tomerbr wrote:

this was a qustion from my instractor in college



Ignore the arguing going on here and suffice it to say that you could
get a PhD in antenna theory and spend a lifetime experimenting and just
begin to have a clear understanding of antennas. That may be
exaggerated, but only slightly.

A few general things to keep in mind. Many cell phones work on two
different bands, the antenna needs to work on both. When you are
interested in signals in a certain direction, in this case out and not
up, the longer the antenna the more gain and the more directional it
will be. Don't mistake that as an indication that you just want as long
as a wire as you can get, it is much more complex than that. For an
omni directional antenna where you want to limit the signal to out and
not up, stacking several antennas and hooking them all together in some
fashion, is one way to do that, but I doubt that any of the little cell
phone antennas do that. About the only time you would want an antenna
that also send the signal up is if you are communicating with low earth
orbit satellites that could be anywhere form the horizon to straight
above you. In that case, the antennas start looking more interesting,
that a vertical element.

--
Chris W
KE5GIX

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On 24 Dec 2005 03:15:26 -0800, "tomerbr"
wrote:

If the ideal lenght of an antenna is 1/4 Wave lenght then way are the
Car Cellular antennas are longer then this ?

this was a qustion from my instractor in college

You have been sidetracked into answering the question, when the
premise that "the ideal length of an antenna 1/4 wavelength" is
unsupported and questionable?

On what basis does your instructor make this general assertion?

You could equally be trying to answer the question "If the ideal
length of an antenna is 1/4 Wave length then why Medium Wave broadcast
antennas commonly longer then this?"

What you learn in answering the question will be useful, but you will
also learn that, subject to the meaning of the term "ideal", the
assertion is unfounded, it does not apply in general.

It is equally valid to turn it around and ask that given so many
antennas are not quarter wavelength, is quarter wavelength really
ideal?

Owen

PS: several reasons come to mind why car cellular antennas are not
quarter wave ground plane antenns, they are (over here) multi-band,
they are often so-called ground independent designs (on-glass, gutter
mounts etc) with elevated feed points, they seek higher gain, some are
a metre long for rural coverage, they appeal to buyers who think
bigger is better.



--

There is a simple answer, and others have given good advice:

The premis of the question is incorrect. A simple antenna such as a
collinear using a quarter wave with a phasing coil and topped with
another half wavelength segment will give power gain by modifying the
radiation pattern.
That statement should work better than OK.



tomerbr wrote:
this was a qustion from my instractor in college

"tomerbr" wrote in message
oups.com...
this was a qustion from my instractor in college

The assumption may be an error to begin with.
For starters I would say a 5/8 wave antenna may be "more ideal".
If you picture the radiation pattern similar to a balloon with a finite
amount of water (the 'power') , the coverage of the antenna can be
represented by the shape of the containment.
If you press down on the balloon center with your hand then the circle of
coverage will be larger - since we assume the cell towers will be on the
same plane as the antenna, you can see that a flat, circular shape will
cover more map area than a spherical shape would cover. A 5/8 wave antenna
is but one of many types which can exhibit 'gain' by distributing the
pattern in more usable directions.
I suspect the antenna the Instructor is referring to is 5/8 wave, and he
tried to throw you off with the 'Ideal antenna" statement.
Merry Christmas