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 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 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 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..
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
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.
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.