[MLB-WIRELESS] pigtail

[John Dalton]

> 2. put a probe on the center pin of the mcx connector and the other probe on
> the center pin of your coax cable.  multimeter should show very small
> resistance ~ 50 ohm.
> 
> 3. probe outer mcx connector and probe outer N connector - multimeter should
> show small resistance again.

This is not quite true, if I have understood the question correctly.  If you
use a multimeter to measure the resistance from centre pin to centre
pin on a 'pigtail' you should see a resistance of the order of milliohms,
which will probably be displayed as zero ohms on your multimeter.  The
same goes for measuring the resistance of the outer 'shield'.

When a cable is specified to be '50 ohms', this is referring to an
impedance at high frequency, not its resistance at DC (as measured by
a multimeter).  A coax has capacitance per unit length and inductance
per unit length.  At high frequencies, the capacitance and inductance
resonate, giving a total impedance of 50 ohms.  Another way of looking
at it is that, in this context, '50 ohms' is simply a specification of how
fast a signal propagates down the cable.

Measurement with a multimeter is the quick way to check a cable, and should
be your first action if a cable is suspect.  Such a test will detect
most problems, which consist of short circuits or broken conductors
(open circuits).

It is possible for a cable to have more subtle problems.  For example, crushing
the cable may not break conductors, but still cause the cable to fail.  In
technical terms, the damage has caused the impedance of the cable to change
(so it is no longer 50 ohms). To detect such problems, it is necessary to
measure the impedance of the cable.  One way of doing this is to terminate the
cable in 50 ohms (for a 50 ohm cable) and connect a signal generator and an
oscilloscope to the other end.  Use the signal generator to inject a short
pulse, or sharp step, into the cable.  The oscilloscope should display a sharp
pulse with no visible echoes or reflections.   The technical name for this
test is 'time domain reflectometry' (TDR).  In simplistic terms, the test relies
on the fact that a change in the impedance of a coax (or more technically a
'transmission line') will cause a reflection.  Any deeper explanation will get
too long.  For a deeper explanation, go to a library and get a book
on electromagnetism (or search google or wikipedia).

Most people will not have the equipment to do TDR.  In any case, you should
start with a simple resistance measurement, as it will detect most problems.

No doubt this is far more information than you wanted.  The simple answer
is 'do a resistance test' (see start of message).

Apologies if I have misunderstood your question.

Regards
John

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