SMA and RP-SMA are so-called coaxial connectors and the standard for antenna connections. With a screwable coupling mechanism, they form the minimum interfaces for coaxial cables.
They are now used in many different devices and applications in the high-frequency range. In the following you will learn what distinguishes the SMA standard from RP-SMA and which version can be used for which purpose.
Adapted to the structure of a coaxial cable, the coaxial connectors also protect the inner conductor from electromagnetic influences by surrounding it with an insulating layer. The so-called dielectric is often made of plastic such as polyethylene and ensures both shielding and capacitive behavior.
In addition, both the characteristic impedance and the impedance influence the structure of a coaxial connector. The line characteristic impedance is mainly influenced by the properties of the dielectric and describes the relationship between a current moving in the same direction and moving voltage waves. The characteristic impedance of the cable depends on the frequency, while the cable length has no influence on it.
The impedance is usually 50 ohms in the high-frequency range and 75 ohms for audio and television technology. The impedance results from the ohmic resistance and the reactance.
The main advantages of coaxial cables are their high transmission speed and simple connection technology. SMA and RP-SMA are among the most important types of coaxial connectors.
The classic SMA connector was developed in the 1960s. The abbreviation SMA stands for “Sub-Miniature Version A”.In order to maintain the transmission speed of coaxial cables, both versions of the corresponding connectors have a coaxial design. They therefore have good electrical shielding and low electromagnetic interference.
Easy to confuse with an SMA connector is the Reverse Polarity SMA design. This variant was originally developed to prevent the unauthorized connection of a stronger antenna. The “reverse polarity” of the RP-SMA refers exclusively to the interface variant and in no way to the signal polarity.
The RP-SMA connector also has an internal thread, but it has the cup in the middle and not the pin. Compatible with this, the socket consists of a female thread with a female pin. Thus an SMA antenna connection fits mechanically on an RP-SMA socket, but not electrically.
SMA and RP-SMA are standard threaded antenna connectors for high frequency applications from 1 GHz to approx. 16 Ghz. They are most commonly used in microwave systems, such as portable radio and mobile phone antennas, but also with WiFi antenna systems.The two versions differ in the reversal of the design. The SMA connector has a union nut and an inner conductor pin, the socket consists of a male thread and an inner conductor tube.
In the area of LTE / GSM / UMTS, an SMA connection is usually used as standard for antennas and devices. RP-SMA, on the other hand, is mainly used for WLAN antennas and devices.
A frequent application is to improve reception by relocating or extending the feed to a WLAN antenna. Due to different application areas and antenna designs, there are also different connections for SMA. If necessary, these must be adapted by adapters.
One advantage that RP-SMA offers over SMA is wear. With frequent unscrewing and unscrewing – normally up to 500 connections are no problem – it can happen that the inner conductors wear out and thus the signal strength decreases. In the case of an SMA built-in socket, it is more complex to replace the connection on the device. With an RP-SMA connector, on the other hand, the cup is located on the antenna or cable, for example, which means that in the event of wear, only the antenna or cable would have to be replaced.
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