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[TowerTalk] Connectors

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Subject: [TowerTalk] Connectors
From: (Tom Rauch)
Date: Tue, 3 Apr 2001 20:33:51 -0400
> With 1500 watts passing through an N connector, a flat 50 ohm load
> gives roughly 275 volts and 5.5 amps. A two to one SWR could raise the

I'm not sure what the failure mode is, because generally by the 
time I know the connector has a problem the guts are "toast".

275 volts is RMS. Our concern is always peak voltage for dielectric 
failures. That is why I suspect voltage more than current.

> voltage to roughly 400 volts or the current to roughly 8 amps, if the
> connector was unfortunately situated in a voltage or current maxima. 

In this case the peak voltage is 550 volts, and that does not include 
the overshoot common with many rigs. Stick an IC-775 on a 
comfortable head-room PA and the transient at leading envelope 
edge will be may be 1.414 times more than that!

Grab the wrong antenna, and you are really in the soup quickly 
with the tiny air gap in an N. The narrowest gap is often at the base 
of the male pin. Who knows what I have out there when I transmit 
on 160 into my 80 meter dipole.

> How much of a safety margin? Two to one? Figure whether you are happy

2:1 is a reasonable margin if the connector is indoors and you 
never screw up. 

> with 16 amperes of RF on that little N pin. Or whether you like 800
> volts of volts of RF on the gap between a BNC center conductor and
> sleeve.

Actually I'd worry about voltage. Once you carbon track the 
insulation, you are on the road to toasted N.

My general rule is if I wouldn't trust the connection to a BNC, I 
won't trust an N connector. They are dimensionally almost the 

> I have N connectors for my 1 1/4 hard line. Wonder if W8JI has posted
> just how he "machined" the hardline N connectors for UHF?

The female N has the correct thread pitch. I remove the center pin, 
bore the inside of the threaded area to the OD of some insulated 
tubing, machine the threaded area to the correct length, cut 
notches in it for the tabs on the male, and use a small chunk of 
thin copper tubing to attach the center from a SO-239 to the old 
pin. I use teflon tubing for the new dielectric, or polyethylene tubing 
like used for water lines.

It costs about fifteen minutes and  $1.00 to convert it, plus the 
connector is repairable...although I've never had one fail yet. I do 
have a milling machine, but I'd bet it can be done with a drill and 
hacksaw if you are careful.

73, Tom W8JI 

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