Antenna Stuff

Recently I spoke to a sales engineer of Kathrein, a prominent German antenna maker that develops and produces all sorts of antenna equipment from TV antennas to sophisticated cellular network antennas. I can still remember how simple antennas were 10 years ago when GSM was first deployed: In many places simple dipol antennas were used and sometimes funny looking trident antennas (that’s how I call them anyway, I am sure they have a more scientific name…). In the meantime we have mostly moved to bipolar antennas that offer a main and diversity output to the base station. On top of that, lots of other things have been developed which are either now being deployed or waiting for that 4G bandwidth push that requires sophisticated antenna features:

Dual band antennas, e.g. 900 + 2100 MHz in one standard casing for sites with GSM and UMTS base stations: Such antennas give themselves away with four connectors at the bottom.

Wideband antennas, e.g. 2.1 – 2.5 GHz to support UMTS and LTE with a single antenna: I am sure those will be in high demand once LTE is deployed in the 2.5 GHz range.

Cable reduction: To reduce the number of expensive coax copper cables between the base station and the antennas, combiners have been developed to combine the signals of several base stations, send them through a single cable and then separate them again before they go into the different antennas.

Remote Electrical Tilt (RET): The size of a cell mostly depends on the angle of the antenna at the rooftop. The more it is tilted towards the ground, the smaller the coverage area. When a new base station is installed to increase the availble capacity in an area, it is necessary to change the tilt of neighboring antennas to reduce interference. Also as capacity in the network increases, it is also sometimes necessary to change the tilt of antennas to improve the overall coverage and bandwidth distribution. Manually changing the tilt of an antenna for these scenarios is expensive and sometimes simply not possible. This is where RET comes in. Instead of physically changing the angle of the antenna, RET changes it by increasing the lengths of the different antenna elements inside the antenna casing. This way, the overall antenna can be electrically tilted by around 10 degrees. Practically, changing the antenna lengths is done with an electrical motor that drives a spindle inside the antenna casing which can change the antenna lenghts of the different modules. The electrical motor is an add on module at the bottom of the antenna.

Antenna auto adjust: It can also be imagined that RET is used in the future to automatically ajust antenna angles based for example on the time of day. This could help to increase coverage in certain areas at certain times of the day by decreasing the cell size. Things could be pushed even further by linking the RET mechanism to the load of the cell and increase the tilt when the cell gets busy to offload some of the borader traffic to neighboring cells.

MIMO and Beamforming: And then there is Multiple Input, Multiple Output and beamforming for further bandwidth increases that requires several antennas at the rooftop. In practice, they are again included in a single casing to look like a single antenna to the outside. The first 2×2 MIMO systems will use little crossed antenna elements that send each MIMO channels with a vertial and a horizontal polarization.

All highly interesting and shows how important antennas have become for increasing bandwidth in the future. Thanks to Kathrein for the interesting information!

3 thoughts on “Antenna Stuff”

  1. Hi Martin,
    thnks for this post on Antennas. Can you tell me what would be the length of an antenna for GSM and UMTS bands. Bcoz when i opened my dual mode phone, i found this antenna is more length and got many twists and turns. Roughly it could be around 7-10cms. What is the critreia behind this length and how they moulded it in bottom of phone?


  2. You have provided some nice information regarding Antenna's. Can you please tell me the main difference between the technology of indoor and outdoor antenna.

  3. Hi Srini,

    antennas in mobile phones are a form of black art. In theory the length of the antenna should be half the wavelength. At 900 MHz, the wavelength is 33 cm, or about 1 inch. In the phones, the antenna is shorter though. I am no expert on antennas so I can’t tell you why and how that is done.


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