[BC] Wave lengths

[Phil Alexander]

On 18 Feb 2006 at 17:10, JYRussell at academicplanet.com wrote:
>I don't know if you saw the thread;  not too far back I had 
>mentioned it would be a bit easier for me to get my brain wrapped 
>around the idea of what's happening when coax is "electrically 
>longer when it is physically" - by literally drawing a picture or two of it....
>
>I was wondering about some more or less real world scenarios , 
>what's  happening when it's right, when it's wrong, with and end fed 
>antenna vs  center fed, and such.
>
>It all started 'cause they finally talked me into taking some of the 
>ham tests, and right off quick I discover 'they' use
>weird numbers.  (Like, 468/F(mHz) rather than 492/f(mHz) for 
>wavelength, and such... )
>
>Have you ever drawn out stuff like this, or do you know where I 
>might find this sort of thing?  I can look at tables of numbers all 
>day, and even remember a few, but somehow stuff just "makes more 
>sense" once I see it as a
>diagram, or picture...

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.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology
(a Div. of Advanced Parts Corporation)
Ph. (317) 335-2065   FAX (317) 335-9037