Dan Hearn wrote:
>Roger, I have heard of problems with N connectors on the end of long
>cables. There is reportedly a different thermal expansion between
>shield and center conductor which tends to pull the center pin out of
>the socket. Apparently the UHF jobs are more tolerent of this.
UHF plugs are more tolerant of end forces because the center pin is much
larger and more firmly anchored.
The main problem with N connectors is that the original MIL-standard
design has a floating center pin, with nothing to retain it but the
center conductor of the coax. However, there should be no problem with
solid-dielectric cables such as the old original RG8, or the later
RG213, provided of course that they are assembled within the correct
tolerance limits.
But the floating center pin can be very bad news with cables such as
9913 that have a floating center conductor, because then there's nothing
to keep the pin from moving. It may pull outward, often causing an arc
on TX; or a long vertical run of cable can allow the center conductor to
slip downward, pushing the pin outward through the plug with so much
force that it splits the center pin of the socket. I've had both of
those happen.
Better designs of N connectors have a captive center pin, and also a
pressure-sleeve cable clamp that grips the cable really firmly. Also,
better designs of cable have an anchored center conductor, and these two
features between them can usually keep the movement problem well under
control.
It may also be important to remember that in high-power VHF/UHF
applications the center conductor of RG213/9913-sized cables can get
quite hot. This may soften the dielectric, and if there is any force on
the center conductor, it may very gradually 'creep' either lengthways or
sideways inside the cable. Depending on the frequency and the power
level, it could be years before anything bad happens, but with 1.5kW at
432MHz at heavy duty cycles there can be trouble in a few hours.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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