RFD 0012 · Why 50 Ohms, RF Cable Types, and Connectors

Summary

Most RF lab equipment, test cables, filters, amplifiers, antennas, and evaluation boards are built around a 50-ohm reference impedance. That choice is not magic. It is a practical coaxial-transmission-line compromise between power handling, loss, manufacturability, and historical standardization.

Once a signal path is electrically long enough to behave like a transmission line, the question stops being only "is this connected?" and becomes:

what impedance is this source expecting,
what impedance does the load present,
how much of the signal is reflected,
how much power is lost in cable and connectors,
and whether the measurement plane is where the engineer thinks it is.

This RFD is a practical field guide for making those choices. It extends the link budget discussion in 0003 and the antenna-system framing in 0004.

Why impedance matching matters

At low frequency, a wire can often be treated like a near-ideal connection. At RF, the geometry of the conductor, dielectric, return path, and load form a transmission line. That transmission line has a characteristic impedance, usually written as $Z_0$ .

If a line with characteristic impedance $Z_0$ is terminated with a load impedance $Z_L$ , the load reflects part of the incident wave. The voltage reflection coefficient is:

\Gamma = \frac{Z_L - Z_0}{Z_L + Z_0}

The reflected fraction matters because reflected energy can:

reduce delivered power,
create ripple in measured frequency response,
move apparent gain or loss depending on cable length,
distort wideband pulses,
heat or stress power amplifiers,
and make measurements disagree between benches.

Two common ways to describe the same problem are return loss and VSWR.

\mathrm{Return\ Loss\ (dB)} = -20 \log_{10} |\Gamma|

\mathrm{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}

Mismatch loss is the power that fails to reach the load because of the reflection:

\mathrm{Mismatch\ Loss\ (dB)} = -10 \log_{10}(1 - |\Gamma|^2)

The important practical point is that VSWR is not only a spec-sheet number. It becomes a source of uncertainty when the reflected wave re-reflects from imperfect source matches, adapters, connectors, launch transitions, and test fixtures.

A few useful mismatch numbers

VSWR	Reflection coefficient	Return loss	Mismatch loss
1.10:1	0.048	26.4 dB	0.01 dB
1.25:1	0.111	19.1 dB	0.05 dB
1.50:1	0.200	14.0 dB	0.18 dB
2.00:1	0.333	9.5 dB	0.51 dB
3.00:1	0.500	6.0 dB	1.25 dB

For many lab measurements, the mismatch loss alone is not the whole story. The bigger problem is often response ripple and measurement ambiguity. A bad adapter stack can make a device look like it has a frequency-dependent problem when the real problem is the measurement setup.

Why RF systems standardized around 50 ohms

For coaxial cable, impedance is set by the conductor geometry and dielectric material. For a simple coaxial line, the relationship is approximately:

Z_0 = \frac{60}{\sqrt{\epsilon_r}} \ln\left(\frac{D}{d}\right)

where:

$D$ is the inside diameter of the outer conductor,
$d$ is the outside diameter of the inner conductor,
and $\epsilon_r$ is the relative permittivity of the dielectric.

Different impedances optimize for different things. In ideal air-dielectric coax, the impedance for highest power handling is closer to 30 ohms, while the impedance for lowest attenuation is closer to 75 to 77 ohms. The 50-ohm standard lands between those two goals.

That compromise is useful because RF systems often need both:

enough power handling for transmitters, amplifiers, attenuators, loads, and test gear,
and low enough loss for receivers, antennas, filters, and measurement paths.

The result is a network effect. Once signal generators, spectrum analyzers, VNAs, attenuators, loads, filters, amplifiers, and lab cables are standardized around 50 ohms, the cheapest and least surprising design choice is usually to stay in that ecosystem.

50 ohms vs 75 ohms vs other impedances

The right impedance depends on the system, not on a universal law.

Impedance	Common use	Why it shows up
50 ohms	RF lab gear, radios, antennas, microwave modules, test equipment	Strong compromise between power handling, loss, and ecosystem support
75 ohms	Video, broadcast, cable TV, some receive-only RF paths	Lower attenuation for a given coax geometry and mature video/CATV ecosystem
93 ohms	Some low-capacitance or pulse/instrumentation uses	Lower capacitance per unit length in some cable families
100 ohms differential	Ethernet, USB, many high-speed digital links	Differential controlled-impedance interconnect, not a single-ended RF coax standard
110 ohms differential	AES/EBU digital audio and similar balanced links	Balanced twisted-pair ecosystem

The common mistake is mixing 50-ohm and 75-ohm hardware casually because the connector shape looks similar. A 75-ohm BNC and a 50-ohm BNC are not interchangeable precision RF parts. They may mate mechanically in many cases, but the impedance discontinuity still exists. For a quick low-frequency bench check, that may be harmless. For a calibrated RF measurement, it is a real error source.

Cable loss, connector loss, and frequency dependence

Cable loss is part of the RF budget. It should not be treated as a small afterthought unless the numbers prove that it is small.

The main practical dependencies are:

frequency: coax loss usually increases with frequency,
length: loss is commonly specified per foot, meter, or 100 feet,
diameter: larger coax usually has lower loss but less flexibility,
dielectric: material affects loss, velocity factor, phase stability, and temperature behavior,
shielding: leakage and ingress matter in sensitive receive paths and high-dynamic-range measurements,
flex life: repeated bending changes cheap or worn cables before it is obvious visually.

Connector loss is often smaller than cable loss, but connectors and adapters are where mismatch, repeatability, and damage often enter. Each interface adds:

insertion loss,
return loss,
possible impedance discontinuity,
mechanical wear,
torque sensitivity,
and another chance to contaminate or damage the mating surfaces.

For calibrated measurements, move the calibration reference plane as close as possible to the device under test. Do not calibrate at the instrument front panel and then pretend a random cable, adapter stack, edge launch, and fixture disappeared.

Common RF connector families

Connector ratings vary by manufacturer and exact part number. The table below is a practical selection map, not a substitute for the datasheet.

Family	Typical impedance	Practical role	Notes
BNC	50 or 75 ohms	Scopes, low-frequency RF, lab convenience	Fast bayonet connection. Check impedance version before using in RF paths.
TNC	50 ohms common	Rugged threaded alternative to BNC	Better vibration behavior than BNC and often better high-frequency performance.
Type N	50 or 75 ohms	Field RF, antennas, higher-power lab paths	Threaded, rugged, common for test equipment and outdoor RF.
SMA	50 ohms	Compact RF boards, modules, lab interconnects	Common default for prototypes and microwave modules. Use care with torque and wear.
SMB / SMC / MCX / MMCX	50 or 75 ohms depending on family and part	Compact internal connections	Useful where size matters, but usually not ideal for repeated bench abuse.
U.FL / MHF-style micro coax	50 ohms common	Tiny internal board-to-cable links	Low mate-cycle parts. Good inside products, poor as a daily lab connector.
F	75 ohms	CATV, video, distribution coax	Belongs in 75-ohm systems, not general 50-ohm RF lab paths.
3.5 mm	50 ohms	Precision microwave measurements	Mechanically compatible with SMA in many cases, but treat it as precision gear.
2.92 mm / K	50 ohms	Microwave measurements above typical SMA use	Higher-frequency precision connector. Keep clean and torque correctly.
2.4 mm / 1.85 mm	50 ohms	mmWave and very high-frequency measurement	Expensive, delicate, and calibration-sensitive. Do not use casually.

The connector should be selected for the real operating conditions:

maximum frequency,
power,
environment,
weather sealing,
vibration,
mate cycles,
cable diameter,
board-launch geometry,
human handling,
and measurement requirements.

The shape alone is not enough. There are poor SMA connectors, excellent SMA connectors, precision 3.5 mm connectors, damaged 3.5 mm connectors, and adapter stacks that make all of them look bad.

Practical selection rules

For lab work

Use 50-ohm SMA or Type N unless the equipment ecosystem forces something else. Use known-good cables with labeled frequency and loss behavior. Keep a small set of high-quality adapters instead of a drawer full of unknown adapters.

For VNA work:

inspect connectors before mating,
clean precision connectors when appropriate,
use the correct torque wrench,
avoid unnecessary adapters,
calibrate at the device plane,
and record the cable and fixture setup with the measurement.

For prototype boards

Use an RF connector that matches the board stackup and launch structure. The connector footprint, ground via fence, trace width, solder mask opening, and reference plane matter together.

For a typical 50-ohm prototype:

use SMA for a connector that engineers will touch often,
use U.FL/MHF only for small internal links or when size dominates,
avoid bringing microwave signals through generic headers,
keep launch transitions short and symmetric,
and verify the controlled-impedance trace with a calculator or field solver.

This is where the AppCAD note in issue #46 fits. Trace calculators are useful for first-pass sizing, but they are not a substitute for board-fabrication stackup data or measurement.

For products

Choose the connector for the user and environment, not just for the engineer's bench.

Questions to answer:

Will users connect it daily?
Does it need to survive vibration?
Does it need weather sealing?
Is it exposed to static, dust, moisture, or torque abuse?
Will field technicians have the right cables?
Is the connector part of a calibrated service procedure?
Does the cable need strain relief?

The right product connector may be larger, more expensive, or less convenient than the development connector. That can still be the correct trade if it prevents field failures.

A simple decision path

Identify the system impedance first. If the system is 50 ohms, keep the whole RF path 50 ohms unless there is a deliberate matching network or transformer.
Set the highest meaningful frequency. Include harmonics, modulation bandwidth, pulse edges, and measurement bandwidth, not only the carrier frequency.
Budget cable loss. Look up loss at the operating frequency and multiply by length. Do this before assuming a receive-path problem is in the receiver.
Budget mismatch. Use return loss or VSWR specs for the device, cables, adapters, and fixture. If the measurement needs to be accurate, reduce adapter count and move calibration closer to the device.
Choose the connector by use case. SMA is a strong prototype default. Type N is strong for rugged lab and antenna paths. U.FL is strong for internal compact links. Precision microwave connectors are for measurements that justify the handling discipline.
Validate the launch. A good cable and connector can still perform poorly if the PCB launch is wrong.
Treat cables as test equipment. Label them, protect them, replace damaged ones, and do not let unknown cables define engineering conclusions.

Common mistakes

mixing 50-ohm and 75-ohm connectors because they mate mechanically,
using U.FL as a daily lab connector,
trusting a long unknown coax cable in a receive sensitivity test,
adding three adapters and ignoring the mismatch,
using a board trace calculator without confirming the actual board stackup,
calibrating a VNA at the instrument instead of the device plane,
using worn SMA cables for high-frequency measurements,
and choosing a product connector only because it was convenient during bring-up.

Why 50 Ohms, RF Cable Types, and Connectors