An SDI cable can have the correct connectors, the correct length and even pass a basic continuity test—and still have problems carrying a high-bandwidth video signal.
One reason is something called return loss.
Return loss is an important measurement in professional coaxial video systems because it tells us how much of the signal is being reflected back toward the source instead of continuing through the transmission path.
For broadcast engineers, integrators and video technicians working with 3G-SDI, 6G-SDI and 12G-SDI systems, understanding return loss helps explain why 75-ohm cable, connectors, terminations and adapters all matter.
WHAT IS RETURN LOSS?
When an SDI signal travels through coaxial cable, the goal is for that signal to continue efficiently from the source to the destination.
If the signal encounters an impedance discontinuity somewhere along the path, some of the signal energy can be reflected back toward the source.
Return loss is a way of describing those reflections.
In practical terms, better impedance consistency generally means less reflected energy and a healthier transmission path.
The SDI signal should travel forward through the cable.
An impedance discontinuity can cause part of that signal to reflect backward.
WHY DOES SDI USE A 75-OHM SIGNAL PATH?
Professional SDI video systems are designed around a nominal 75-ohm coaxial transmission path.
That includes more than just the cable.
The complete path may contain:
- 75-ohm coaxial cable
- BNC connectors
- HD-BNC connectors
- DIN 1.0/2.3 connectors
- Patch panels
- Couplers
- Adapters
- Equipment inputs and outputs
Each component becomes part of the transmission path.
This is why AV-Cables uses professional components designed for 75-ohm video applications in our SDI and BNC cable assemblies.
WHAT CAUSES SIGNAL REFLECTIONS?
Reflections can occur when the electrical characteristics of the transmission path change.
Potential sources of impedance discontinuities can include:
- Improper connectors
- Poor connector termination
- Damaged coaxial cable
- Inappropriate adapters
- Poor-quality couplers
- Patch points
- Physical deformation of the coax
- Components not designed for the required video application
One small discontinuity may not immediately cause a visible problem.
But SDI performance depends on the complete path—not one component in isolation.
RETURN LOSS IS NOT THE SAME AS INSERTION LOSS
These two terms are related to transmission performance, but they describe different things.
| Measurement | What It Describes |
|---|---|
| Insertion Loss | How much signal is lost as it travels through the transmission path |
| Return Loss | How much signal energy is reflected because of impedance discontinuities |
Both can matter when evaluating a professional SDI cable path.
A longer cable naturally introduces more attenuation, while impedance discontinuities create reflections.
WHY CONNECTORS MATTER
A connector isn't simply a mechanical device that keeps the cable attached to the equipment.
It becomes part of the transmission line.
That is why professional SDI assemblies use connectors designed for the appropriate impedance and coax construction.
A connector also needs to physically match the cable being terminated.
Mini RG59, standard RG59 and RG6 have different physical constructions and require appropriately designed connectors.
Using a connector because it happens to fit mechanically does not automatically create a properly matched SDI transmission path.
WHY TERMINATION QUALITY MATTERS
Even when the correct cable and connector are selected, termination quality still matters.
The transition from the coax into the connector should maintain the intended geometry of the transmission path as consistently as possible.
This is one reason professional cable assembly involves more than simply attaching two connectors to a piece of coax.
WHY RETURN LOSS BECOMES IMPORTANT AT HIGHER SDI DATA RATES
Higher-bandwidth SDI formats place greater demands on the complete transmission system.
This helps explain a frustrating situation technicians sometimes encounter:
The cable did not suddenly become electrically disconnected.
Instead, the higher-bandwidth signal may expose weaknesses in the transmission path that were less significant at the lower data rate.
That is why 12G-SDI systems deserve careful attention to the cable, connectors, terminations, adapters and total path.
AV-Cables builds Belden 4855R Mini RG59 12G-SDI assemblies for compact professional video applications.
CAN A CABLE PASS CONTINUITY AND STILL HAVE AN SDI PROBLEM?
Yes.
A basic continuity test can tell you whether there is an electrical connection from one end of the cable to the other and help identify shorts or open conductors.
That's useful information.
But continuity alone does not characterize how the cable assembly behaves as a high-frequency transmission path.
A cable can therefore pass continuity testing while still having a performance problem that affects SDI.
This distinction becomes increasingly important when troubleshooting higher-bandwidth professional video systems.
DO BNC COUPLERS AFFECT THE SIGNAL PATH?
Every component inserted into an SDI path becomes part of that path.
That includes a BNC female-to-female coupler used to join two cables.
If a coupler is necessary, use one designed for professional 75-ohm video applications and the required signal performance.
AV-Cables offers the Canare BCJ-JK 12G-SDI 75-ohm BNC coupler for professional video applications.
A coupler does not reset cable distance or regenerate the SDI signal. The two cable lengths and the connection between them remain part of one complete transmission path.
WHAT ABOUT HD-BNC AND DIN 1.0/2.3?
Traditional BNC is not the only connector used for professional SDI.
High-density broadcast equipment may use smaller connector formats such as HD-BNC or DIN 1.0/2.3.
The smaller physical size allows manufacturers to place more connections into limited rear-panel space.
But the same basic principle applies: the connector is part of the transmission path and must be appropriate for the signal and cable.
AV-Cables offers HD-BNC cable assemblies as well as professional BNC-to-DIN and DIN-to-DIN configurations for high-density broadcast systems.
DOES CABLE TYPE AFFECT SDI PERFORMANCE?
Yes. Different coax constructions have different physical and electrical characteristics.
Professional SDI systems may use:
- Mini RG59
- RG59
- RG6
Mini RG59 can be especially useful for compact rack wiring and high-density installations.
Standard RG59 provides a larger construction that can be appropriate for many professional video applications.
RG6 is often selected where lower attenuation over longer transmission paths is valuable.
The goal is not to choose the largest or smallest cable automatically.
The goal is to match the coax construction to the required SDI data rate, distance and installation environment.
RETURN LOSS AND PHYSICAL CABLE DAMAGE
The internal geometry of coaxial cable matters.
Severe mechanical damage can deform the cable construction and potentially create an impedance discontinuity.
Watch for:
- Sharp permanent kinks
- Crushed sections
- Damaged connector terminations
- Flattened coax
- Excessive pulling at the connector
A cable with obvious physical damage should be inspected and tested before being trusted in a critical SDI signal path.
TROUBLESHOOTING A MARGINAL SDI PATH
If an SDI connection is intermittent or works at one data rate but fails at another, don't focus on only one component.
Check the complete path:
- Verify the coax is appropriate for the SDI format.
- Check the total cable length.
- Inspect the BNC, HD-BNC or DIN terminations.
- Check for unnecessary adapters.
- Inspect couplers and patch points.
- Look for crushed or damaged cable sections.
- Confirm equipment inputs and outputs support the required format.
- Test questionable cable assemblies with appropriate equipment.
THE BOTTOM LINE
Return loss helps explain something that is easy to miss when looking at an SDI cable from the outside:
Not all of the signal necessarily continues forward through an imperfect transmission path.
Impedance discontinuities can create reflections, which is why the quality of the coax, connectors and terminations matters.
As professional video systems move from 3G-SDI to 6G-SDI and 12G-SDI, maintaining a well-designed 75-ohm signal path becomes increasingly important.
That's also why a professional SDI cable should be viewed as a complete engineered assembly—not simply a piece of coax with connectors attached.
BUILDING A PROFESSIONAL SDI SIGNAL PATH?
AV-Cables builds professional SDI cable assemblies using 75-ohm coax and connectors selected for broadcast and professional video applications.
Choose from Mini RG59, RG59, RG6, BNC, HD-BNC, DIN 1.0/2.3 and multi-channel configurations.
Hand assembled in the USA and individually tested.
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