The SMA vs RP-SMA trap
This is the single most expensive connector mistake in the field.
- Standard SMA: plug carries the pin, jack carries the socket.
- RP-SMA (reverse polarity): plug carries the socket, jack carries the pin.
- The thread is identical. An RP-SMA plug screws smoothly onto a standard SMA jack, feels seated, and looks correct — but the two centre contacts never touch. The result is a DC-open feed: infinite VSWR, a "dead" antenna, and a return that tests fine mechanically.
Where each one lives: retail Wi-Fi routers and APs almost always use RP-SMA; industrial modules, radios and test equipment use standard SMA. When you specify an external antenna, state the connector as "SMA male (plug, pin)" or "RP-SMA male (plug, socket)" — never just "SMA male".
Regulatory nuance, worth getting right: 47 CFR 15.203 does not name RP-SMA. It prohibits "a standard antenna jack or electrical connector" and permits "a unique coupling". RP-SMA is the industry's customary way of complying. Writing "the FCC requires RP-SMA" is factually wrong and a knowledgeable buyer will notice.
Frequency-ceiling caveats
- SMA — the 18 GHz figure is the interface rating. Amphenol publishes VSWR limits only to 12.4 GHz for flexible-cable groups (RG-58 / RG-174 / RG-178). Say "DC–18 GHz interface, 12.4 GHz typical in flexible-cable assemblies."
- MCX — there is a genuine vendor split: 6 GHz on most catalogues, 12 GHz on some Amphenol pages. Design to 6 GHz.
- SMC — 4 GHz standard, 10 GHz on extended-range designs.
- U.FL — a 30-cycle part with VSWR 1.3 max to 3 GHz and 1.4 max from 3–6 GHz. Design it as assemble-once: remove the plug with a proper tool, never by pulling the cable. Any port a customer will plug and unplug must be SMA, BNC or N.
Choosing a connector
| Situation | Choose | Why |
|---|
| Board-to-internal-antenna jumper, <0.3 m | U.FL / IPEX (MHF1 or MHF4) | Smallest, cheapest; 30 cycles is fine for factory assembly |
| External antenna port on a plastic IoT enclosure | SMA (standard, unless you are matching consumer Wi-Fi gear) | 500 cycles, weatherproof options, universally available |
| High-vibration or outdoor industrial | TNC or N-type | Threaded coupling resists vibration; N handles large low-loss cable |
| Gateway or mast feeder, long cable run | N-type | Lowest loss at low frequency, IP67 variants, lightning-exposed masts |
| Automotive or multi-antenna vehicle harness | FAKRA | 14 codings prevent mis-mating across GNSS/cellular/DAB ports |
| Lab or video equipment | BNC — but confirm 50 Ω vs 75 Ω | Cheap and fast; the impedance ambiguity is the hazard |
| Wearable or very small handheld | MMCX or U.FL | Smallest footprint; rotatable MMCX reduces cable stress |
Insertion loss per mated pair is frequency- and grade-dependent — roughly 0.1 dB at 10 GHz and 0.2 dB at 18 GHz for an SMA pair, about 0.05 dB at 10 GHz for an N pair, per published references. Do not budget a single universal number; state the frequency you measured at.
Related
Sources
- Amphenol RF connector families (SMA, SMB, SMC, MCX, MMCX, TNC, N, BNC) — https://www.amphenolrf.com/zh-cn/products/rf-connectors/sma-connectors (accessed 2026-10-26)
- Hirose U.FL-R-SMT-1(80) specification sheet — https://www.hirose.com/product/download/?distributor=digikey&type=specSheet&lang=en&num=U.FL-R-SMT-1(80) (accessed 2026-10-26)
- eCFR 47 CFR §15.203, antenna requirement — https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15/subpart-C/section-15.203 (accessed 2026-10-26)
- Amphenol RF FAKRA / USCAR product specification — https://www.bce.it/wp-content/uploads/2012/02/Amphenol-RF-FAKRA-revb-3-BCE10-11.pdf (accessed 2026-10-26)
Frequently Asked Questions
Can I convert RP-SMA to SMA?
Yes, with a purpose-made adapter. Never force-mate two connectors that "fit" — the RP-SMA/SMA pair is precisely the case where it fits and does not connect.
How many times can I unplug a U.FL?
About 30 times per the Hirose specification, and practice is less forgiving. Treat every disconnection as a consumable event.
Is SMA weatherproof?
Standard SMA is not. Use IP-rated assemblies, a sealed bulkhead connector, or move to N-type with a weatherproof boot for outdoor installations.
Why does my Wi-Fi router antenna not fit my industrial radio?
Almost certainly RP-SMA (router) versus standard SMA (radio). Check the centre contact, not the thread.
What is the minimum order quantity and lead time?
Standard models ship in 7–15 working days after order confirmation; samples of in-stock models can ship within 3 days. MOQ depends on the model — contact us for the exact figure for your part number.
Can AntennaHome customize an antenna for my device?
Yes. We provide customized design including frequency tuning, connector and cable length, housing and mounting method. Send us your enclosure drawing, available ground plane size and target bands, and our RF engineers will propose a design.
What certifications and test reports are available?
Products are RoHS and REACH compliant. We operate an in-house 3D anechoic chamber and can provide VSWR, gain and radiation pattern test reports for the specific part number you order.
