SMA Antenna Connectors


RF Connector Conventions

This tutorial attempts to explain the confusing naming conventions for SMA connectors. If you would rather not understand why the convention is the way it is, you can just look at the 4 pictures and move on. Otherwise, have fun with the read!

SparkFun uses SMA connectors on boards that need a 50 Ohm impedance connection to an external antenna (GPS, Bluetooth, cellular, Nordic, and XBee). However, some of these boards use different genders and polarities of the SMA connector. Therefore, we need different antennas to match the specific gender or polarity of the RF connections.

There are 4 different types of SMA connectors using a combination of gender, which refers to the center pin and polarity, which refers to…..uh, this is where it gets confusing. Wikipedia tries to explain it. But from what I have found there was an original “old” design for SMA connectors.

The original SMA design called for two compliant connectors:

http://www.sparkfun.com/tutorial/SMA_Antenna_Connectors/SMA_Malesmall.JPG

SMA Male

http://www.sparkfun.com/tutorial/SMA_Antenna_Connectors/SMA_Femalesmall.JPG

SMA Female

The above two connectors were designed to be used together, but there was a problem with this configuration and the FCC started moving towards Part 15 compliance. All this means is that all the SMA RF connectors are changing gender (center pin). Really annoying for those of us who need to mate an antenna to an RF device. The FCC gender change was instituted to prevent home users from damaging RF equipment (think home WiFi) when screwing on an antenna. If all antennas are female, there is no way to damage the center connector.

There is one consistancy however; all antennas, cables or anything was being attached to a potential stationary object used an outer nut or inner thread design and all stationary devices used the outer thread design. This applies for all SparkFun products. All of our antennas are either SMA male or RP-SMA female. All of our boards are either SMA female or RP-SMA male.

The only thing that changed with the Part 15 compliance was the center pin, thus reversing the polarity of the connection and forming a “new” standard; the reversed polarized SMA (RP-SMA).

The next two photos are considered reversed polarized (RP-SMA).

http://www.sparkfun.com/tutorial/SMA_Antenna_Connectors/RP-SMA_Malesmall.JPG

RP-SMA Female

http://www.sparkfun.com/tutorial/SMA_Antenna_Connectors/RP-SMA_Femalesmall.JPG

RP-SMA Male

SparkFun RF boards and antennas use a combination of  the old (SMA) and new (RP-SMA):

Anything 2.4GHz (Bluetooth, ZigBee, WiFi, and Nordic) use the new convention: RP-SMA male on the antennas and RP-SMA female on the modules.

Cellular and GPS (900/1700/1800MHz and 1.57542GHz respectively) use the old convention: SMA male for the antennas and SMA female for the  modules.

Really, you can ignore the gender descriptor. If you have a RP-SMA board or module, you need a RP-SMA antenna and so forth for SMA. Pretty simple, right?!

And just in case if you happen to find the old and new mixing, we sell a SMA male to RP-SMA male and a RP-SMA female to RP-SMA male connector that will most combinations of antenna and connector to be mated.

I hope you are not thoroughly confused!

-Aaron

Comments 14 comments

  • “The above two connectors were designed to be used together, but there was a problem with this configuration and the FCC started moving towards Part 15 compliance."
    I think what you mean is that the U.S. Government (FCC) decided to reverse the connector polarity to prevent consumers (you) from attaching high gain antennas. About twenty milliseconds after the first reverse polarized WiFi box came out, reverse SMA adapters fell from the sky.
    This is what happens when you let the Government control your life. And now we’re getting Government controlled health care shoved down our throats in the U.S. I’m looking forward to waiting six months for my first reverse polarized colonoscopy.
    David

    • Gee, Drone, should be even madder now that all our hard work as Democrats has come to fruition. Perhaps Sparkfun should start selling fiber-optic equipment so he can get a head start on his own do-it-yourself colonoscopy kit.
      David, judging by your wack-a-loon comment in regards to government, it becomes immediately apparent what the actual source of your colonic blockage is.
      _richard

  • Yes, it’d be far better to have no regulation regarding the RF spectrum, because it’s completely unacceptable that you as a free person should be told what you can and can’t do. Nobody “owns” the airwaves, right?
    Anybody should be able to run as much power as they’d like on any frequency for any reason. If someone’s interfering with me, I’ll just bump up the power. Then I’ll buy a new amplifier. And who cares if that amplifier is throwing huge gobs of interference up and down the band for 100MHz in either direction? If it was really that important, they’d make devices so that they could tolerate it. I mean, it’s not going to kill anyone… there aren’t any medical devices or anything on the ISM bands, are there?
    [sarcasm off]

    • In case it’s not obvious, the above is a response to Drone’s message on 2009/07/02. I hit the nifty “reply” button but it didn’t work. =(

  • “The FCC gender change was instituted to prevent home users from damaging RF equipment when screwing on an antenna."
    Not quite. The issue is that 47 CFR 15.203 requires that, "an intentional radiator shall be designed to ensure that no antenna other than that furnished by the responsible party shall be used with the device. The use of a permanently attached antenna or of an antenna that uses a unique coupling to the intentional radiator shall be considered sufficient to comply with the provisions of this section."
    In plain English, the FCC wants to prevent untrained home users from being able to buy an off-the-shelf high-gain antenna that lets them exceed the Part 15 EIR Power limits.
    Rather than design a completely new RF connector, WiFi manufacturers opted to simply flip the polarity of the standard SMA connector. Over time RP-SMA has become a de facto standard for 802.11 home gear, subverting the intent of sec 203. I have to question the practicality of requiring a unique connector for every such product anyway.
    Thanks for all the great work on the tutorials!
    Eric MacIntosh
    MindTribe Product Engineering

    • A unique connector isn’t actually required ? just “sufficient”, and it happens to be one of the easiest ways to achieve the goal. There are others, but they generally involve far more complex methods.
      And since the folks who make 50-ohm amplifiers or ultra-high-gain antennas aren’t going to bother to change them for this, it at least helps prevent truly accidental use of such gear (along with the excuse “I had no idea”).
      And no, it doesn’t stop the folks just smart enough to know how, and not smart enough to remember that triangulating a mostly-stationary signal source is a matter of a few minutes with a car and a directional antenna.

  • Will using an adapter degrade or attenuate the signal at all, since the impedance will then be different?

    • Most connector are impedance matched for their application, even if they are not, the effect is negligible for most amateur application unless you daisy chain a lot of them. An important factor is the RF losses which is caused by the weak electrical property of the contact itself, Gold, Silver and Browse if frequently used to improve the electrical property of the contact.

      The principal factor that will affect your transmission quality and range are;

      -Cable length to the antenna (keep it short)

      -Poor quality connector and presence of corrosion between contact

      -High amount of RF noise (Use a spectrum analyser to determine the less occupied frequency)

      -High amount of EMI noise (Make sure the current going to your radio is clean, use filters or separate power supply for motor and radio, learn how to use torroid and shield your radio equipment’s)

      -Harmonic, If your setup use more than 1 radio on different band you will need low pass filter on the lowest band radio TX to avoid harmonic saturation on the highest band radio RX

      -Polarization, both TX and RX need matched polarization (not talking about RP-SMA vs SMA, read light or wave polarization)

      -Antenna gain, center frequency, bandwidth, radiation patern…

      -Diversity radio (2 radio can complement them-self on different frequency, a simple circuit that monitor the RSSI can switch to the best feed automatically)

      With these in mind, I made a RC car with video feedback that can go several miles in a dense urban area without exceeding the FCC ERI limit on 433, 900 and 2400 ISM bands…

      Good luck

  • I have a bunch of old RF wire that contains SMB and SMC connectors.. and of course I have tons of stuff with UHF, N and F type connectors too..
    There are a LOT of different RF connectors out there..
    This web page seems to cover many of them:
    http://www.wa1mba.org/rfconn.htm

  • Has anyone else noticed a completely different labeling system regarding SMA connectors from Wiki?
    http://en.wikipedia.org/wiki/SMA_connector
    Will the real SMA labels please stand up?!

  • Your images are labelled incorrectly. The RP-SMA male and female are backwards. The RP-SMA Female connector has threads on the outside and a pin on the inside.