How important is maintaining mechanical tolerances for mil spec connectors?

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Discussion Overview

The discussion revolves around the maintenance and cleaning of military specification (mil spec) connectors, particularly in the context of corrosion and mechanical tolerances. Participants explore various cleaning methods, the implications of mechanical integrity on performance, and the specific challenges associated with RF/microwave connectors.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant inquires about effective cleaning methods for lightly corroded mil spec connectors, expressing concern that strong acids may damage the plating.
  • Another participant suggests using mild electric contact cleaner sprays as a successful alternative for cleaning connectors.
  • There is a viewpoint that for RF/microwave connectors, cleaning is insufficient and replacement is necessary due to potential damage affecting electrical performance.
  • A participant emphasizes the importance of mechanical tolerances in maintaining transmission line performance, noting that even minor changes can significantly affect impedance and return loss.
  • Discussion includes the frequency of connect-disconnect cycles and their impact on connector integrity, particularly for high-performance connectors.
  • Concerns are raised about the consequences of mechanical damage, including effects on measurement accuracy and power efficiency in transmission lines.
  • Some participants mention the use of torque wrenches to ensure proper connection without damaging the mechanical structure of connectors.
  • There is a distinction made between different types of connectors, such as BNCs and Type N, regarding their specifications for return loss and performance.
  • Participants highlight the practice of using sacrificial adapters to protect expensive equipment from damage during frequent connect-disconnect operations.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of cleaning versus replacing RF/microwave connectors, indicating a lack of consensus on the best approach to maintaining connector integrity.

Contextual Notes

Participants discuss various cleaning methods and their potential risks, but there are unresolved questions regarding the specific impacts of cleaning on different connector types and the mechanical tolerances required for optimal performance.

dingpud
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Aside from replacing a lightly corroded mil spec connector, does anyone have any experience in a process for cleaning them up? Myriadic or hydrochloric acid have been recommended, but my thoughts are that they will eat away at the plating...
The connectors are either

Aluminum
Cad Plated Steel
Stainless

Anyone who has actually done this, I would really appreciate info on your trials and tribulations.
Thanks,
pud
 
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dingpud said:
Aside from replacing a lightly corroded mil spec connector, does anyone have any experience in a process for cleaning them up? Myriadic or hydrochloric acid have been recommended, but my thoughts are that they will eat away at the plating...
The connectors are either

Aluminum
Cad Plated Steel
Stainless

Anyone who has actually done this, I would really appreciate info on your trials and tribulations.
Thanks,
pud

I have had success with just regular electric contact cleaner sprays:

http://www.google.com/images?hl=en&...ic+contact+cleaner&gbv=2&aq=f&aqi=g1&aql=&oq=

They are generally pretty mild in their action. You could try the CRC brand for example, to see if it worked for you. You can find them at many auto parts stores and some motorcycle shops.
 
Yep, that helped...

Thanks!
 
It depends however.

If it's a RF/microwave connector, you should toss/replace it. Cleaning won't help either the electrical match or the mechanics damage. You'll still likely have return loss.
 
jsgruszynski said:
It depends however.

If it's a RF/microwave connector, you should toss/replace it. Cleaning won't help either the electrical match or the mechanics damage. You'll still likely have return loss.

Could you say more about this? I'm not familiar with what you are saying (yet).
 
Mechanical tolerances are key to maintaining the transmission line performance of the connector. This is the primary reason why microwave connectors and adaptors are so pricey. You can look at the equation for http://www.microwaves101.com/encyclopedia/coax.cfm#Z0" and see it has the inner, d, and outer, D, diameter terms.

Calculate the wavelength at UHF and microwave frequencies and you see the wavelength is at nearly the same physical size as transmission line diameters. This is why just a little change in mechanicals can change impedance which in turn change return loss of the interface between connector and coax line. So if you are expecting 1% accuracy wavelength of 5 mm and coax dimensions of ~3 mm, a little damage could ruin your day

Every time you disconnect and connector a connector like an SMA, SMB, APC 3.5, 2.4mm, etc. you are causing some mechanical damage. The typical connect-disconnect cycles before the connector exhibits significant return loss changes can be as low as 20-100 cycles for high performance connector (3.5 mm, 2.4 mm) depending on the return loss and mismatch uncertainty you can tolerate for your system performance.

The issue is that because of http://www.eetimes.com/design/analog-design/4017988/Improving-Measurement-Accuracy-by-Controlling-Mismatch-Uncertainty" , you can have a significant effect on measurement accuracy (for an instrument) or power efficiency (for a transmitter) or sensitivity (for a receiver) if you bollux the return loss of components in your transmission line chain.

This is part of why you have to be http://www.microwaves101.com/encyclopedia/connectorcare.cfm. When I worked at HP T&M/Agilent there was (probably still is) a "connector care" training course sold to customers (for several $1K)! It was based on this [URL="http://cp.literature.agilent.com/litweb/pdf/5954-1566.pdf" (PDF)]application note[/URL] but went into more gory details.

This is also where the connector torque wrenches come in: they put exactly the most tightening possible without damaging the transmission line mechanicals. That little pin in the male side of the connector is fitting into a female receptacle and you can overdrive it in and cause mechanical damage. Not connecting tightly enough leaves a gap that causes return loss. Altering the mechanic dimensions of the entire coax line by overdriving will cause return loss.

Also if you've misthreaded a connector like an APC 3.5 or 2.4 or similar M-F connector and then tried to force it even a little, it's probably already had some significant damage (on both sides) that is easily visible with a network analyzer.

Often you'll see people using sacrificial adapters in situations that have frequency connect-disconnect cycles - the idea being, destroy the $400 adapter rather than the built-in connector on the $200,000 network analyzer that will cost your a few days of $300/hour repair and calibration to fix.

Connectors like BNCs don't really matter so much but only because they have absolutely no spec for return loss or any other transmission line performance even defined. They are low-frequency, lumped-element connectors. Thus if you are doing anything where return loss, impedance matching or reflections might matter, absolutely do not use a BNC. Yes, some commercial firms selling instruments or products get this wrong.

Other connectors like Type N or UHF do have some specs for this defined but they also have cut-off frequencies defined.

Of course the main of idea of worrying about connector care in the first place, even if your application doesn't need super good return loss or isn't affected by connector damage, is that connector handling is a habit which should be done well for the times when it does matter.
 
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