[BC] Wave lengths

[Alan Alsobrook]

Jason,

I'll try another analogy, don't know if it'll help.  But different 
materials do slow the wave down. Keep in mind that RF travels on the 
surface of a conductor. Even air slows it down to some extent (not 
much). This is referred to as the Velocity of Propagation (VP). The 
Formula you referred to 468/F(MHz) Assumes a 95% VP, which pretty close 
for RF traveling on the surface of an antenna. (Note that is for a half 
wave, and 234 is the constant most used for 1/4 wave).

To try to help you with this, I'll try this analogy, assume you are the 
RF and on pavement you can run the 40 meter dash in 8 seconds (air). The 
you go down to the beach and run it on loose beach sand it'll take you a 
bit longer (foam dialetric). Lastly you hop in a waist deep pool and try 
to run it it'll really take some more time (poly). Now well change the 
rules just a little bit and say that where ever you are when you've run 
7.8 seconds you will stop and turn around. That point where you stop and 
turn around will represent the half wave point. As you can see in all 
circumstances you will not have reached the end of the course (the free 
space 1/2 wave point) before you had to turn around.

Now if that made any sense at all I think you'll be able to visualize 
the reasoning for the difference in numbers.

Now to pick on Phil just a bit <G>, Phil if those wave crests were 
further apart wouldn't that mean the frequency had been reduced?

Phil Alexander wrote:
> Jason,
> Look at it this way. Sound traves much faster in some mediums than in
> others. So it is with transverse wave radiation (e.g. RF);.  In free space
> the velocity is C, or about 300,000,000 m/s. In another medium, say
> polyethelyne, is is only 0.66 x C or about 200,000,000 m/s.
> 
> Thus, if two waves are passed from point A to point B, the one in the
> poly is going to take about half again as long.
> 
> Phase is a matter of time. If transition from the coax goesinta (A) to
> comesoutta (B) takes half again as long, the phase will lag a wave that
> traveled in free space (or air insulated coax) from A to B.  Look at it
> this way: The waves are longer - IOW, wave crests further apart - in
> poly than in free space. It really is that simple. Foam being mostly air,
> falls between air and poly.

-- 
Alan Alsobrook CSRE AMD CBNT
St. Augustine Fl. 32086 904-829-8885
aalso at Bellsouth.net