MadSci Network: Physics

Re: How do internal atenna boosters for cellular phones increase the reception?

Date: Sat May 5 12:58:22 2001
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Physics
ID: 988875697.Ph

Hello Duke,

An interesting question, and one that has a couple of answers; depending 
on certain assumptions.

Designing an antenna is a very complex science in itself, but I will try 
to keep it simple.  The purpose of an antenna is to gather radio frequency 
signals from the air and connect them to the input stages of the 
receiver.  The input stages are compromised of an oscillator (usually user-
tuneable, as in a transistor radio) which allows you to tune to a selected 
station, and one or more I.F. (intermediate frequency) amplifier stages 
which take the weak signal and amplify it to a usable level. In a cellular 
telephone, it is not necessary to be able to tune the oscillator because 
the frequency range is preset by your service provider, and one of the 
many individual frequencies, or channels, available are automatically 
selected, and allows the transmitting cell to select a currently unused 
channel for a call to you.  Your 'phone is able to accept signals on any 
of the available channels.

While it is theoretically possible to add many I.F. amplifiers to gain the 
necessary signal strength, the problem with that approach is that they not 
only amplify the desired signal, but all the other frequencies (noise) as 
well. Seperating the noise from the signal is no easy task, so it is far 
better to use an antenna which provides gain at the correct frequency, and 
tends to eliminate the other frequencies.

To illustrate antenna gain, think of it as a length of clothesline, 
stretched between a fixed point at one end and your hand at the other.  As 
you move your hand up and down, you will create a wave in the line, which 
will travel from your hand to the other end and return to your hand. This 
represents the radio signal.  If you move your hand up and down at just 
the right speed, the returning wave will reach your hand in exactly the 
same relationship as your hand movement, causing the resulting wave to be 
the sum of the two waves.  In other words, you have just amplified the 
original signal by 2, discounting any losses that may be incurred, and the 
receiving (fixed) end will see a signal that is approx. twice the original 
signal strength.  This amplification is dependent on the length of the 
antenna (clothesline), and the frequency of the applied signal.  If the 
antenna is the wrong length, the returning waves may partially or even 
completely cancel out the desired signal.  That's why "rabbit ear" 
antennas for tv's were adjustable in length; so you could tune it for the 
channel you were watching.

Getting more specific, radio waves travel at the speed of light, or 
186,000 miles per second. If you convert this to inches/sec, you get 
11,784,960,000 which, when divided by the frequency used by a normal cell 
phone of 900MHz, you get a wavelength of 13.0944 inches.  This is the 
distance between upper-most and lower-most points of ONE cycle.  
Therefore, the ideal length for a 900MHz cell phone antenna would be 
13.0944 inches.  Because this is not practicle, a sub-multiple of the 
ideal length is used - such as 1/2, 1/4, 1/8, etc. Any such fraction 
(within limits) is o.k. as long as it is divisible by two.  If you use a 
factor such as 1/3rd, it will cause the incoming waves to collide with 
returning waves, creating what are called "standing waves", and the 
efficiency of the antenna will be significantly impaired.

Now, as to using metal strips: If the strips are the correct length, it is 
POSSIBLE that they can improve reception given that one of two facts are 

1. The strip is electrically connected to the built-in antenna at the 
antennas attachment point (the length of the connecting wire becomes part 
of the equation) or

2. The strip is inductively coupled to the antenna or I.F. circuit.  This 
inductive coupling is based on the fact that when an electric current is 
passed thru a wire (the "strip antenna" in this case), it will induce a 
current in another wire running more-or-less parallel to it.  If the 
two "wires" are perpendicular to each other, no inductive current is 

To summarize: As the inductive current flow is inversely proportional to 
the square of the distance between the wires, you can see that the 
placement and positioning of the strip is critical, and not knowing where 
the proper wire is located in your phone and its' direction of run, any 
advantage you would see would be purely coincidental.  Also, since high 
frequency radio signals travel in a line-of-sight manner only, you would 
gain no advantage in those areas commonly known as "Black Holes".

I hope this helps you understand this antenna "snake oil".

Your not-so-mad scientist,


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