I didn't claim anything about voltage rating of N connectors, but the
stand-off voltage of air corresponds more or less to 50 volts per mil.
(mil=.001").
Calculation for breakdown voltage in N or BNC (where also a teflon
dielectric is present) is left to individual calculation as well as to
evaluate if the obtained number insures a satisfactory degree of safety
with normal powers.
Incidentally, the voltage at 1500W is less than 275 V on 50 Ohm.
High SWR doesn't lead to so high voltages along the whole line.
Very high values occurr in peculiar points in case of unloaded
resonance, in other words if zero or infinite termination is present on
the other edge of the line and when the line itself is almost lossless.
In such a case there is no chance also for PL259/SO239 that may easily
suffer, in addition to the specified, also because of moisture and
oxidization that may penetrate inside, whose fact dramatically decrease
the isolation and the current capability
Also I didn't generalyzed saying that the inner pin of an N carries
more current than a pin and receptacle of an UHF connector.
I told that at reasonably higher frequency than DC or several MHz the
mechanical precision of an N pin doesn't decrease its current
capability like instead happens with the UHF type.
PL259 & SO239 are non standard connectors, some of them are often
machined poorly and mounting mistakes (extra tin, over heat, etc.) make
thing worse reducing the practical current capability and reliability
of the connector.
Checking the material and the shape of an inner in a 7/8 EIA connector,
not only a specialist realize that not only the size but mechanical
precision is important to carry RF current.
In a short cut, since less than 5.5A is the current at 1500W RF, using
an N connector mounted properly one get's a safe, standard weathrproof
connection able also to withstand high SWR and up to the UHF.
In the other end (inner) size or isolation aren't a reasonable point to
support the choice of PL259 or SO239 versus N while the point of a
bigger power capability with UHF is questionable if generalized, and
out of discussion if the word reliability has a sense.
In spite of the gaps, PL259 or SO239 are still the choice of most
people because they appear an acceptable and cheap solution below 30
MHz at similar power of an N.
73,
Mauri I4JMY
> ----- Original Message -----
> From: "Tom Rauch" <w8ji@contesting.com>
> To: <towertalk@contesting.com>; <i4jmy@iol.it>
> Sent: Tuesday, April 03, 2001 1:20 AM
> Subject: Re: [TowerTalk] Connectors
>
>
> > > rated for RF power well exceeding the Kilowatt. The bigger size
of the
> > > UHF central pin connector lead to believe that a UHF connector
> > > withstand more current than N but this is true only at DC or in
the
> > > lowest HAM band where mechanical precision is a less stringent
matter.
> >
> > Dimensions of the N are almost identical to the BNC connector. As
> > a matter of fact, if you remove the locking bayonet from a male
> > BNC it works nicely as a "quick connect" for 50 ohm N females.
> >
> > Normal N connectors are rated for about 1000 watts, and they are
> > living on borrowed time even at 1500 watts. This is especially true
if
> > SWR is high, or if there is moisture ingress into the connector.
> >
> > I am very interested in the claims that the much smaller pin of the
> > N, and the much smaller spacing from the conductor to ground can
> > handle more current and voltage than the larger pin and larger
> > spacing of the UHF connector.
> >
> > I am especially interested because that runs contrary to my
> > experience and the experience of many others that I know over the
> > past 30 years.
> >
> > All hyperbole and conjecture aside, please explain the mechanism
> > that allows a smaller conductor to safely dissipate more heat, or
> > have less surface resistance as frequency is increased, than a
> > conductor several times larger.
> >
> > Also, please explain the mechanism that allows a much smaller air
> > gap to hold off more voltage than a larger airgap.
> >
> > I would be interested in the physical reason for this, and some
> > evidence or explanation why the mechanical holding ability of the N
> > is better.
> >
> >
> > 73, Tom W8JI
> > W8JI@contesting.com
> >
>
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