Most people will have seen the familiar white rectangular antenna panels mounted on telecom towers, rooftops and other structures. They are a visible part of the mobile network, but what is actually inside one of these panels is rarely seen.
In a recent Wireless Future video, Prof. Emil Björnson carries out a teardown of a cellular base station antenna that was discovered in a box when his research division moved offices. The antenna operates in the 3.5 GHz band, which is widely associated with 5G deployments, but it is important to understand that this is not specifically a ‘5G antenna’.
It is a passive sector antenna. It can be used with either 4G or 5G, depending on the radio signal connected to it.
The antenna is designed to cover a 60-degree sector and would be installed vertically using mounting points at the top and bottom. A conventional macro site may use multiple sector antennas pointing in different directions to provide coverage around the site.
At the bottom of the antenna is a radio frequency connector through which the transmitted and received signals pass. The antenna does not create or process the radio signal itself. The signal is generated by an external radio unit and carried to the antenna through a feeder cable.
There is also a small drain hole at the bottom of the enclosure. Although antenna panels are designed to operate outdoors, condensation or small amounts of water may still enter the enclosure. The drain allows this moisture to escape rather than accumulating inside.
After removing the enclosure, the internal antenna structure becomes visible. The antenna contains eight radiating elements arranged vertically. Each of these elements is a dipole antenna.
The radio signal enters through the connector and is distributed through a feed network to all eight elements. The elements transmit the same signal coherently. Their individual radio waves combine through constructive interference to produce a stronger and more directional radiation pattern.
The physical dimensions of the antenna are closely related to the wavelength of the radio signal. At around 3.5 GHz, one wavelength is approximately 8.5 centimetres. Each dipole is therefore around half a wavelength long, or approximately four centimetres.
Behind the dipoles is a metal reflector positioned approximately a quarter of a wavelength away, which is around two centimetres at this frequency. The reflector reduces radiation behind the antenna and helps direct more of the energy forwards.
Metal structures along the sides of the antenna further shape the horizontal radiation pattern, limiting the main coverage area to approximately 60 degrees. Instead of transmitting energy equally in every direction, the antenna concentrates it towards the geographical area that the sector is intended to serve.
The eight dipole elements are separated vertically by approximately eight centimetres, which is roughly one wavelength. Arranging the elements in a vertical column produces a narrow beam in the vertical, or elevation, plane.
This is useful because a base station normally needs to transmit signals across the surrounding area rather than towards the sky or directly into the ground. The horizontal beam provides coverage across the 60-degree sector, while the narrow vertical beam concentrates the radio energy towards users within the intended coverage area.
According to the antenna specifications shown in the video, it has a gain of 17.5 dBi. This corresponds to a peak power density roughly 56 times greater than that of an ideal isotropic antenna transmitting the same total power.
The antenna is not amplifying or creating additional power. Antenna gain is achieved by concentrating the available radio energy in particular directions while reducing the energy transmitted in others. This improves coverage and signal strength within the sector.
The maximum input power shown in the specifications is 150 watts. The data sheet also includes horizontal and vertical radiation patterns, mounting instructions and information about wind loading. These mechanical considerations are important because antennas must remain safely mounted and correctly aligned while exposed to wind, rain and changing temperatures.
The antenna was manufactured by the Italian company Sira Sistemi Radio. Its relatively simple internal construction demonstrates how passive base station antennas have traditionally worked. One radio signal is distributed across several antenna elements to create a fixed directional radiation pattern.
The same antenna could carry a 4G or 5G signal because the antenna itself does not understand the mobile technology being used. Provided that the radio frequency falls within its supported operating range, it simply converts electrical RF signals into electromagnetic waves and performs the reverse process when receiving signals.
This is different from many modern 5G Massive MIMO antenna systems.
In the passive antenna shown in the video, the eight radiating elements collectively behave as one large directional antenna. They receive the same input signal and produce a largely fixed beam.
An active Massive MIMO antenna has many individually controlled antenna elements or subarrays, together with multiple radio frequency chains. By adjusting the phase and amplitude of the signals supplied to different elements, the system can electronically form and steer beams, serve several users simultaneously and adapt the radiation pattern as network conditions change.
Modern active antenna units may also integrate the radio electronics into the antenna enclosure. This reduces feeder losses and allows closer coordination between the radio and antenna functions, although it also makes the equipment more complex than the passive panel examined in the teardown.
The video provides a useful reminder that an antenna is not simply an empty white box. Its dimensions, element spacing, reflector, feed network and surrounding metal structures all contribute to how radio energy is transmitted into the coverage area.
It also illustrates the progression of cellular infrastructure. Traditional passive sector antennas use carefully designed physical structures to create a fixed coverage pattern. Modern active antenna systems build on the same electromagnetic principles but add multiple radios, digital signal processing and dynamic beamforming.
The complete teardown and explanation from Prof. Emil Björnson can be viewed in the video below.
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- Telecoms Infrastructure Blog: Ericsson's Massive MIMO Handbook(s)
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