In message , Wes Stewart
writes
On Sat, 08 Jan 2005 19:30:03 GMT, "Phil" wrote:

|
|"art" wrote in message
groups.com...
| I was just given a coil of 7/8 hardline coax that I have not seen
| before.
| It has plastic discs inside to separate the core from the outer
| aluminum tube which I suppose uses air as the dielectric. Anybody got
| any info on this stuff
| and any advantages it may have over the normal hardline cable?
| Regards
| Art
|
|
|Look on Google for "fused disc" AND "cable".
|
|Most Cable TV companies stopped using it when they started using frequencies
|above 300 MHz.
|
|This cable acts as a filter around 350 MHz.

How so?


The discs have a higher dielectric constant than the air. There is
therefore an increase of capacitance where there are (so the Zo dips
slightly).

As the discs are located at regular interval, where this spacing is one
wavelength, the repetitive higher capacitance adds up in parallel
(although buffered somewhat by the losses loss of the cable). The effect
is to produce a sharp suckout at the frequency of that wavelength (and
at multiples thereof). This effect is also referred to as 'Structural
Return Loss' (well, it's one of the causes). It sets the maximum
frequency at which the coax can be used.

The closer the spacing of the discs, the higher will be the suckout. If
the spacing is zero, the frequency will be infinite. However, you will
have just invented solid dielectric coax!

Ian.
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