On Sat, 08 Jan 2005 19:30:03 GMT, "Phil" wrote:

|
|"art" wrote in message
roups.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?

Ahh! But amplification is used for very long spans
of cable TV.
'Doc

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.
--

On my links page I have some links on how to adapt normal so239 and N
ends onto this cable.
Can be used for TX just fine.
www.qsl.net/w2rac

On 7 Jan 2005 18:49:53 -0800, "art" wrote:

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

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