Red Wave Radio

Hi,
    What is it about an antenna that makes it a good antenna for a given
frequency range?

    Now, I know the basics about electromagnetic waves inducing a
current in a conductor but what shape and how big is the most
appropriate antenna for a given job? I'm thinking mainly in terms of
television signals but I guess radio and cell-phones are all covered by
the same principles.

    Maybe I'm asking a lot (there are probably whole text-books covering
this stuff ??) but are there some relationships between the dimensions
of the antenna (say for simple-geometry antennae), the frequency of the
EM wave, the orientation of the antenna, the input impedance of it(which
I can't say I understand very well) etc.

    eg Why aren't coat-hangers (or "cat's ears" \_/ ??) a good solution?

    Any advice or pointers would be very welcome,
thanks,
John.

PS: Please feel free to send a copy of your reply directly to me at

    [without the ANTI SPAM of course :-)]

The size of the antenna is limited by the frequency band you're operating in.  
For example, consider a simple dipole antenna which  is just a straight wire.
In order for it to transmit at it's peak efficiency neglecting a few other
details, its length should be at least half of the wavelength of the
frequency that it's transmitting or receiving.  That's a simplification of
course, but that's pretty much it.  That's why you see those CB radios on cars
with such long antennas. They're operating at the lower frequency range which
means a long wavelength.  If not then you'd have to increase up the power
output which is not good efficiency wise and the FCC doesn't like it much
either.

Quote:>    Maybe I'm asking a lot (there are probably whole text-books covering
>this stuff ??) but are there some relationships between the dimensions
>of the antenna (say for simple-geometry antennae), the frequency of the
>EM wave, the orientation of the antenna, the input impedance of it(which
>I can't say I understand very well) etc.

If you want to know these things I'd suggest you buy a text titled "Antenna
Theory and Design" by Stutzman and Thiele.  Not for the average person, but if
you're that interested then knock yourself out.

Quote:

>    eg Why aren't coat-hangers (or "cat's ears" \_/ ??) a good solution?

There are people who spend their careers on the subject.
John Kraus probably wrote the most definitive work on
the subject, but there are hundreds of others. And there is
no simple answer.  An antenna that works great for long
distance communication on a given frequency may work
lousy for short range communication, and vice versa.
An antenna that won't "play" well at a given height may
play real well when it is LOWER!  Usually, as a very
general rule, the higher the better, but not always.
A directional antenna pointed say due north may not
work to pick up signals originating in that direction,
but may work to pick them up if aimed east or west,
as the signal may be reflected.

There's all kinds of shapes and sizes for antennas
for a given frequency. And when you consider
the whole RF spectrum, the range of antenna
configurations boggles the mind.

There are definite relationships between size
and frequency.  F=L/C where F is the frequency,
L is Lambda (the wavelength) and C is the speed
of light. There are also relationships between size
and resonance and impedance.  These vary according
to the antenna configuration. Just as an example,
a beam antenna with 3 elements has some characteristics
of interest - gain, front to back ratio, front to side ratio,
stuff like that.  The size of it plays a direct role in those
characteristics.  Changing the spacing between the elements
affects them.  A higher gain 3 element beam will have greater
spacing between the elements than a lower gain 3 element
beam, when the elements are the same size.

You have asked a question that thousands of people ask,
and experiment to find the best solution for their interests.

Getting down to your reference to TV antennas:
a coat hanger will work just fine if it is close enough
to a typical TV transmitting antenna and if there is
not a lot of reflected signal to cause ghosting.  But
when faurther away and/or when there is a lot of
reflection, the need for a typical roof mounted TV
antenna arises.  These are usually in a configuration
called "log periodic", and exhibit gain over your
coat hanger antenna, as well as a greater degree
of rejection of reflected signals coming it to the antenna
from the side.  If you look in catalogs, you'll see that
TV antennas with greater gain have more elements and
longer booms than those with lesser elements.

Finally, the halfwave dipole antenna, mentioned earlier.
That is an antenna which has, according to the
texts, a characteristic impedance in free space
of about 72 ohms.  It is made with 2 "poles".
Each "pole" is an element about 1/4 wavelength
long.  The two "poles" are run in a (roughly) straight
line, end to end for a total distance of about 1/2
wave - thus the term halfwave dipole. A transmission
line (coax for example) is connected in the center
of the antenna - the center conductor connects to
one pole and the braid connects to the other.
This antenna's impedance will vary depending
on how close it is to other objects and the
ground.  (The impedance is discussed at the
frequency where the antenna is resonant. Resonance
is the point at which the inductive and capacitive
reactances cancel each other out, and is frequency
dependant.)

This long winded reply is actually an over simplification!
You can spend a lot of years studying antennas and just
scratch the surface.

Quote:>The size of the antenna is limited by the frequency band you're operating in.  
>For example, consider a simple dipole antenna which  is just a straight wire.
>In order for it to transmit at it's peak efficiency neglecting a few other
>details, its length should be at least half of the wavelength of the
>frequency that it's transmitting or receiving.  That's a simplification of
>course, but that's pretty much it.  That's why you see those CB radios on cars
>with such long antennas. They're operating at the lower frequency range which
>means a long wavelength.  If not then you'd have to increase up the power
>output which is not good efficiency wise and the FCC doesn't like it much
>either.

CB whips are 1/4 wl long.

When the antenna is worked against a large counterpoise (like a car)
1/4 wl is the shortest resonant length.

Actually the question he asked would require a good-sized book to be
answered accurately.

73 Tom

I can't remember the book, it was a long time ago, but it was about
antennas.  On the title page there was a caption "Antennas are funny
people".  It is the shortest summary I have ever seen which covered just
about everything -- and truthfully!   All the rest you will learn about
antennas is theory and conjecture!

--
Irv Finkleman,
Grampa/Ex-Navy/Old Fart/Ham Radio VE6BP
Calgary, Alberta, Canada

Thanks for the feedback, guys :-)

I guessed that it's a tall order to find a *simple* overview in the
space of an email.

I'll try out the library for some books and see if they have the books
mentioned before or similar ones...

the intros were helpful in any case...
bye for now,
John.

--
"The man who goes alone can start today; but he who travels with another
must wait till that other is ready." :-))

What make's a motorized vehicle good???
It is designed to do what you need it to do and it does it better than any
other vehicle. For example: would you drive a bulldozer, across country, to
enjoy your vacation? would you plow a field with a helicopter?
No way!
Every antenna has it's designed use. It is optimized for that use and there
are all kinds of trade offs.
Lets look at rabbit ear antennas. They are small, portable, light weight.
Just the thing you would want to put on top of your TV. The major advantage
is size. The trade off is your reception is not as good. If your main
concern is reception then you will have to look at other designs. But the
trade off here is you will probably do not want to put a yagi on your TV,
because of a yagi's size and looks.(a yagi antenna is the TV antennas you
would put outside and up on a pole). If you put rabbit ears out side and up
in the air your reception would improve just because of its location.
However There are far better antennas designed to be used for that location.
My best advice is, ask a expert for advice on the best antenna for your
needs.
I hope this helps.
Frank Dedon

a good antenna is within 3db of the best antenna for the job.  Leave the book
on the shelve.