Graham,
#1. Correct. There must be absolutely no air gap! The cable is produced in a
way, that the copper tube is pressed on the dielectric. Any sign of air gap
means a risk that the dielectric will move on inside of the tube. I have
been working with semirigid cables for many years, and I've seen
unbelievable things. In one case, we could 'pour' the dielectric out of the
tube by holding the straight part vertically and knock on the copper!
#2. Depends on the frequency and the requirements for a clean signal or
pulse. Normally, one mil is not a problem, BUT, in principle, with an air
gap, you get a DISCONTINUITY, which can cause reflected signals, deformed
pulse form and spurioses. I can't tell you what the breakpoint is, depends
on the cable properties, the speed of the signal, rise/fall and such things.
If you have no microwave gure at hand, I can help you.
/Inge
----- Original Message -----
From: "Graham Collins" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, December 15, 2009 12:43 PM
Subject: [TN] Semi-Rigid coax cables
A question for the TechNet cable gurus. We are putting together some
semi-rigid coax cables and as this is new ground for me I'm not clear on
some of the inspection criteria. IPC/WHMA-A-620 is helpful, but
expresses things in absolutes. In particular, two things are being
questioned:
1) Air gap between dielectric and cable shield is not allowed.
Figure 13-62 shows it. But how extreme is that "not allowed"? Under
magnification there can easily be a gap that is not substantial, and is
not visible to the naked eye. What sort of magnification should we be
using?
2) Dielectric below flush with the connector face - again there is
no stated wiggle room on this. Any tips on arriving at a reasonable
compromise? If I can see a .001" gap do I reject it?
regards,
Graham Collins
Halifax Production Engineering
(902) 873-2000 ext. 6215
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