All,
I have also personally seen the center pin of a type N
connector that was pulled back in to the shell and had
broken contact with the mating connector. This happened
during the winter months, and was at the end of a 100'
length of 9913 coax. I was alerted to this problem because
I could no longer get into the local DX cluster via the
radio that was connected to an antenna using the
coax/connectors in question.
An investigation revealed that the center pin of the
type N connector on the far-end of the coax had indeed
retracted into the back shell of the type N connector by
approximately 1/16 – 1/18 inch breaking the connection to
the antenna.
I attribute this to the contraction of the center
conductor of the 9913 coax due to the cold. Since the type
N center pin is soldered to the center conductor of the
coax, and effectively floats through the insulation
material of the connector shell, the center pin naturally
followed the center conductor of the coax and contracted
in.
As was mentioned below, this does not happen as often
(at all ?) with PL-259 connectors. The difference is
that the PL-259 has a captive center pin where the center
pin is solidly part of the actual connector assembly.
This gives a strong anchor point for the center conductor
of the coax.
As was also mentioned below, if you must use type N
connectors on long lengths of air dielectric coax, try to
find connectors with captive center conductors.
73,
Gene K1NR
On Mon, 16 Oct 2006 22:49:38 +0100
Ian White GM3SEK <gm3sek@ifwtech.co.uk> wrote:
> 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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