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about RF connectors/adaptors - RF Cafe Forums

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Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: about RF connectors/adaptors Posted: Fri Jun 30, 2006 7:53 pm


Joined: Tue Apr 18, 2006 10:40 pm
Posts: 9
hello....i'm new to using RF lab equipment so i've a basic question...

an N-connector has cutoff frequency of ~11 GHz and SMA ~25 GHz. when an N-to-SMA adaptor is used, what's meant by this? That the cutoff frequency moves upto 25 GHz, meaning there will be 50 ohm match upto 25 GHz?

or the cutoff frequency is 11 GHz after using this adaptor? i don't think it will be 11 GHz. for one, i've two pieces of equipment that say in their manuals that they're compatible w/ each other but one has N connector and the other has SMA/3.5 mm connector because it operates upto 26.5 GHz---so i'd thinkt he cutoff will be around 26 GHz and not 11 GHz (for N) otherwise the two equipment are not completely compatible. thanks


Post subject: Posted: Wed Jul 05, 2006 10:27 am


Joined: Thu Apr 13, 2006 9:53 am
Posts: 2
Location: Florida
The cutoff freq of a standard N connector is 11 GHz whereas a precision N is 18 GHz. The cutoff freq of a standard SMA is 18 GHz and a 3.5mm is 26.5 GHz. These values are for connecotrs made to the MIL standard. Many manufactures control the machined tolerance much better than defined in the MIL spec so they can boost the frequency performance of there version to a higher frequency range (SMA to 22-26GHz vs 18GHz from a "standard" for example).

The frequence range of a between series adapter (N to SMA etc.) is limited to the frequency range of the lowest type's freq range. In a standard N to SMA the freq range is limited to 11GHz ortherwise moding can occure and for sure it will have higher loss at freq above 11GHz. The actual loss will have to be measured since it woun't be specified on any spec sheet. This is also true of cables made with sifferent series connectors.

best source of info are the App-notes from M/A-Com, Maury Microwave and other connector Mfgr. Also look for connector design articles in back issues of RF Design, MW&RF etc.

Hope this helps



Post subject: about RF connectors/adaptorsPosted: Thu Jul 06, 2006 5:37 am


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
I can't add much more to what RFTEJerry has said, except a note about the cables to which the connectors are attached.
A coaxial cable to which the N type is attached (and even the connector) will contain dielectric insulation. This means that a typical 12 to 15mm diameter (id) coax cable will have a set of performance parameters that include Reflection Coefficient and loss attenuation (S11 and S21).

There is a non-TEM waveguide mode that can be set up in a coaxial structure, and I think this can propagate at frequencies above about 7GHz. The effect of the dielectric insulation is that the lowest cut off frequency for the rogue non-TEM mode is lower than the 11GHz stated.

If these are present in one part of the system, the part terminated in the N-type, then when they encounter the transition to a much smaller cross section coax (the SMA part) they encounter quite a high Ref Coeff. Thus the combination of 2 different cable sizes and a BSA type adapter imposes a Ref Coeff spec that sets the frequency of application to a max of around 11GHz.
A lot depends on how bad an S11 spec you can live with


Post subject: Posted: Sun Jul 09, 2006 1:45 pm


Joined: Tue Apr 11, 2006 8:13 pm
Posts: 34
Most test equipment (spectrum analyzers, VNA's) that I have worked with have an N-connector, even though they are meant to be used at frequencies way above the cut-offs mentioned in the previous posts. How much of an impact would that have on the measurements?


Post subject: Posted: Sun Jul 09, 2006 3:49 pm


Joined: Thu Apr 13, 2006 9:53 am
Posts: 2
Location: Florida
The Type-N connector on the spectrum analyzer will handle the frequency range of the analyzer with the loss and VSWR accuracies specified by the instrument manufacture.

When you reference a Type-N, SMA, 3.5mm it is understood to be the standard MIL-SPEC connector and it will be limited to the MIL-SPEC frequency range. A connector mfg can make a mechanically equvialent connector with an extended freq range by having the manufactured dimentions and tolerance more precisely controlled. Any connectors frequency range is controlled by its mechanical dimensions and the dielectric, if used. A 3.5mm does not have a dielectric so it can go to 26.5GHz whereas the SMA can only go to 18GHz (it has a dielectric), but they can mate. Many instrument manufacturets and cable manufacturers also make their own versions of Type-N and SMA connectors just so they can use them on their instruments and cables at extended frequencies.

Visit connector mfg and cable mfg web sites and read their App notes. They will explain just how they are able to extend the freq range of a Type-N from 11GHz to 18GHz and the SMA from 18GHz to 26GHz.

I know this can be confusing because it was to me when I first started in RF test. But after researching and reading it became clearer.

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