[TowerTalk] N or UHF?

[Ian White GM3SEK]

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