Mobile phone antennas and their propagation characteristics

mobile phone antennas and their propagation characteristics

This page is split into several sections on issues related to mobile phone antennas and propagation. As a licensed radio amateur for over 31 years I want to dispense with much of the theory on electromagnetic and radiowaves and focus on the 'headlines'. The 'headline' for microwave radiation can be stated very clearly: if you are line of sight to a transmitting antenna and within a distance of about 450m, you are being exposed to potentially dangerous levels of microwave pulsed radiation. It is also important to understand that for some individuals it is not the intensity of the signal that has damaging effects on their biology, but purely the pulsed nature of microwave radiation. The pulsed component of the microwave signal is the 'intelligence' that is superimposed with the microwave carrier signal. There are many different types of intelligence modes for data, voice and moving images. Suffice to say, some of these signals are of extremely low frequency and are similar in frequency to varying consciousness or alpha states typical of brainwave activities.

Figure 1: Typical radiation pattern from a mobile phone tower under ideal conditions(1)

Figure 1 illustrates a typical radiation pattern from a mobile phone transmitter. These ideas will be developed later in this article but for now we need to visualise and understand the shape, pattern and distribution of a radiowave as it is emitted from a mobile phone tower. The figure also illustrates signal intensity i.e. the most intense signal is represented by the dark blue areas and signal intensity decreases for the lighter blue areas with distance from the transmitting antenna.

Firstly, however, notice that the signal intensity of microwave radiation directly under the antenna appears very low and this is because the signal needs a certain distance from the antenna (based on its wavelength) to develop. The signal gets stronger from about 3m directly away from the transmitting antenna. Highest signal intensities occur between 50 m - 300 m from the transmitting antenna. This obeys the inverse square rule i.e. signal intensity decreases with distance from the transmitting antenna. It is also important to note that there is a maximum distance between the transmitting antenna and its roaming target audience of individuals using mobile phones. The distance of about 200 - 300 m between transmitting antenna and receiving antennas ensures a good signal yet if this was obeyed in reality, it would mean the distance between mobile phone transmitting antennas should be no further apart than about 300 m. This means a mobile phone mast on almost every corner.

Also note that most mobile phone manufacturers tilt their transmitting antennas 5 degrees towards the ground relative to the horizon. In general, this ensures maximum signal intensity towards ground level and not towards the horizon. The physical conditions for efficient operation of signals emitted in the microwave spectrum are (a) height is might i.e. install mobile phone transmitting antennas as high as possible and (b) there should be nothing in between the transmitting antenna of the mobile phone mast and the receiving antenna inside the mobile phone. These represent ideal physical conditions for the transmission and reception of pulsed microwave radiation. The reality is that signals are enhanced or denuded due to atmospheric conditions and these factors will be discussed shortly.

Figure 2 illustrates a case study, which occurred in Mumbai (India) when a consortium of mobile phone companies installed 4 mobile phone transmitting antennas on a rooftop. Directly across the road from these antennas was a tower block in which individuals lived at various heights above ground. They were exposed to 'line of sight' pulsed microwave radiation at high signal intensity. Six cancer cases were discovered and they corresponded with height above ground to the radiation pattern from the 4 mobile phone transmitters.

Figure 2: Case study of induced cancers following installation of mobile phone transmitting antennas

Propagation modes for microwave frequencies, diffraction, refraction and reflection

As noted previously, the signal transmission characteristics of microwave frequencies are relatively short and out to about 350 - 400 m from the transmitter antennas. The signals propagate most efficiently through 'line of sight'. Under certain atmospheric conditions, signals can be enhanced or reduced. When it rains, raindrops decrease signal strength between two points and under exactly the same atmospheric conditions, the same signal can be enhanced via 'rainscatter' (see Figure 3 below). Similar phenomena can also be observed for TV/satellite transmissions in which fading or complete breakup of the picture occurs during rainstorms. Frequencies around 2.4 GHz are attenuated whilst frequencies around 10 GHz and above lead to both signal attenuation or signal enhancement.

Figure 3: Rainscatter of microwave frequency signals

Microwave signals can also be reflected off the ground, buildings and other environmental substances to both enhance the signal intensity at a much further distance than intended. In other circumstances this can lead to 'multipath' effects i.e. two radiowaves 'collide' with each other and cancel each other out (see Figure 4 below). Much however, depends on what is actually between the transmitting and receiving station and this will be explored a little later.

Figure 4: Signal reflections off the ground or environmental media

Besides signal reflections, we also have signal diffraction and refraction. Figure 5 below illustrates these environmental factors.

Figure 5

Figure 5 illustrates how signals can either be enhanced in intensity beyond their stated communication range or significantly decrease in signal strength. The radio signal has been diffracted within what is known as 'knife-edge diffraction' off a mountain top. In effect, the radio signal has been 'bent' back towards ground and in doing so, the range of this signal has been increased. Diffraction can also occur within the lower atmosphere i.e. diffracted off dust particles, rainscatter, tall buildings etc. and it is almost impossible to model or theorise how good or bad the signal might be between transmitting and receiving antennas.

The final image I show relates to 'line of sight' and its associated Fresnel Zone. The Fresnel Zone is all physical and environmental objects, trees and other biota between the transmitting and receiving antennas. In order to visualise this, look at Figure 6 below.

Figure 6: Line of Sight Communication and the Fresnel Zone

The top image illustrates 'line of sight' (LOS) between a transmitting and receiving antenna. The Fresnel Zone is below the LOS which means there is little reflection, diffraction or refraction of signals into the Fresnel Zone. The middle and bottom images show some profound effects on the LOS as the microwave signal is emitted towards the receiving antenna. These two images are more representative of the urban environment in which millions of people find themselves.

To sum it all up, there are physical and environmental factors involved in trying to elucidate a signal path between a microwave transmitting antenna and its receiving antenna. There are too many variables to make sense of and if you include absorption of microwave signals as they pass through wood, glass, concrete, buildings and trees etc., it is clear that there is only one way in which to measure microwave signal intensity and that is with use of a suitable meter. Characterising microwave signals will be discussed later, but first I finish this section on a brief description of the antennas found in mobile phones.

Typical antenna design used in mobile phone handsets

The wavelength of the microwave bands is very short and antennas lie within the range of several centimetres long. Correspondingly, antennas found in mobile phones are short and can be made of fixed strips of aluminium. Some mobile phones operate over several frequencies e.g. generally around 900 MHz, 2.2 GHz and even at 5 GHz. The higher up in frequency you go, the shorter the antennas. As the physical size of antennas decreases, their relative capture area also decreases. The various antennas must also be matched to their frequency of operation and besides automatic switching of antennas to frequency, there must also be a means of ensuring that as antenna efficiency drops (for whatever reason), the output power of the phone decreases accordingly. The technology behind these devices is complex and beyond the scope of this article. Suffice to say that antennas and their operation need to be well thought out because if the wrong antenna is switched in during transmit or the antenna itself shorts across itself, transmitted power will flow back down the antenna and heat up the mobile phone itself. This obviously means that if the mobile phone is next to your skull, this part of your face will also heat up beyond power densities that were authorised. A typical design is shown below.

Figure 7

Mobile phone antenna design

A typical design of mobile phone uses an antenna called an IFA or inverted F antenna. If you look at the drawing on the right from Figure 7 you can see the big copper coloured area comprising the ground plain and at the bottom of this is a strip of metallic wires in the shape of an F that has been rotated 90 degrees to its left. This particular antenna is known as a dual band because it is used at 900 MHz and 1.8 GHz frequencies. The matching impedance for these antennas is nominally 50 ohm and the VSWR (voltage standing wave ratio) varies to about 75 ohm or 1.5:1 VSWR. The drawing on the left is an actual drawing of the antenna setup itself. You can see how they utilise a longer arm on the F structure for the 900 MHz antenna.

My only question on these types of antennas relate to a change in antenna impedance when using long leads from a headset. If the negative side of the headset cable earths itself into the ground plane of the antenna system itself, will this alter antenna impedance? In effect, does the headset represent lumped inductance? If the addition of a long wired headset does alter antenna impedance, what is the magnitude of change within its impedance and whether or not the transmitted signal ends up as heat within the mobile phone itself?

Mobile Base Station Transmitting antennas

Figure 8 gives a glimpse of what is found inside the normally closed outer case of a mobile base station antenna. There are 6 antennas or radiating elements fed in phase to produce an electromagnetic wave.

Figure 8

Sector antennas are so-called because these types of antennas are directional with a beam width between 60 degrees and 90 degrees. They are mounted on a tower in a particular way because each sector of 120 degrees has 3 antennas, normally one transmit antenna in the middle and two receive antennas either side (see Figure below). Note, RX means receive antenna and TX is the transmit antenna.

Figure 9: Layout of sector antenna used as mobile phone transmitting and receiving antennas

There is really not too much to say on mobile phone base antennas other than a requirement that the antenna be installed as high as possible. Please see image below of a mobile phone installed on top of a church roof. Some months back I sent a letter of protest to the highest orders of the church or those people responsible for the maintenance of church buildings. I sent a lot of information (see here) on the latest developments with regard to medical evidence of cancer induction, microwave frequencies being classified as a Class 2B carcinogen etc. The response from the church was "so what". In other words, it is we the people who have to look after our health in relation to irradiation of our bodies from mobile phone antennas and WIFI etc. We must bring to account all those organisations that currently take monies from the telecom giants to install mobile phone antennas on properties they are responsible for. The same goes for English Heritage. They are taking monies from the telecom industries to allow them to install mobile phones and satellite links on properties supposedly managed by this organisation. A petition should be launched in which people refuse to visit these historic buildings unless and until they take these cancer-inducing pulsed microwave emitting antennas down. This is not rocket science and later on I will produce a legal document that anyone can send to their local church vicar and organisations like English Heritage which will hold them personally accountable for any adverse biological effects from mobile phone transmitting antennas installed on their properties. Its that simple and if we do not do anything, who will? If not you, who?

Bibliography

(1) Radiation Hazards from Cell Phones/Cell Towers, Prof. Girish Kumar, Electrical Engineering Department, IIT Bombay, Powai, Mumbai (022) 2576 7436 gkumar@ee.iitb.ac.in

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