It's interesting that the N connectors would burn up "every so often".
Presumably they were always OK when tested immediately after
installation, and then failed sometime later. That begins to sound like
creepage of the inner conductor in the cable, leading to a poor contact
pressure as the pins move apart. Pulse power increases the momentary
I-squared-R losses much more severely than the peak voltage, and the
higher frequency increases the RF skin resistance... then up she goes.
If that line of speculation is correct, then suspicion may fall on the
"MIL-spec" N connector with the floating center pin and very poor
anchoring of the cable to the connector body.
With 1-1.5kW on the old 432MHz moonbounce system, I had some N
connectors survive for years, but have also had a 7/16 break down. The N
connectors that survived were the ones on the coax relay, where I had no
other choice; but those plugs were the modern style with a captive
center pin, and I tried hard to assemble them with absolute perfection.
At that combination of frequency and power level, N connectors that are
not perfectly assembled WILL burn up - if not immediately, then sometime
later, because the cable will always move and 'settle' after
installation. It only takes a tiny amount of movement to make a very big
change in the contact pressure... and then...
The complete connector pair then has to be replaced, so wherever
possible I replaced failed N connectors with 7/16 DIN. (These were cheap
at the time because the British cellular system was undergoing a refit,
and nobody else at hamfests had the slightest interest.) The huge 6mm
center pin is very securely anchored into the Teflon insulator, so
problems due to conductor migration became a thing of the past.
However, there was one 7/16 plug that did fail, due to moisture ingress
and voltage breakdown across the exposed face of the Teflon. The
breakdown had etched a 'fractal' branching pattern into the Teflon,
though surprisingly there was no sign of burning. The failure was a
half-wavelength away, a hole blown through the side of the 9913-type
cable.
Looking back, many of my N-connector failures have been my own fault,
for assembling connectors 'by hand and eye' out in the field, failing to
wrap exposed connectors in this damp climate, or other bad installation
techniques such as failing to anchor the cable at both ends of a rotator
loop (so that no twisting and pulling stresses can reach the connector).
But I feel strongly that the biggest mistake anyone can make with an N
connector is to use the unimproved "MIL" design, because it punishes
exactly those kinds of errors that we amateurs are most likely to make.
73 from Ian GM3SEK
>-----Original Message-----
>From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of John
>Lyles
>Sent: 29 December 2013 03:12
>To: amps@contesting.com
>Subject: [Amps] "N" Connector
>
>When I came into my current job, there were some cavity amplifiers
using
>RCA 7651 tetrodes, running 5 kW pulsed power, about 600 watts average.
>This was at 200 MHz. The N connectors on the input to the next stage
(an
>RCA 4616 tetrode) would burn up every so often. Since the RCA cavity
amp
>input was tuned, VSWR could wander up when fingers twirled knobs too
>quickly. We replaced the N with 7/16 DIN and never had another failure.
>It was clearly too much for N connectors, the VSWR and the freq x
power.
>73 and happy new year,
>John
>K5PRO
>
>
>> Message: 1
>> Date: Thu, 26 Dec 2013 12:40:45 -0500
>> From: "Mark Bitterlich" <markbitterlich@embarqmail.com>
>> To: "N1BUG" <paul@n1bug.com>, "Amps reflector"
><amps@contesting.com>
>> Subject: Re: [Amps] "N" Connector
>> Message-ID:
><B8CB101417974FA2B7987EC7449B4E35@PC144745932200>
>> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>> reply-type=response
>>
>> I've put 10 KW PEP (SSB) through a standard Milspec N connector
>attached to
>> RG-393. Also with L44W 1/2" Heliax connectors (solder on center pin
as
>> well) . However, this was with a perfect match.
>>
>> And for Peter .... this was not on the ham bands, so you can rest
easy.
>>
>> Mark
>> WA3JPY
>>
>> ----- Original Message -----
>> From: "Carl" <km1h@jeremy.mv.com>
>> To: "N1BUG" <paul@n1bug.com>; "Amps reflector"
><amps@contesting.com>
>> Sent: Thursday, December 19, 2013 9:43 AM
>> Subject: Re: [Amps] "N" Connector
>>
>>
>>> I run 1500W on 144, 222 and 400W on 432 thru N connectors.
>>> The 1000W AM CARRIER (4000W PEP) 432 military cavity Ive been
>working on
>>> has an N.
>>>
>>> All Mil Spec US made of course.
>>>
>>> Carl
>>> KM1H
>>
>
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