SMA Connector Selection Guide SMA VS RPSMA.
What happened to make the SMA connectors that we use on our FPV Transmitters and antennas so confusing? I have struggled for the longest time trying to figure out some sort of rhyme or reason to these connectors, and hopefully this post will help you wrap your head around it.
How to think about SMA connectors
To think about RP-SMA and SMA adapters, I have developed this method….
First, think conventionally. Forget about RP-SMA for a minute-because it goes against normal convention, which is what makes it so darn confusing.
So the basic thing to remember about connectors is that (usually) if there is a center conductor sticking out in a manly fashion, then you have a male connector. If there is a hole in the center conductor, then you have a female connector.
So the convention is: SMA Male looks like this:
SMA female looks like this:
So that is pretty easy to remember….. I like to think of the cable box or Satellite receiver (or VCR for you mature folks….lol)
Female connectors are on the cable box, males on the cable.
SMA connectors do the same thing…. females have threads and holes, males have collars and poles.
Why then is RP-SMA so hard to think about?
Along came the FCC, that’s right, another government standardizing agency that has good intentions, but those intentions sometimes don’t make any sense without knowing the history behind the regulations. You do have your ham license right? Right?? It’s time to get one if you don’t…ARRL…
The FCC mandated that the standardization for newer WIFI devices be made so that high-gain antennas couldn’t be put onto newer Wifi devices. Manufacturers of these Wifi devices had to make a new non-standard antenna connector that was hard to find replacements for (as they were all regular SMA before that). The aim was to prevent consumers from connecting antennas with gain and therefore breach compliance (from Wikipedia).
We all know that Wikipedia is a terrible source of credible information, so to dig deeper, this change was implemented to meet the requirements of 47CFR15.203:
“…a permanently attached antenna or of an antenna that uses a unique coupling to the intentional radiator…”
so, yes, the reason was to limit the options for alternate higher gain antennas used on part 15 (unlicensed) equipment.
So how do you wrap your head around RP-SMA?
Think “reverse polarity” RP-SMA…..REVERSE POLARITY-SMA. Take the Female RP-SMA adapter for instance. Start out thinking about what a normal Female SMA connector is like….. A normal female SMA adapter has a hole in the middle and threads on the outside……… NOW an RPSMA female is the same connector with threads on the outside, but the center conductor has a sex change….so it is like the SMA female, but with a male center conductor…….That’s not confusing is it??
Lets try again….
Male RP-SMA……. Think Male SMA…. collar with pole for center conductor………now RP it! Swap the male middle for a female… and you end up with a Male RP-SMA.
The RP-SMA connector……It’s the transvestite of the Sub-Miniature Version A world!
What is the standard?
There is one consistency; 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 should use the outer thread design.
We are working with our manufacturers to try and make sure that video transmitters have a female SMA connector, and all Antennas have a male SMA, but it is going to take a while to get everyone to catch on! If ordering a transmitter, make sure that you look at the connector to make sure it will work without the need for SMA adapters.
SMA Connector Selection Guide SMA VS RPSMA
If you still can’t wrap your head around it, just print this image and put it in your tool box!