MobileWorld's Antenna Information

Car Antenna

Getting energy out from a transmitter and in to a receiver is critically dependent upon the ability of the transmitter to pass energy (radio signals) from its antenna to free space, similarly the same is true of a receiver.

There are a number of  factors involved including:-

• Frequency (wavelength).
• Gain.
• Impedance.
• Polarisation.

Frequency

Each antenna has a resonant frequency, the frequency at which it is most efficient at either transmitting or receiving energy. The resonant frequency is set by the physical length of the antenna.  Frequency and wavelength are related, the wavelength (in metres) is equal to the speed of light (in metres/sec) divided by the frequency (in Hertz - Hz). 

Similarly the frequency is equal to the speed of light divided by the wavelength.  So in the good old days when Radio 4 was the Long Wave it transmitted on a wavelength of  1500m.  The speed of light is 300,000,000 metres a second so 300,000,000 / 1,500 = 200,000Hz or 200 kHz.  Go find an old radio and you will find 1500m on the dial, newer ones have 200 kHz (and yes, thank to some interfering French politicians Radio 4 is now on 198 kHz which took away a lovely stable frequency reference - but that’s another story).

A frequency of 1800 MHz (GSM 1800) equates to a wavelength of:-

300,000,000 / 1,800,000,000 = 0.167m

or a wavelength of about 16.7cm.  At 900 MHz everything is twice as big, so 900 MHz gives a wavelength of 33.4cm. Antennae are usually referred to by the fraction of a wavelength represented by their physical length, so a full wave antenna at 1800 MHz would be 16.7cm long (In practice it would be a slightly different length to allow for corrections for end effects).  A half wave antenna at 1800 MHz would be 8.4cm and so on.  Most phone antennae are about 1/4 wavelength long.

Gain

The basic pattern of energy coming from a “perfect” antenna with no gain is a bit like a ball (with the antenna in the middle), the antenna radiates equally in all directions (the “isotropic” antenna).  This isn’t always what is wanted.  In most mobile phone antennae you want most of the energy coming out near the ground and not too much going vertically into space.

A standard dipole radiation pattern is not isotropic - it looks bit like a doughnut with the antenna in place of the hole. 

An antenna can only put out what is put in to it, so when you see adverts for antennae with “gain” (for example 3dB gain) what it means is that the energy is being directed more in one direction than others (It also means the area the energy was redirected FROM will get less.)

Going back to the doughnut, if you press down on the top of the ball it gets wider and shorter, the wider axis is showing gain, the shorter one loss.

You can also put directivity in the azimuth pattern - but for phones this is not a good idea!  The most common antenna with gain in azimuth is the common TV antenna (a Yagi antenna design for the curious) which typically has a beamwidth of about 15 to 20°.

Antenna gain is usually expressed in decibels and refers to the gain of the design over the radiation in that direction given by a perfect isotropic antenna or a dipole.  As the isotropic antenna and dipole differ anyway it is important to know which is being referred to when comparing antennae.  Usually if antenna is described as having “3dB gain” it means compared with a dipole. If it says “3dBi gain” it means compared with an isotropic radiator.

The most common mobile antenna design to show gain is the co-linear.  In most cases this will give about 3dB gain over a dipole.  Treat all claims for greater gain from non directional antennas with severe suspicion!

Impedance

Impedance is to AC circuits roughly what resistance is to DC circuits (OK - I know that’s a shelf full of text books dismissed in one line!).   It isn’t just the length of the antenna which matters but also how you get power into it.  For maximum transfer of power the source, transmission line, and load must all have the same impedance   In the case of your phone this means the phone, antenna lead, and antenna should all have the same value of impedance. 

This value is 50 ohms for most phones so the transmitter and receiver in the phone have a 50 ohm characteristic impedance, the cable is 50 ohms and the antenna impedance should be 50 ohms. 

At the base of a 1/4 wave antenna the impedance is indeed about 50 ohms, however at the base of a 1/2 wave one it is several thousand ohms.  Making dual frequency antennae (for use on both 900 and 1800 MHz) is a compromise between length, thickness (which also affects impedance) and gain.  Nearly all dual frequency antennae will work quite well at one of the frequencies and less well at the other.  All are outperformed by single frequency antennas.

Polarisation

Polarisation is the alignment of the electrical part of the radio frequency energy in space.  A vertical antenna produces a vertically polarised signal, a horizontal one a horizontally polarised one, and a spiral antenna a circularly polarised one (left or right hand depending upon the way the spiral goes).  In theory a horizontal receiving antenna will receive no energy from a vertical transmitter antenna (and this works - many continuous wave tracking radar’s use a left hand circularly polarised signal to transmit and a right hand one to receive so they can transmit and receive on the same frequency at the same time.

However we all know the phone still works lying on the table - so what happens?

The signal from the transmitter strikes many objects along its way and is reflected from them, these reflections are often twisted because of the irregular nature of the reflecting object.  By the time the signal reaches you it has lost much of its initial polarisation and become scattered.  However it will usually still be the case that most of the signal will maintain its original polarisation and the more vertical you keep the antenna the better your chances of a good signal.

Special Antennas and Signal Amplifiers.

The Co-linear

The true co-linear design is a series of dipoles stacked end to end and fed by different cables such that the radiation patterns inter-react to give a lower angle of radiation with more power in the lower angles than the higher.  The antenna called a colinear in mobile phones achieves a similar effect by being partial multiples of wavelengths long and having tuning and loading coils built in ( the single coiled twist in the 1800 MHz antenna shown above and the thicker tube about 1/3 of the way up the 900MHz antenna.  The extra length of the co-linear explains why your antenna is longer than you expected based on the calculations at the top of this page.

The Yagi

The Yagi antenna design is probly the most common antenna with gain - nearly all TV antennae are Yagis.  Its use in mobile phones is very limited because it gives directional gain in azimuth - you need to know where the base station is and point at it!  However it does have its uses, models for 900MHz are made mainly for the Nordic market where mobile phones are the communication method of choice for the popular remote weekend houses.  Fitted to a house and pointing at the nearest base station it gives excellent gain and will often turn a no hope signal into a strong one.

Signal Amplifiers

Touted by some as the secret panacea for all ills the linear amplifier (AKA “Burner”, Power Booster, Power Amplifier) came to infamy in the heyday of CB radio when they were brought over from the USA and fitted illegally to Ford Capris and Cortinas by numbers of CB enthusiasts.   In general there were two main effects - the car battery ran down very quickly and every receiver for miles around was jammed by the spurious out of band emissions.  Some of these amplifiers were quite impressive - 1kW (yes - 1000 Watt) linears sitting on the boot of ratty Fords were not unknown!

Somewhat more civilised amplifiers were fitted to car kits for analogue phones taking their power up to 5 Watts.  However since the advent of GSM and PCN the benefits to be gained from these quite expensive boxes have become much less. 

As far as PCN is concerned the only benefit is to overcome losses in installations where long cable runs must be employed, for example if you need an antenna on the roof of your house.  In this situation the amplifier incorporates both a received signal pre-amplifier and a transmitted signal power amplifier.  It is designed to overcome the quite significant losses which occur in co-axial cables at 1800MHz.

Putting one in your car will usually have little or no significant effect.

If you use your cellphone in your motor-vehicle, an external cellphone antenna is a must !!

That’s because cars insulate cellphones from the external GSM signal, an unwanted artifact known as the "Faraday Cage." This Cage can sometimes result in poor voice quality and even dropped calls. A well-installed external car antenna usually fixes the problem. And if you’re in a rural area that’s on the periphery of the GSM coverage range, or even in a building that tends to block GSM signals, there are some novel antenna solutions available.

External antennas are available at around US$20, but are invariably professionally fitted as part of the complete installation of a cellphone car kit.

Your installer is likely to provide you either with a semi-permanent stick-on antennae design that simply sticks onto the front or back car windows using adhesive tape, magically transferring the GSM signal through the glass and then via a cable to and from the external antenna socket on your GSM cellphone or it’s special car kit.

Some installers also provide permanent boot (trunk) and car roof mount designs, but these tend to be more expensive as they invariably necessitate some car-body drilling and additional wiring. There are however many do-it-yourself clip-on designs available that don’t really require any detailed technical knowledge and can be installed in minutes. The most popular though are the installed stick-on types.

These external antenna housings usually consist of a base with a screw-on antenna rod. Some rods have an enclosed coil in the middle. Antenna efficacy is usually measured in decibels (dB) - the higher the rated dB specification, usually the better it’s performance. Longer rods of around 50cm usually have a dB level of around 5dB, the smaller 9cm types around 3dB or less.

Whatever their length, the antenna rods should be attached to a positionable, swivel-type joint on the base to allow the rod to be positioned backwards and forwards, left to right to optimize efficacy. The base positioning of the antenna on the car is also important: some installers prefer the front or back window, while others drill on the car boot or roof. The back window however is the most popular antenna position, although this invariably depends on the vehicle’s shape.

Your external antenna should also feature a position lock "memory" to ensure that the antenna rod stays in the position you set it - especially when used in high-wind areas. Some of the longer antennas tend to create high-pitched whistle effects in winds. Make sure that you can also unscrew the rod: this feature is especially useful if you want to prevent the antenna being mangled by a car-wash behemoth. Smaller, low profile 9 cm front-window mounted antennas are perfect for avoiding these situations.

There are also a number of easily fitted removable/portable car antenna solutions. One design simply clips on to the top of a wind-up side window, allowing you to switch cars and still have external antenna support. Once you’ve placed the clip onto the top of the window, you then simply plug (hard wire) the attached antenna cable into the phone’s antenna socket. You can use it with the window open or closed as the cable signal is relayed to the antenna’s external rod via the window clip-on.

There is however another side window clip-on design available that does not use any hard wiring. Instead it uses a special cordless pick-up rod inside the car - also connected to it’s external rod via the window clip-on - to "passively" relay the GSM signal to and from the cellphone. With both these clip-on designs, the back right passenger window is recommended.

There is yet another flat "patch" passive antenna type that simply sticks down flat onto any window. This solution, although not the most effective, is useful in offices where GSM signals may be blocked by an abundance of concrete and steel in the wall.

If you’re in a fixed rural location on the coverage fringe, there are special 10dB corner reflector antennas that can be attached to poles or buildings.

No field assembly or tuning is required and they easily attached to your cellphone using ordinary cable connections.

If necessary, you might also want to consider special booster devices that increase the power of you cellphone from the average 2W to up to a powerful 8W.