On 18/03/2019 18:18, Terry Casey wrote:
In article
,
says...


Perhaps I am missing something here,

I think you are! I was trying to emphasize that the allowable
tolerance of the inner dielectric has to be much tighter than
for the outer sheath

Of course - the outer does not have any significant effect on the cable
impedance, where as, the radius of the dielectric, and the material it
is made from does.

but it seems to me that you won't
be altering the spacing between inner and screen... at the point the
short length of dielectric is exposed, it is protruding beyond the foil
screen anyway

Is it? it isn't apparent in your pictures which appear to show
the foil screen extending the full length of the dielectric,

Indeed they do

(although I note that different plug manufacturers give subtly different
examples of best termination practice - some show removing the foil at
the point the outer is striped, and others continuing it a little
further up to (or at least close to) the point where the inner wire is
exposed - I favour the latter).

as it should do. Perhaps you meant to say the exposed length
of the centre connector? In that case, the screening function
is the responsibility of the F female which completely
encloses it when the two are mated.

Right, and that is the same size regardless of the size of the rest of
the plug.

In fact if you imagine preparing the cable as normal, and then
connecting it to the socket *without* any plug at all, you would still
have the same fundamental screening geometry. (ignoring the lack of
electrical continuity of the braid for the mo).

- and the screen is now being provided by the threaded
part of the plug

Which would be completely wrong.

Sorry, me being sloppy - provided by the threaded barrel of the socket
not the plug. Either way the spacing/material of screen to inner in the
socket will not exactly match those of the coax with either size cable,
however they should both (in combination) approximate the same
characteristic impedance.

If you cut cleanly through
the cable and view it end on you have the centre conductor
surrounded by the dielectric with its foil covering. This is
the coaxial cable. Additional screening is just that and the
outer sheath protects the cable but they are not
electronically part of the coaxial cable.

Yup, I agree with that.

the size of which is the same for all sizes of connector.

In which case, why did you find it neccesary to wind tape on
the cable to increase the size

Because the rear section of the F plug's (i.e. the bit with the knurled
outer and internal threads), only purpose is to facilitate attachment of
the plug to the cable. Its entirely outside of the screen on a foil
screened cable.

When sized for a normal CT 100 style cable, is too large to engage with
the outer jacket of a shotgun cable.

So for example, on the left some Webro WF100, and on the right some
Labgear R09W42, with the matching plugs:

http://wiki.diyfaq.org.uk/index.php/...g-shotgun8.jpg

Cross section of the cables:

http://wiki.diyfaq.org.uk/index.php/...-shotgun10.jpg


and would you have to do the
same if you used the correct connector?

No, because it would fit without. All the tape is doing on the smaller
cable is making the outer jacket wide enough to allow the plug to grip.

I'd love to know how you would get either of those connectors
to fit over the sheath of RG11 (10.3mm nominal).

You wouldn't - its insulating tape, not a TARDIS!

You can make a plug fit the "correct" size of cable or one a bit
smaller, you can't as easily make it fit one that is too large to enter
even enter to cable grip section at all.

What I am struggling with here, is what the actual thrust of your
argument is... is it that the hole though the end stop in the plug is a
different diameter? Vis:

http://wiki.diyfaq.org.uk/index.php/...g-shotgun9.jpg

Since it seems to me that part of the plug is outside the foil screen
anyway, and so should not fundamentally change the properties of the coax.

Looking at the layers of the cable side on where it attaches to the F
plug you have something like:

==== knurled barrel of plug
---- braid folded back
pppp outer plastic jacket
---- braid
ffff foil
dddd dielectric
iiii inner copper conductor
dddd dielectric
ffff foil
---- braid
pppp outer plastic jacket
---- braid folded back
==== knurled barrel of plug


I am assuming that the characteristics of the coax are controlled by the
spacing of iiii from the foil, and what material dddd is made from.

So altering the arrangement to:

==== knurled barrel of plug
---- braid folded back
pppp outer plastic jacket - paddding
pppp outer plastic jacket - paddding
pppp outer plastic jacket
---- braid
ffff foil
dddd dielectric
iiii inner copper conductor
dddd dielectric
ffff foil
---- braid
pppp outer plastic jacket
pppp outer plastic jacket - paddding
pppp outer plastic jacket - paddding
---- braid folded back
==== knurled barrel of plug

Will not alter the critical defining characteristics of the coax at all.



----
====





--
Cheers,

John.

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