The cables in question all used braided shield construction and were
aerospace quality. The individual center conductors are insulated, of
course, but there is nothing else between the insulated wires and the shield
braid. The shield braid itself appears to be Kapton coated. It is indeed
true that the twisted pair as a center conductor has an irregular shape
relative to a single coaxial conductor. However there is an expression for
the characteristic impedance of just such a cable (except for the twist) in
the Reference Handbook for Radio Engineers. In my 1953 copy this type of
transmission line is termed wires in parallel - sheath return. It is a
complex formula but the point is that even with an irregular center
conductor there is a defined transmission line characteristic impedance,
which means that whatever the impedance actually is, if properly matched,
mismatch losses may be bounded. If it were mismatch loss wouldn't the
loss be strongly frequency dependent in a periodic fashion? The losses I
measured increased with increasing frequency, and the nature of the increase
was identical to that specified for the RG-XX coaxial types (with larger
magnitude as previously noted). I can send a test data attachment to anyone
interested.
--
From: Wan Juang Foo f...@np.edu.sg
To: Ken Javor ken.ja...@emccompliance.com, emc-p...@majordomo.ieee.org
Subject: Re: Stumped (TSP or FTP (Foil screened Twisted Pair) )
Date: Mon, Oct 14, 2002, 9:11 PM
Ken,
I am only making a guess in the absence of the cable's specifications. Is
your TSP a FTP (Foil screened Twisted Pair) type of cable constructed from
copper wires coated with polyethylene and wrapped by Mylar tape (a
transparent and mechanically tough film) between the TP and the screen? I
think it is just mismatch losses due to the eccentricity of the cable.
Imagine what kind of performance one would expect from a coax with an inner
conductor that exhibit irregular cross sectional radius.
IMHO, it's the inherent 'mismatch' losses of the 'cable' to CM signal.
Given the inherent twisting (eccentricity) of the conductors within the
'screen', it is more like a coax with a screw shaped (how would I describe
it???) distribution of Zo along the length of the cable. The TP
configuration in the CM situation should be low loss only to circularly
polarised electromagnetic waves (if there is such a phenomenal in
electromagnetic propagation within a Tx line). For propagation modes of
anything else approaching something that may represent a substantial
fraction of Lambda would be, (or shall I say, should be) presented (or
seen) as a lossy line due to the changing cross sectional characteristic of
a FTP.
I expected losses that would be on the same order or lower than
that associated with off-the-shelf coax types like RG-58. Instead my
losses
were dramatically higher.
In terms of CM performance (e.g. input impedance, losses, etc... ), a FTP
due to it's eccentricity, I suppose cannot be compared to a RG-58. I have
come across some FTP with 0.52 mm i.d. copper with a final diameter of 6.1
mm for the cable. Let me know if this description fits your bill. I have
some information on their fabrication.
Just another of my 2 ¢ ...
regards
Tim Foo
Ken Javor
ken.javor@emccomplian To:
emc-p...@majordomo.ieee.org
ce.comcc: (bcc: Wan
Juang Foo/ece/staff/npnet)
Sent by: Subject: Stumped
owner-emc-pstc@majordo
mo.ieee.org
10/15/02 05:31 AM
Please respond to Ken
Javor
Forum,
I have made some measurements and gotten results which are at odds with my
intuition. I am wondering if someone out there can shed some light on this
subject.
I was interested in the losses associated with rf traveling on a twisted
shielded pair cable. The scenario is that a length of this TSP cable is
exposed to an rf environment (as in a test chamber during IEC 1000-4-3
testing) and then the cable penetrates a bulkhead using a grounded
connector
that provides excellent shield termination, and the cable continues on the
other side in the pristine rf environment of a shielded control chamber,
say
for several meters. The question is, how much rf signal is at the final
destination point vs. at the bulkhead. The concern is common mode, not
differential mode. Meaning that the twisted pair can be looked at like
coax, with an identical signal on both inner conductors relative to the
shield. I expected losses that would be on the same order or lower than
that associated with off-the-shelf coax types like RG-58. Instead my
losses
were dramatically higher.
Following is my measurement technique.
snip
