Antennae

Antennae

You will find below notes on how antennae work at the two UK phone frequencies of 900 MHz (Vodafone and Cellnet) and 1800MHz (Orange and 121) and descriptions of some different types of antenna. 

How to solve specific siting and installation problems in boats, cars and houses can be found here

 

900 and 1800 MHz

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:-

  1. Frequency (wavelength).
  2. Gain.
  3. Impedance.
  4. 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 (The GSM1800  frequency used by Orange and Mercury 1 to 1) 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.  Looking through the side of the doughnut it would be a bit like the diagram below (where the black thing in the middle is the antenna).

MiniMag Antenna Picture
Picture

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 as is illustrated below.

Picture

The Allgon MiniMag Colinear magnetic mount antenna for 1800 MHz

You can also put directivity in the azimuth pattern - but for mobile 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 (a dipole has 3dB gain over an isotropic radiator) it is important to know which is being referred to when comparing antennae.  Usually if antenna is described as having "3dB gain" it should mean compared with a dipole. If it says "3dBi gain" it should mean compared with an isotropic radiator.  Unfortunately by the time the advertising guys have got their sticky mitts on things the difference is often blurred.

900/1800Mhz Aerial length comparison

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!

Size comparison between the Allgon 900 MHz and 1800 MHz antenna (on the left) for the bodymount Base 2000 system

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.  A yagi is nearly always preferable to a signal amplifier if you can manage to point it in the right direction because gain from the yagi is signal gain whereas in an amplifier both signal and noise are amplified equally.

Dual Frequency Antennae

Dual frequency antennae usually cover both the 900 and 1800MHz frequency bands.  They are increasing in popularity with the greater use of dual band phones such as the Nokia 6100 series and with more continental networks using both 1800 and 900MHz transmissions.  As always there is a compromise and their performance is usually about 3dB less than a single frequency co-linear design.  Glass mouint dual frequency antennae are even more difficult to get to work efficiently and have slightly greater losses..

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 in the boots of ratty Fords were not unknown!  Normally they could be detected by the car slowing down every time the transmit button was pressed.

Somewhat more civilised amplifiers were fitted to car kits for analogue mobile (TACS) 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 real 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 (in the case of the Allgon version) 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 (other than making you somewhat poorer) although they can be useful in vans and caravan installations.

 

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