficiency and the highest ratio of
antenna size, so commonly used quarter wavelength antenna.

Third, how to calculate the quarter wavelength

A quarter wavelength can be calculated by
frequency, the formula is: the length of the antenna (in meters) = (300/f) *
0.25 * 0.96, f indicates the frequency in MHz, 0.96 is the wavelength
shortening rate (electromagnetic wave propagation in the antenna, some factors
lead to wavelength shortening, that is, shortening effect, engineering
generally think shortening 5% or so).

→ Following this formula, we give two
simpler examples below.

VHF 144.00MHz corresponds to a quarter wavelength of (300 / 144.00MHz) * 0.25 * 0.96 = 0.5
(m)