RE: 1/4 wave (108") antenna

From: The Man From Utopia (tmfu@home.com)
Date: Mon Oct 09 2000 - 21:43:15 EDT


> Ok... one more question..... after what I have read and what
> folks here
> have said, it is best to go with a 1/4 wave whip for best
> results. All
> other antennas are a compromise.. (FWIW: I have yet to find a
> true 9ft
> antennae... the longest truck mountable one I have found is
> 102" or 8ft 6in
> long... wth?)

Ok basic antenna theory...(BIG snooze)

Radio energy waves are waves of energy that are similar to light waves. In
fact signals (radio waves) travel through the air at the speed of light. To
understand how an antenna works you must first have a basic idea of the
makeup of a radio wave. A radio wave can be visualized as a sine wave. The
distance a wave travels to complete one cycle is known as the wavelength of
the signal. A CB signal completes a cycle as it travels through the air at
roughly 36 feet. For every 36 feet the wave travels it has completed one
cycle. Since we have defined wavelength as the distance a wave travels to
complete one cycle, we now know that one wavelength on CB frequency is
approximately 36 feet.

A good way to visualize a signal as it travels through the air. We can now
visualize an actual distance that a wave must travel to cycle. This distance
is around 36 feet. If you convert 36 feet to meters you get approximately 11
Meters. This is why the CB band is sometimes called 11 Meters!

How Antennas Work In General

When your transmitter puts a current (Radio Frequency (RF) energy) into an
antenna, your antenna responds by producing a magnetic field surrounding the
antenna (this is the signal). When this magnetic wave strikes another
antenna (the receiving station antenna), it induces a current on the
receiving antenna surface (that current is then converted by the receiving
stations receiver to sound).

The length of the antenna structure plays an important role.

The magnetic field that your antenna puts out will produce an electric
current on any metal surface that it strikes, however if the metal that the
signal strikes has a length relation to itself the induced current will be
much stronger on the object. We stated before that as a CB signal travels
through the air, it completes a cycle in approximately 36 feet. For
instance, if the object that the magnetic wave strikes is 18 feet long (1/2
wave length), 9 feet long (1/4 wavelength) or 36 feet long (1 full
wavelength), then the induced current will be much higher than if the signal
struck a metal object that was not some appreciable fraction of the
wavelength of the signal. If you have ever heard people say they want to
"tune" their antenna, they usually mean make it have a length relation to
frequency they are trying to receive.

This has a special name, it as known as antenna resonance. Every antenna has
at least one exact resonance point.

Antenna resonance is the frequency (in MHz) where the antenna is in a state
of electrical balance, which is determined by the length of the antenna
(every antenna has an exact frequency it is resonant on). To use some
numbers to demonstrate what we are explaining here, let us look at a very
simple formula. You can calculate the distance of a wavelength in freespace
for any frequency using this formula:

One Wavelength, in feet = 984 / Frequency in Megahertz (MHz)

Lets look at an example:

CB Channel 40 uses the frequency 27.405MHz.

One Wavelength for 27.405, 35.906 Feet = 984 / 27.405

So we want to make a antenna that is resonant on channel 40 and not too
large in size. Lets cut a straight piece of aluminum rod to be 1/2 a
wavelength long. One Wavelength for 27.405 is 35.906 feet (from the formula
above), and we want 1/2 of that, or 17.593. So, we cut our piece of aluminum
rod to 17.593 and we have made an antenna that resonates on 27.405 (or close
to that). This piece of rod should pick up (receive) on channel 40
(27.405Mhz) well.

Ok, now lets look the most basic antenna most CBers are familiar with, the
102" whip, also known as the 1/4 wave whip. Why is it known as the 1/4 wave
whip? As we can see if we get out our calculator, 102 inches is
approximately 1/4 of 35.906 feet (1 wavelength). This antenna should also
perform well on the CB band because its length relates to that 36 foot
wavelength signal we are trying to receive!

To further simplify things, we have been speaking strictly about receiving -
but these same principles apply to transmitting. Our antenna also transmits
a strong signal if the antenna we are transmitting through is resonant on
the frequency on which we are transmitting.

>
> So.... what's better, the steel whip or the fiberglass? After
> checking it
> out, I could mount one on the rear bed rail by the tailgate
> and bend it
> down to tie off at the front of the bed without losing any
> clearance, but
> can pop it up when I really want to reach out and touch someone or am
> roadtripping....
>

I would go with a steel whip... More mass(the glass one's have only a thin
wire going through the core of the glass. Plus the steel whip is really more
durable...

Greg
95 Dakota Sport CC v6 5spd
Rahway NJ
ICQ: 283886
http://24.6.89.18/dakota/dodge.htm



This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 11:55:55 EDT