Return Loss Testing of Mixed Gender RF Adapters
By Dave Fant – W5SWL
Introduction
Mixed gender RF adapters (for example, N-male to N-female, BNC-female to SMA-male, etc.) are commonly used to interface test equipment, antennas, feedlines, duplexers, and laboratory devices. While they appear simple, poor-quality or worn adapters can introduce measurable impedance mismatch, degraded return loss, and unwanted reflections into an RF system.
This tutorial outlines proper return loss testing of mixed gender RF adapters using an HP 8935 RF Communications Test Set. Although the HP 8935 is primarily a service monitor, it provides accurate RF source and measurement capability suitable for return loss evaluation when used correctly.
Special emphasis is placed on the proper use of qualified test cables, as the test interface is often the largest source of measurement error.
1. Understanding Return Loss
Return loss quantifies how well a device matches a specified impedance (typically 50 ohms).
It is defined as:
Return Loss (dB) = -20 log |Γ|
Where Γ (reflection coefficient) represents the ratio of reflected voltage to incident voltage.
Key relationships:
• Higher return loss = better match
• 20 dB return loss ≈ VSWR 1.22:1
• 26 dB return loss ≈ VSWR 1.10:1
• 14 dB return loss ≈ VSWR 1.50:1
A high quality RF adapter should typically exhibit return loss better than 20 dB across its rated frequency range.
2. Equipment Required
• HP 8935 RF Communications Test Set
• Directional coupler or return loss bridge (if not using internal reflection capability)
• Precision 50-ohm termination (low VSWR)
• Qualified RF test cables (low-loss, phase-stable, known performance)
• RF torque wrench appropriate for connector type
3. Importance of Qualified Test Cables
The accuracy of return loss testing depends heavily on the quality of the cables used between the HP 8935 and the adapter under test.
Qualified test cables should have:
• Known return loss performance
• Low VSWR (preferably < 1.15:1)
• Phase stability under flexing
• Proper connector geometry
• Recent calibration traceability if in lab environment
Poor cables can introduce reflections that mask the true performance of the adapter.
Never evaluate a precision adapter using unknown, worn, or consumer-grade patch cables.
If possible:
• Verify cable return loss independently
• Avoid sharp bends
• Maintain consistent cable routing during test
4. Test Configuration Overview
The HP 8935 provides an RF source and power measurement capability.
Basic reflection measurement setup:
HP 8935 RF Output → Directional Coupler → Adapter Under Test → Precision 50-ohm Load
The coupled port measures reflected power relative to forward power.
If a return loss bridge is used:
HP 8935 RF Output → Return Loss Bridge → Adapter → Precision Load
The bridge separates forward and reflected components for measurement.
5. Step-by-Step Test Procedure
Step 1 – Warm-Up
Allow the HP 8935 to warm up per manufacturer recommendation (typically 20–30 minutes).
Stability is critical for accurate reflection measurements.
Step 2 – Establish Baseline
Before inserting the adapter:
-
Connect HP 8935 → Coupler/Bridge → Precision 50-ohm Load directly
-
Set frequency (example: 146 MHz or desired test frequency)
-
Set output power to moderate level (example: 0 dBm to +10 dBm)
-
Measure reflected power
This establishes system baseline.
If baseline return loss is 30 dB, your measurement system is adequate.
If baseline is only 18 dB, your cables or load are limiting accuracy.
Step 3 – Insert Adapter
Now insert the mixed gender adapter between:
Coupler/Bridge → Adapter → Precision 50-ohm Load
Do NOT change cables or reposition them significantly.
Measure reflected power again.
Step 4 – Calculate Return Loss
Return Loss (dB) = Forward Power (dBm) – Reflected Power (dBm)
Example:
Forward Power: +10 dBm
Reflected Power: -15 dBm
Return Loss = 10 – (-15) = 25 dB
This indicates good impedance match.
6. Frequency Sweep Testing
Adapters may behave differently at different frequencies.
Recommended test points:
• 30 MHz
• 146 MHz
• 440 MHz
• 800 MHz (if applicable)
• 1 GHz (if within rating)
Return loss should remain consistent across rated frequency band.
Degradation at higher frequency often indicates:
• Poor connector geometry
• Dielectric inconsistency
• Manufacturing tolerance issues
7. Identifying Problem Adapters
Signs of poor performance:
• Return loss < 20 dB
• Large frequency-dependent variation
• Unstable readings when lightly flexed
• Measurement drift
Common causes:
• Worn connector plating
• Deformed center conductor
• Improper dielectric spacing
• Low-quality internal construction
Adapters are often overlooked but can degrade entire systems.
8. Mechanical Best Practices
• Use proper torque specification
• Avoid over-tightening
• Avoid stacking multiple adapters during evaluation
• Keep mating surfaces clean
Connector repeatability directly affects return loss measurement.
9. Measurement Error Considerations
Potential error sources:
• Cable reflections
• Directional coupler directivity limits
• Load VSWR
• Instrument calibration status
• Cable movement during test
The HP 8935 is not a full vector network analyzer, so reflection accuracy is dependent on coupler quality and setup precision.
For laboratory-grade measurement, a VNA is preferred. However, properly configured, the HP 8935 can provide reliable service-level return loss evaluation.
10. Acceptance Criteria
For general RF service work:
Return Loss > 20 dB = Acceptable
Return Loss > 26 dB = Very Good
Return Loss < 14 dB = Marginal
Return Loss < 10 dB = Unacceptable
Adapters used in precision lab or repeater duplexer systems should meet tighter standards.
11. Documentation
Record:
• Adapter type and gender configuration
• Frequency tested
• Forward power
• Reflected power
• Calculated return loss
• Cable type used
• Date of test
Maintaining traceable records supports system integrity and quality assurance.
Conclusion
Mixed gender RF adapters may appear insignificant, but they can materially affect system performance through impedance mismatch and reflection.
Proper return loss testing using the HP 8935 requires:
• Stable instrument warm-up
• Accurate reflection measurement method
• Precision 50-ohm load
• Qualified test cables
• Controlled mechanical practices
Without high-quality test cables and a verified baseline, measurement results are unreliable.
Accurate return loss testing ensures adapter integrity, protects downstream equipment, and preserves RF system performance.
Dave Fant – W5SWL
RF Systems Evaluation and Testing
Questions/errors - let me know, sometimes I know what I mean but no one else does ...