If you follow towertalk you will read about how terrible N connectors are.
Many comment on the small pin being the same size as a BNC, how could it
possible be good for a kilowatt. I agree that they do not look impressive,
especially on the end of 1 5/8 Heliax, however I have a number of N
connectors at my station and I can run legal limit RTTY though them all
day long. Jim Lux W6RMK brilliantly explained how N connectors really
connect. The small pin is mostly just for alignment. While I always tried
to install them properly I never thought how critical the pin and socket
settings really are. I have a lot of precision measuring tools but
measuring the pin's step depth is still pretty tough without specialized
equipment.
John KK9A
From: jimlux <jimlux@earthlink.net>
Date: Tue, 26 Jun 2018 07:46:15 -0700
The UHF connector has a larger diameter center pin (0.156") than the N
(0.1197"), so its current handling capability is (slightly) higher.
__
True enough. I think, actually, a better thing to look at would be the
"mating contact surface". In a N connector, I think the contact area is
between the faces of the two inner conductors - the pointy pin on the plug
is just to center into the pin on the receptacle side.
Take a look at the *RF* mating surfaces in the N
https://www.amphenolrf.com/media/wysiwyg/N-Type_Interface_Drawing.JPG
Skin depth in Copper at 7MHz is about 25 microns (0.001"). The center pins
are probably gold plated beryllium copper, and the conductivity is
probably about the same.
If we assume that most of the current is carried in a layer about 2-3 skin
depths deep, just the outer surface of that 0.120" pin in the N connector
is carrying the current, and more importantly, the contact area is a thin
annulus, 0.120" OD, 0.116" ID - .002"* .75" (.118*2*pi)
If that connector is a bit dirty, or the mating surface is damaged or off
angle, then you've got a problem.
If you have an N where the female side center conductor is spring loaded
(2 slot or 4 slot), then you have the problem of the "prong" being
slightly bent, and not making good contact with the pin on the male side.
In the UHF connector, it's more like a pin plug in a banana jack. The
contact path is through the *side* of the pin on the plug. There's no
illusion of trying to carry the current in a constant diameter (constant
impedance) path, so the wiping and contact area is much larger.
And then, the similar thing applies on the shield side - there are
slotted/springy and non-slotted versions here. From an RF standpoint, the
mating surface is that little ledge inside the female, and the face of the
tubular part sticking out of the male. That face can be damaged or roughed
up, or even bent in a bit, so the contact area is reduced.
For the "drive across it in the mud" UHF connector, the shield contact
area is those sawteeth interlocking, and with an installer applying force
with pliers, that will deform to make good contact, even if there's crud
in there. and, again, no illusions about "constant impedance".
>From a "lightning protection" standpoint, the outside of the cable carries
the current, and that's the screw thread and barrel, in both cases.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
