Which RF connector should I use?

SMA (Sub-Miniature version A): the most common RF connector. DC to 18 GHz (standard) or 26.5 GHz (precision). 50 ohm impedance. Threaded coupling with 5/16-36 thread. Torque: 5 inch-pounds (hand-tight, never use pliers). 500 mating cycles. Used for: lab cables, PCB-to-cable, most general RF connections. Not waterproof. N-type: medium-sized connector, DC to 11 GHz (standard) or 18 GHz (precision). Both 50 ohm and 75 ohm versions exist (not interchangeable). Threaded coupling. Weatherproof with proper sealing. Higher power handling than SMA (300W average). Used for: outdoor installations, base stations, antenna feeds, test equipment front panels. BNC (Bayonet Neill-Concelman): DC to 4 GHz. Quick-connect bayonet coupling (quarter turn). 50 or 75 ohm. Lower performance but fast connection/disconnection. Used for: oscilloscopes, test points, video, low-frequency RF. 2.92mm (K connector, Anritsu trademark): DC to 40 GHz. Compatible with SMA (same thread, but precision bore). Used for: mmWave measurements, 5G FR2 testing. 1.85mm (V connector): DC to 67 GHz. 1.0mm: DC to 110 GHz. Smallest standard coaxial connector. Expensive, fragile.

What determines connector frequency limit?

The frequency limit of a coaxial connector is determined by the onset of higher-order modes. A coaxial line supports only the TEM mode below the cutoff frequency of the first higher-order mode (TE11): f_c = c / (pi times (a + b) times sqrt(epsilon_r)), where a is the inner conductor radius and b is the outer conductor radius. For air-dielectric connectors: smaller diameter equals higher frequency. SMA: outer diameter approximately 4.1mm, f_c approximately 19 GHz. 2.92mm: outer approximately 2.92mm, f_c approximately 43 GHz. 1.85mm: outer approximately 1.85mm, f_c approximately 69 GHz. 1.0mm: outer approximately 1.0mm, f_c approximately 115 GHz. The connector must maintain 50 ohm impedance through the mating interface. This requires precise dimensions: the ratio b/a = 2.302 for 50 ohm in air. Any discontinuity (step, gap, misalignment) at the mating interface creates a reflection. Precision connectors use air dielectric through the interface region and have center conductor alignment sleeves to minimize discontinuity. The connector is often the limiting factor in measurement accuracy above 10 GHz.

What about PCB-mount connectors?

Edge-launch SMA: solders to the PCB edge with the center pin contacting a microstrip trace. The ground pins (usually 2 or 4) solder to the ground plane. Critical design parameters: launch pad dimensions, via placement, ground clearance. A well-designed edge-launch SMA achieves return loss greater than 20 dB to 18 GHz. End-launch connectors (like Southwest Microwave 292 series) provide better performance: return loss greater than 20 dB to 40 GHz by using a precision-machined housing that controls the launch geometry. Vertical-launch (through-board): center pin goes through a via to an internal stripline layer. Higher parasitic inductance than edge-launch. Return loss: 15 to 20 dB to 10 GHz. U.FL and IPEX: ultra-miniature snap-on connectors for internal PCB-to-PCB connections. DC to 6 GHz. Used in: WiFi modules, cellular modules, GPS. Very small (2mm diameter) but limited mating cycles (approximately 30). SMPM (GPPO): push-on micro connector, DC to 65 GHz, blind-mate capable. Used in modular test systems and phased array modules.

Coaxial Interface

RF Connector

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Precision coaxial interface. SMA: DC-18G, 500 cycles, 5 in-lb. N: DC-11G, weatherproof, 300W. BNC: DC-4G, bayonet. 2.92mm (K): DC-40G, SMA-compatible. 1.85mm (V): DC-67G. 1.0mm: DC-110G. Frequency limit: f_c=c/(π(a+b)). 50Ω: b/a=2.302. Edge-launch: RL>20dB @18G. U.FL: 6G, snap-on, 30 cycles. SMPM: 65G, push-on.

SMA: DC-18G

1.0mm: DC-110G

Z₀: 50Ω

Understanding RF Connectors

RF connectors are deceptively critical components. A poorly chosen or improperly torqued connector can limit the performance of an entire system. At microwave frequencies, the connector interface is often the most significant discontinuity in the signal path, creating reflections that degrade return loss and introduce measurement uncertainty.

The evolution from N-type (11 GHz) to 1.0mm (110 GHz) connectors has tracked the push to higher frequencies. Each generation uses a smaller diameter to push the first higher-order mode cutoff above the operating band, at the cost of reduced power handling and increased fragility.

Connector Equations

TE₁₁ cutoff (frequency limit):
f_c = c/(π(a+b)√ε_r)
Smaller diameter = higher frequency

50Ω impedance:
Z₀ = (60/√ε_r)ln(b/a)
50Ω in air: b/a = 2.302
75Ω in air: b/a = 3.497

Connector VSWR budget:
|Γ_total| ≤ |Γ₁| + |Γ₂|
Two 1.2:1 connectors: worst case 1.44:1

RF Connector Comparison

Type	Freq (GHz)	Power	Cycles	Use
BNC	DC-4	80W	5000	Test, video
N-type	DC-11	300W	5000	BTS, outdoor
SMA	DC-18	100W	500	Lab, PCB
2.92mm (K)	DC-40	50W	500	mmWave test
1.0mm	DC-110	5W	100	THz, metrology

Common Questions

Frequently Asked Questions

Which one?

SMA: DC-18G, most common, lab/PCB. N: DC-11G, outdoor/weatherproof, BTS. BNC: DC-4G, quick bayonet, test. K (2.92mm): DC-40G, SMA-compatible, 5G FR2. V (1.85mm): DC-67G. 1.0mm: DC-110G, fragile, expensive. Choose by freq range, power, environment, mating cycles.

Freq limit?

TE₁₁ cutoff: f_c=c/(π(a+b)). Smaller = higher freq. SMA 4.1mm: 19G. 2.92mm: 43G. 1.85mm: 69G. 1.0mm: 115G. Must maintain Z₀=50Ω (b/a=2.302). Mating discontinuity = reflection. Precision: air dielectric + alignment sleeve. Connector often limits system at >10G.

PCB mount?

Edge-launch SMA: solder to edge, RL>20dB @18G. End-launch (Southwest): RL>20dB @40G, machined housing. Vertical: through via, 15-20dB @10G. U.FL: 2mm snap-on, 6G, 30 cycles (WiFi modules). SMPM/GPPO: push-on, 65G, blind-mate (phased array modules).

Related Terms

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Interconnects

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