SMA, RP-SMA, MMCX and U.FL connectors are widely used in FPV transmitters, receivers and antennas, it’s important to understand the differences to avoid getting incompatible equipment. The gender of SMA and RP-SMA connectors are just as confusing.
Some people find it difficult to get their heads around the different types. Even manufacturers could sometimes get it wrong. When buying FPV equipment with a SMA connector, we urge you to double check if the product description matches product image. If not, confirm with the seller to avoid surprises.
Differences of RP-SMA and SMA Antenna Connectors
SMA and RP-SMA connectors are the original connectors used in FPV equipment. They are still very common due to their robustness and versatility. However they tend to be bulky and heavy.
SMA stands for Sub-Miniature Version A. These are coaxial RF connectors developed in the 1960s.
RP-SMA stands for Reverse Polarity SMA). It is a variation of the SMA connector which reverses the gender of the interface.
There is no difference with video/signal quality between these connectors, but you’ll read further down why we even have these 2 different types. Generally speaking, SMA appears to be more popular choice of connector especially in the mini quad (racing drone) industry.
Here is a comparison table of the SMA antenna and RP-SMA antenna connectors.
For most of us, it would take some time to get to know which was which off the top of our heads. For those who are still getting used to remembering them, an SMA antenna basically has the pin in the middle while the RP-SMA antenna is the exact opposite with a small hole in the centre. It’s not that hard so I’m not coming up with a nursery rhyme to remember it like some others do. :-)
SMA to RP-SMA Adapter – Angle Connector
If you happened to have antennas of the wrong connector, you can get an adapter for it. Some even come with angle to make antenna mounting more flexible.
However note that there is some performance loss to every adapter you use. So avoid it whenever possible.
Removing/Changing SMA Connectors
Many will often replace the connector with the ones they prefer or may choose to add a “pigtail”extension” on the video transmitter to save the components from breaking off.
Whether you are just adding a pigtail or directly soldering the antenna to the VTX, the same steps apply to both. This is a very useful trick and quite easy to do with a bit of soldering skill.
For replacing RP-SMA to SMA or vice versa (on video transmitter), simply cut off the bulk of the connector with some wire cutters or dremel cutting wheel, and use your iron to remove the remaining little legs off the VTX surface. If you’re really handy with an iron or have a heat gun, you can remove it without needing to cut anything off.
An important thing to keep in mind is that an antenna coaxial cable is made up of a ground shielding and signal wire. The ground shielding is the conductive outer layer of the antenna and is used as a shield to help keep interference and RF voodoo magic in line. The signal line runs inside along the middle separated from the ground shielding by a silicone or plastic layer. Do not just solder one end to the other, ground must connect to ground and signal to signal.
When doing the pigtail mod
- Cut off the end you don’t need and strip back approximately 3-5mm of the outer coating exposing the first ground shield layer
- Pull the few mm of ground wires back to expose the middle section of the cable
- Strip just the tip of the signal coating about 1mm to expose just the tip of the signal wire
- Tin both the ground and signal separately, you can split the ground into 2/3/4 separate strands depending on how many ground pads there are to solder to.
- Be sure there is enough length so the grounds won’t touch the signal wire; Be sure to note which pins/pads are the grounds and the signal
- Solder the signal first to the corresponding pad (usually the centre pad) and then start soldering on the surrounding grounds.
- Last step is to find a suitable place for the connector, most usually mount it to the frame away from the receiver antenna on the quad. Voila, a crash proof VTX connector!
VTX’s that come with pigtail
If you’re not so handy with the soldering iron or don’t even want to consider such a task, there are many vendors out there now that sell VTX’s with pigtails already installed or have a variety of mounting and connector options to suit your builds.
AKA IPEX connectors, U.FL has been a popular connector choice in many small size video equipment and antennas. They have actually been the most popular type of connector on radio receivers too due to their compact sizes.
However, they are a lot more fragile than standard SMA/RPSMA connectors and they have a very limited mating cycle.
But in the never-ending search for shaving weight, this may be a great choice. Current options with this type of connector including the TBS Unify Pro HV and the Immersion Tramp HV.
MMCX connectors are slightly smaller and lighter than SMA connectors, but much tougher than U.FL. More and more VTX and antennas are picking up this connector and we can only expect it to play a bigger role in FPV.
Some Interesting History about the RP-SMA Connectors
The intent behind creating RP-SMA was to comply with specific local regulations. The manufacturers had to make a new non-standard antenna connector for their WiFi devices that was hard to find replacements for (as they were all regular SMA before that), to prevent consumers from connecting antennas with gain and therefore breach compliance (from Wikipedia). So they just simply swapped the locations of the centre pin and hole between the male and female connectors.
The term “reverse polarity” here basically means the change of gender. It might confuse those who are not WiFi engineers, as polarity elsewhere in radio frequency literature can refer to the electromagnetic polarity, not to a change of gender.
Hopefully this helps clear up the differences and provides some useful tips. If you have any suggestions or comments please leave them below. Thanks for reading and happy flying!
- Jan 2014 – Article created
- Aug 2016 – Added UFL connector info
- Jul 2017 – Added converter info