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Open in coax

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Subject: Open in coax
From: (John Brosnahan)
Date: Tue, 15 Oct 1996 06:55:39 -0600
>>Interesting things happen to connectors with aging.  Last year I had a 
>>of RG-213 look "open" and it was driving me crazy finding the problem. 
>> I
>>had a cable splice in a termination box, joining a cable-end male-N to 
>>cable-end female-N. There was no strain on this splice.  It turned out 
>>over time (with lots of hot weather in San Diego), the center 
>>dielectric of
>>the cable had migrated (pulled back) relative to the connector body.  
>>male-N pin thus had retracted about 3/16 inch, and separated from the 
>>pin, even though the male pin at one time had been in the right 
>K1VR:  Curious.  I discovered the same problem last winter here.  When
>things got REALLY cold last February, my 40 meter beam stopped working.
>The problem turned out to be that an N connector splice in the K1XX
>asynchronous transformer (to feed a 50 ohm beam with 75 ohm hardline) had
>experienced this "n-connector pullback".  The inner body of the N
>connector, on the female side, had retracted and was no longer making
>contact.  Instead of attributing it to warm weather, as Glen K6NA does, I
>had attributed it to cold weather.  However, at that time, someone wrote
>me that he had discovered the same problem in several places with N
>connectors on RG-213 at KN8Z, where the N connectors were taped
>vertically to a tower leg (as was true in my case).  So it may be an N
>connector problem, not a weather problem.

This problem is not limited to N-connectors, nor to RG-213.   But the "old"
N-connectors are very susceptible to this problem in even short runs.
The captivated contact N-connector--the UG-1185 is an improvement.  And
the new style of N-connector that is a two part device more closely resembling
a UHF style assembly should also help with limiting the center pin movement 
on cables with braid as oppossed to hardline.

But the forces can be very large when dealing with long runs of heavy duty
cable.  We had many runs (when I was the engineer for the University of 
Colorado's Radio Astronomy Observatory) of 1000 ft or more of 7/8 inch hardline
(100 ohm stuff that was picked up surplus).  The temperature varied over a wide
range at our almost-9000 ft elevation.  I have seen times where the center
had EXTENDED so much that it acted like a hydraulic piston and destroyed a
100:50 ohm transformer and cracked the cast aluminum box.  We ultimately had 
to design a non-coaxial style of jumper that would tolerate some movement, on
occasion more than 1.5 inches on the longest runs of close to 2000 ft.   This 
movement was easily much greater than a typical length of engagement
of a center pin in a standard connector.  (But this was RX only and we had
figure establishing preamps and the antennas, and the minor impedance bump was
not a problem at HF and low VHF.

Hardline in most forms is made with the center bonded to the shield through the
dielectric and isn't suppossed to expand and contract differentially--the
shield should expand and contract to compensate but we were using corrugated 
shield hardline and it WAS a problem.

John   (expanding the scope of the problem without offering viable
John Brosnahan  W0UN
24115 WCR 40
La Salle, CO 80645

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