On Sun, 9 Jan 2005 16:59:10 +0000, Ian Jackson
wrote:

|In message , Wes Stewart
writes
|On Sat, 08 Jan 2005 19:30:03 GMT, "Phil" wrote:
|
||
||"art" wrote in message
egroups.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).

Okay, I guess this stuff doesn't use the compensation that I am
familiar with where either/or/both conductor diameters are modified to
account for the dielectric constant difference and the locations are
*not* at regular intervals.

Some of the missiles I've worked on used beaded coax at 10 Ghz.

|
|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 "suck out" you refer to is primarily due to mismatch loss. For
the case at hand (ham operation, i.e., a narrow-band system) I don't
think this is an issue.

Wes