Cell Phone Antennas: a look at the history and design of vhf antennas

A look at the history and design of VHF antennas

Daniel Mouw, Student Member, IEEE

The average consumer in our society has come to have very high standards when it comes to cell phones. Cell phones, which is short for Cellular Telephone, is a phone that is not tied down with a cord or the limited range of a base station. When they were first introduced in the early 80’s(www.privateline.com), they were large, bulky and had very limited service areas. Their antennas were very large and often had to be pulled out even further during operation. By contrast my current cell phone while folded up is 2cm by 4cm by 8cm. The antenna sticks out a whole 3 cm further. Even while unfolded it is 14 cm which is only because it would be uncomfortable to talk if it was any smaller. This dramatic decrease in size is the result of many different reasons. One of the major reasons is because of the shrinking size of the antenna required. In this paper I will go over the history of cell phones, go over the many issues that have to be overcome in the design of an antenna, go over basic antenna types, lastly I will show how the technology has allowed the size of antennas to shrink on cell phones.

Cell phone antennas may seem like a fairly harmless issue that has no repercussions beyond the size of the phone and the quality of signal received. This is not so. The more efficient the antenna, the less power consumed by the device which in turn means that a smaller battery can be used. Most batteries are filled with toxic chemicals. Unfortunately in the US we are not very good about recycling, so many of these will end up in landfill where they might leak and end up in ground water. Secondly, if they are efficient, they save time because the owner then does not have to search around for a good signal. While there are not any current studies, all you have to do is look at the number of people who are standing around outside buildings because they cannot get a good signal to get an idea of the size of the problem. Lastly, there is the question of whether the radiation from cell phones can cause cancer, or other ailments such as Alzheimer’s disease. With intelligent antenna design it is possible to control the output of cell phones so that the majority of signal is directed away from the head and instead to the side opposite the head. In addition, with better design it is possible to have to transmit less energy then with a different antenna and still have not have any errors creep into the data

As Christians, we are called to be stewards of the world the Lord has given us. There is many different ways in which the design of cell phones we can try to be good stewards. Because of how often new features and new protocols are introduced into the market, cell phones have become a disposable appliance. As a Christian engineer all aspects of the cell phone should be considered from durability to recyclability, to environmental friendliness. There is a very fine line between making the item overbuilt and more expensive to manufacture and too cheap where a there are unreasonable amounts of failures in the field. The bigger the antenna is the more resources it takes to build it. On the other hand, all other things equal, the larger the antenna the higher the gain is, so the more efficient it is.

The cell phone has come a long way since its introduction in the early eighties. There are two distinct types of mobile phones. Satellite phones and Cellular phones. Satellite phones have the advantage of having coverage over the entire globe. The disadvantages though are much larger then cellular phones. They also require much larger antennas. They also require quite a bit more power to operate because they have to communicate to satellites in orbit. Last, the monthly service is much more expensive because of the cost to launch satellites. The other type, Cellular phones are what most people think of when they hear the term mobile phone. These phones work by communicating with a cellular tower, which is ground based. The disadvantage is that in many rural areas, coverage is spotty at best, although in the US it is getting difficult to find an area that is not covered. The advantages of cellular phones are that they are much cheaper, require a lot less power to operate, have many more channels and lastly they can be made very small.

Satellite phones will never be as popular as cellular phones. There is many reasons for this, but the biggest one is the limited number of channels. The biggest advantage of cellular phones as shall be seen is the ability to reuse frequencies. Satellites because of the large transmission area can never do this. So while satellite phones have uses in areas such as Africa, or in really rural areas such as northern Canada, they will never be effective solution to the problem of staying always connected.

There has been several “generations” of cellular phone technology. Currently the highest tech is generation three, but they are already talking about generation four cell phones. The first generation is aptly named generation 1. Generation 1 is an analog standard. The reason for this is that previous transmission technologies tended to be analog and because integrated circuits were not small enough or cheap enough. Some examples of previous transmission technologies include FM and AM radio among other technologies. The second type of cellular phones is generation II. This generation is the first digital technology. This technology is about 10 years old. With this transition to digital came three competing standards: CDMA, TDMA, and GSM. This is the technology that is behind most of the phones that people own today run on. The next big thing is Generation III. With this new set of standards comes much higher data rates which gives phone companies opportunities to sell extra features like never before. This is just coming out right now. Lastly on the horizon is Generation IV. Right now there is no silicon that implements generation IV technology, but they are working on developing the protocols that will support all of the features they feel they need for the “killer apps” which drives consumers to buy the phones and pay the phone companies ever larger sums of money.

The first commercial cell phone system started in the USA in July of 1978 in Newark, New Jersey and in trials around Chicago.(privateline.com). This is the first time that AMPS was used. AMPS stands for Advanced mobile phone service. AMPS is the technology that most people think of when they think of analog phones. In the USA this was the standard, but in other countries they used many different standards including Aurora-400 in parts of Canada, C-Netz in German, Austria, Portugal and Africa, Comvik in Sweden, ETACS in U.K., JTACS in Japan, RadioCom in France, TRMS in Italy, and TACS in UK, Italy, Spain, Austria and Ireland.

AMPS works from 824 to 894 MHz. This is split up into two separate bands from 824 to 849 MHz and 869 to 894 MHz. The lower band is for transmitting from the phone to tower and the upper band is for transmitting from the tower to the cellular phone. This bandwidth is split into 832 channels 30 kHz wide(Privateline.com). Each channel has a transmit and receive frequency that is exactly 45 MHz apart. So if you get make a call on a cell phone and are assigned channel 333 you transmit on 879.990 MHz and you receive on 834.990 MHz. In figure one below, it shows the layout of the channels.
Figure 1 Layout of the channels in the AMPS Cellular system. The A system was assigned to one provider and the B to a competitor to prevent a monopoly.

On top of being split into 832 channels, the spectrum is also split up into A and B channel sections. When the frequency spectrum was allocated, it was split into two sections in each area. One section was given to a Baby bell the other one was given to another company that met all of the requirement and won the lottery. The reason behind splitting the spectrum was so that there was competition between the carriers unlike the monopoly that had developed on landline telephones. The lottery came about because there were so many companies that had met all of the requirements set out by the FCC that they had to just randomly assign the spectrum to a lucky qualified company.

What made this system work was frequency reuse. In previous systems, the transmitter power was very large and one antenna would cover a whole city. The problem with this is that there is very few channels available for use at one time. This means for example that only 800 people in all of Chicago could be on a cell phone at one time. Another problem with this method is power use. Because the communication tower covered such a large area, it required high power transmission to guarantee a good signal. While this would not be a problem for the tower, this would be a problem for the handset. As a

Figure 2 this is an example of an early cellular phone. The main thing in the suitcase is the battery.

result the efficiency of the antenna had to be greater, and the battery was used up much quicker. This is the reason for the large size of phones before 1980. In figure 2 you can see an example of a phone that has the battery in a suitcase.

In comparison in a cellular system, each tower will only cover a few miles, but if you place enough of them close enough together, you are able to get the same coverage. In addition, the secret behind how cellular works is that while adjacent cellular towers cannot reuse frequency, the cellular towers beyond the neighboring cellular towers can. It is not that simple of course, A lot of work has to be done to ensure that there is no overlapping of the channels which will cause

interference and dropped calls. This makes it possible to field hundreds thousands of calls in a large city instead of merely thousands of calls. It can do this by using ever smaller cells. There is a limit to how many channels it is possible to add to an area though. Also it takes quite a bit of effort to add channels to any cellular tower. As Mark van der Hoek says on www.privateline.com: Re-tuning a site in a congested downtown area is not trivial! An engineer may work for weeks on a frequency plan just to add channels to one sector. It is not unusual to have to re-tune a half dozen sites just to add 3 channels to one”.

Another important part of the technical detail that was determined during the analog stage was the design of cells. When thinking of the different cells that make up a cellular system, it is easy to think that the cell consists of the area around a cell tower. Such as the solid lines in figure 3

But actually the cell is the dashed line that connects all three towers. The reason for this is that in each cell certain channels are available. When you switch from tower to tower, the channel stays the same so there is less for the phone to do, but when you switch cells, you stay with the same tower, but different channel.

There are several reasons that a hexagon is used for the cell. The most important reason is for frequency reuse and for efficient handoffs while driving (www.Privateline.com). With a hexagon it is very easy to make a frequency reuse pattern of seven. This is approximately 60 channels per cell per provider for analog service. The second reason is because cell phones are often used on roads and either the cell phone is forced to hand off frequently between the two towers or there is not efficient use of the frequency spectrum.

The second generation of Cellular technology was a switch from analog circuitry to digital circuitry. There is three main standards for generation two: GSM, TDMA, and CMDA. Each of these is a digital standard and to understand these technologies, first you have to understand why the switch to digital.

There is three main reasons for the switch to digital. First, the phones use less power. Second, to many people the signal seems to be better quality. Third, technology had advanced enough to make digital transmissions practical. Fourth, and most important to the phone companies, it is possible to have many more people on a digital system.

The first advantage of a digital cellular phone system is that the phone use less energy. The is reason for this is because a digital signal does not have to be transmitted at anywhere near as much power because the signal does not have to drown out the noise. In analog cellular technology, as the signal strength got weaker, the noise went up, to counteract this, the transmitting power was much higher. When you use digital technology though, you can quite a bit of noise and still not even hear it. This lower power is the biggest reason why cellular phone antennas are smaller than for analog phones.

The third reason for the switch to digital is because the technology had advanced enough that it was practical to build digital phones. Before this time it was impractical to build digital phones because the size of the circuit board would have been so large to fit all of the computing power necessary to make the phone unwieldy.

The fourth and most important reason by far is the fact that going to digital technology increased the number of channels three fold or even more depending on the technology used. (www.priviteline.com) This is important because cells were running out of room. As you can see on any college campus in the nation, the cellular phone business is booming.

Generation II phones are one of three different standards: CDMA, TDMA, or GSM. CDMA stands for “Code Division Multiple Access”. Code Division Multiple access works by everyone using the same frequency, but assigning each phone a different code and spreading the transmission over a much larger frequency spectrum, 1.25 MHz compared to 30 KHz. TDMA stands for “Time Division Multiple Access”. This technology works by splitting the transmission channel up into three equal bands and each phone only transmits in one of these time divisions. This had the affect of tripling capacity, while using the same channels as the earlier AMPS service. Staying within the channels used by AMPS is very important so that the switch from a analog service to a digital one could be made in steps instead of all at once. GSM is the standard in Europe and the acronym has two meanings. Its original meaning was “Groupe Speciale Mobile”, but now it is known as Global System for Mobile Communications.(www.privateline.com) This is a standard that was developed by European countries in the early 80’s in order to consolidate the fragmented cellular market that had developed with analog cellular technology. GSM uses a type of TDMA. GSM is the most advanced of the three competing standards

TDMA is the simplest of these three standards. In order to maximize the use of the spectrum, each of the AMPS channels is split into three separate channels except instead of splitting the frequency in three, each cellular phone only transmits one third of the time. Below in figure four you can see how the time is split up among the three transmitters.

Figure four shows one from frame from a TDMA signal. Each call would consist of two blocks. The reason it is split into two blocks is most likely to make sound output smoother.

While it is called TDMA whenever it is referenced in popular literature, the actual format is called IS-136. TDMA is merely the style of transmission used by IS-136. There is many different implementations of TDMA (GSM is actually one as well), IS –136 is the technical document that states what the phones have to do to connect and to transmit on these networks.

GSM is a specialized version of TDMA. GSM was developed because when cellular phones started appearing in Europe each country had its own standard if not multiple standards that were not interoperable. This made roaming on these networks very difficult. In 1982 it was decided by a consortium of 26 countries to build the next generation standard so that you could roam anywhere in Europe and get good coverage. GSM is probably the best developed standard because of the number of countries and amount of time spent developing this standard.

CDMA is the most complex and spectrally efficient of the cellular technologies standards at the moment. (www.arcx.com). The official name name of the CDMA scheme used in cellular telephones is IS-95B. This system works by having everyone transmit on the same frequency. Every new person just adds more noise to the system. One of the problems with this scheme is that there is no defined maximum to this scheme. Every person just keeps on making the system a little noisier. The channel is defined as full when it cannot add more users without making the circuit too noisy.

One of the best features of this system for the user is soft handoffs. Since all towers operate at the same frequency, the only thing that has to change in a handoff is the pseudo random code. This significantly reduces the number of dropped calls during handoffs.

The next evolution for cellular phones will be generation III technology. These are a set of technologies that enable much higher data rates, so such features as streaming video will be possible. The three different competing standards are CDMA2000, WTDMA, and a new version of GSM. Each of these schemes has data rates in the 100 of kilobits per second that will enable many new features and make such features as cameras easier and more enjoyable to use. While increased bandwidth comes at the cost of fewer users per MHz, cellular companies are finding out the real money is in extra features.

Even though generation III phones have not yet hit the market, they are already trying to develop generation four technologies. This will most likely involve even higher data rates and possibly even video phones. The days of Dick Tracy style communicators might not be far off.

Antenna Design

Antenna Design is a complex and difficult task. There are several major decisions that have to be made. First step is to determine what the transmission circuit has to do. How far does it have to transmit? How much power needs to be at the receiver to determine the signal? Is it mobile? How many receivers? How big can the receiver be? Where does it get its power? This is a sampling of the many questions that have to be answered before the design of an antenna can even be started. In order to make the design of antennas easier, the different properties of antennas has been the simplified to equations and to graphs so that the many tradeoffs inherent in antenna design can be effectively weighed.

After the introductory questions have been answered and you have an idea of what is required of the transmission system, the first step is to choose the type of antenna selected. This is dependant on several things including space available, wavelength, desired directivity and many other things. The second area that has to be looked into is the transmission line. There are also many different types available from twisted pair to coaxial cable to fiber optics to various strip transmission lines. The impedance of the transmission line should be matched to the antenna to minimize the Standing Wave Ratio (SWR). In cell phone applications, the most obvious choice would be one of the different strip transmission lines because they are very easy to integrate into printed circuit boards. This is not an easy task because every trace on the board has an effect on the ability for the strip to transmit the signal without loss. The other technologies such as coaxial or twisted pair would most often be used for longer transmission distances (greater than one foot). . The last step in antenna design is to match the impedance of the transmission line to the impedance of the antenna. This is the most important step in the design of antennas. Even if you have the best antenna in the world, if all of the energy that is sent to it is just reflected back because of a bad impedance match, then your system is worthless.

There are literally hundreds of different types of antennas that can be designed. From whip antennas to microstrip antennas to dipole antennas to helical antennas, each of these has advantages and disadvantages. There are a couple of important features for each antenna. The first is gain. This is the maximum amount of radiation compared to a average power in all directions times the efficiency (Setian p. 311) or in equation form:

The second important feature is directivity. This is the measure of the how even the radiation pattern is. The higher the directivity is the smaller the pattern it radiates. A good example of this is a 60 Watt light bulb has a really low directivity while a laser point has a really high directivity. The equation for directivity is:

and when the efficiency of the antenna is 100% then the gain is equal to the directivity. A third important feature is the bandwidth of the antenna. All antennas transmit one frequency best, but the width of their 3 dB points can vary remarkably with the type of antenna selected and with the method selected to match the impedance of the transmission line. The last feature that is important for all antennas is the radiation pattern. Directivity and gain only give the value for one point while the radiation pattern gives shows all values along a plane. Normally this plane is selected to be normal to the desired direction of the antenna.

A third important feature of an antenna is the bandwidth of the antenna. With today’s swamped airwaves, it is often necessary to be able to use multiple frequency band in order to minimize noise. An example in the cellular telephone arena is that often phones are built to be able to use many different networks that will operate at different frequencies. In order to maximize coverage phones will be able to receive both lower frequency analog and digital signals and higher frequency PCS digital signals. The impedance of the device depends on the frequency that is being received. Antennas include capacitive and inductive reactances and these change with frequency.

In order to compare the various antennas the bandwidth factor is used. To get the bandwidth factor the equation below is used: (Setian p. 145)

To get the high frequency and low frequency normally a maximum SWR is used. SWR as mentioned before is the standing wave ratio. This is the ratio of the highest voltage to the lowest voltage. The best SWR is a SWR of 1. There are a couple of reasons why you want to keep this value as low as possible. This ratio gives a good idea of how much energy is being reflected back to the source instead of being transmitted (or in the case of a receiver, received). So the higher the value of the SWR, the less energy that is being used efficiently by the circuit. Also if this value is too high then there is a chance that you might damage your transmission line. The voltage could get high enough to break down the insulation of the transmission line and then short out the transmission line or start a fire. For example, for a broadband antenna an acceptable SWR is 2. Below is the SWR graph for an actual antenna

Figure 5: This is an example SWR curve for an actual antenna for sale in the USA

The center frequency of this antenna is around 158.5 MHZ and if an acceptable SWR of 1.5 is chosen, it has a BWF of 3.4 %

Another important feature of an antenna is whether it is electrically short or long antenna. Depending on the size of the antenna, it is either electrically short or electrically long. An electrically short antenna is one that the length of the antenna is short compared to the wavelength of the transmitted wave. Normally to be considered an electrically short antenna, it should be ¼ wavelength or shorter. Electrically long antennas are any antennas that are not electrically short, so any antenna that is over ¼ wavelength. Whether an antenna is electrically long or short has a huge effect on the properties of the final antenna.

Electric length is not always the same as the physical length of the antenna. Because the velocity of the wave is dependant on the electrical permittivity and the magnetic permeability of a material, Two antennas could be the same length, but one could be electrically short and the other electrically long. This number is also important because the electric length is used in many equations. The equation below relates the electric permittivity and magnetic permeability to velocity.

Using this equation, the wavelength of the signal in the antenna can be calculated and the electrical length calculated. One of the most important things that can be inferred by the length of the antenna is whether the antenna is capacitive or inductive. If it is an electrically short antenna, it is capacitive, while an electrically long antenna is reactive (has an inductance associated with it).

One last important aspect of antenna is the radiation resistance of the antenna. The lower the resistance, in general, the lower the power transmitted over the air wave. Instead of the energy being transmitted it is instead turned to heat energy in capacitors and inductors in the tuning circuitry. The ideal radiation resistance is equal to the impedance of the transmission line that connects the antenna to the circuit.

Antenna Types

One of the simplest antennas is a dipole antenna which you can see if figure 6 below.

The performance of this antenna depends mostly on the length of the two poles. In the figure above they are ¼ wavelength, but they can be any length. Common lengths include ¼, 3/8 and ½ wavelength. As a general rule, the larger the antenna is the greater the radiation resistance. Radiation resistance is defined as (Setian p. 265):

The equations for radiation resistance even for this small subset of antennas is different depending on the ratio of antenna length to wavelength. For very short dipoles the equation is (Setian p. 267):

This equation is useful because often wavelengths are in meters and it is impractical to make an antenna near the wavelength.

Figure 7 Elevation pattern for 1.5 wavelength antenna

The radiation pattern is also highly dependant on the length of the dipole. On the previous page is the Elevation pattern for a 1.5 wavelength dipole (Setian p. 327) and here is a Azimuth pattern for a ½ wavelength dipole

Figure 8: Azimuth Pattern for ½ wavelength dipole

The equation for this pattern is

As you can see the directivity and gain of the antenna is highly dependant on the size of the antenna.

damage and it is becoming unfashionable to have to pull out an antenna, so they are going out of style.

One of the most popular antennas on the market today for cells phones is the Helical Antenna. A helical antenna is formed by bending a antenna in the form of a helica. In figure 9 it shows a drawing of what a helical antenna would look like if you could see the radiating elements.

The two biggest advantages of helical antenna is size and radiation pattern. Some disadvantages they have is that the SWR is not as good as you can see in figure 10 below.

Figure 10: This is the SWR for ¼ wavelength Helical Antenna. It is designed to be used around 460 MHZ

Insert figure.

Figure 11: SWR of ¼ Helical

The whip antenna is one of the simplest and most popular antennas. In figure 12 you can see an example of a common whip antenna. These antennas are often used in wireless applications, especially in 900 Mhz cordless phones and in early model cellular phones. They are not as popular today in cell phones because they are quite large.

Two of the most common sizes used in actual products are

F
igure 12: Whip antenna and Helical Antenna. Not equal scale

¼
is much worse then both dipoles and compared to the 3/8. Figure 13 SWR for 3/8 whip antenna

Figure 14 SWR ¼ wavelength Whip Antenna

In the late 90’s, the most popular antenna was the retractable antenna. This antenna combined small size with reasonable gain as well. The biggest problem with these antennas is that they do not stand up well to the abuse that many people dish out to them, so they had problem with breaking. The other problem is that people are often do not care about the technical merits of a device as much as they do what it looks like. These antennas when extended have very good reception and they have a low SWR. Unfortunately when down they suffer from either high SWR or bad gain. Figures 15 and 16 in appendix A show one example of a ¼ wavelength retractable antenna while Figures 17 and 18 in appendix A show a 3/8 wavelength antenna.

The most important part of antenna design is matching impedance of the circuit, the transmission line, and the circuit. Every antenna has a characteristic impedance. This is dependant on many different factors from the physical resistance of the antenna to the stray impedances to the capacitance between the antenna and ground. According to the Max Power Transfer theorem, the most power is transferred to the load when the source and load impedance are equal. Another reason to match impedance is to minimize the SWR or standing wave ratio. If impedances are not matched, then some of the power will be reflected back to the source. There is a direct correlation between SWR and power absorbed by the load. The Standing Wave Ratio is the ratio of the max voltage on the line to the input voltage. If this gets too high, then it is possible to damage the transmission line.

To match the impedance of the transmission line and the antenna capacitor and inductor networks are used as well as resistors. The problem with using these is they absorb energy, so the efficiency of the circuit will not be as high as it could be. The other method to change the impedance of the circuit is to physically change the circuit. Such as adding a top to the antenna or making the antenna longer.

In conclusion, I have learned a lot about electromagnetic from this research project. I have also learned that the reason cellular phone antennas have got smaller is not because of changing frequencies, but because of the switch to digital technologies and the ever increasing sophistication of electronics. Designing an antenna is a very difficult task that involves a lot of mathematics. There is a multitude of different parameters that have to be balanced together in order to produce the best antenna possible. This paper also gave me an appreciation for all of the work that the IEEE does in forming standards. This is an important duty, and the state of analog cellular in Europe in the late 80’s show what happens when there is no standard.

[0]Setian, Leo. “Antennas with Wireless Applications”, Prentice Hall 1998

[2]Johnson, Richard C. “Antenna Engineering Handbook”, McGraw-Hill 1984

[3]Hayt, William. “Engineering Electromagnetics”, McGraw Hill 6^th edition 2001

[4] “Privateline.com: Cellular Telephone Basics” by Tom Farley http://www.privateline.com/Cellbasics/Cellbasics.html

[5] “Poynting - Helical Antenna Design Curves” http://www.poynting.co.za/tech_training/helical.shtml

[6] “Privateline.com Mobile Telephone History: Introduction” http://www.privateline.com/PCS/history.htm

[7] “MA 2-1 K”

http://www.ers.fr/gbma21k.htm
Biography

Daniel Mouw received his BSE from Calvin College in May of 2004 with a concentration in Electrical. He has previous experience working at Innotec Lighting Group. He is a student member of the IEEE