The proliferation of non compliant UHF series connectors is getting nothing but worse. We sell real UHF standard designed connectors, not the metric designs. These will not mix!
1. Standard UHF Connector (PL-259 / SO-239)
This is the original American standard UHF RF connector used on most ham and CB equipment.
Thread specification
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Thread: 5/8-24 UNEF
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Major diameter: 0.625 in (15.875 mm)
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Threads per inch: 24
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Thread pitch distance: 1.058 mm
Used on
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PL-259 (male plug)
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SO-239 (female jack)
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Amateur radio HF/VHF equipment
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CB radios
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HF amplifiers
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antenna switches
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most American and Japanese RF gear
Characteristics
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Designed by Amphenol in the 1930s
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Not constant impedance
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Usually acceptable to about 300–500 MHz
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Very rugged mechanically
This is the true UHF connector standard.
2. Chinese Metric UHF-Style Connector (SL16)
Some Chinese RF equipment uses a connector often labeled:
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SL16
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M16×1
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sometimes incorrectly labeled merely UHF
It looks almost identical to an SO-239 but the threads are metric.
Thread specification
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Thread: M16 × 1
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Major diameter: 16.0 mm
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Pitch: 1.0 mm
3. Direct Mechanical Comparison
| Parameter | Standard UHF | Chinese SL16 |
|---|---|---|
| Thread type | UNEF (imperial) | Metric |
| Diameter | 15.875 mm | 16.0 mm |
| Pitch distance | 1.058 mm | 1.0 mm |
| Threads per inch | 24 | ~25.4 equivalent |
The diameters are extremely close, but the pitch is different.
Because of this they start to thread but do not mate correctly.
4. What Happens When Mixed
If you try to screw a PL-259 into an SL16 jack (or vice versa):
Typical behavior:
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Threads start normally.
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After 1–2 turns they bind.
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Connector feels gritty or forced.
Possible results:
Cross-threading
The mismatched pitch damages threads.
Loose RF connection
If forced together:
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poor ground continuity
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unstable SWR
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intermittent contact
Connector damage
Often ruins the threads on:
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the radio jack
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the cable connector
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the antenna connector
5. Why This Exists
The SL16 design appeared because:
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metric machining is easier in China
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close enough visually to copy the UHF connector
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some factories assumed compatibility
Many export listings incorrectly label it “UHF connector”, which causes confusion.
6. Where SL16 Is Often Found
Examples include:
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inexpensive Chinese antennas
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export radios
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low-cost RF switches
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some HF amplifiers
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surplus RF equipment
It is uncommon on U.S. ham gear, which almost always uses true SO-239.
7. Quick Identification Rule
If a PL-259 only screws in one turn and stops, the jack is almost certainly:
SL16 (M16×1) rather than SO-239.
8. Solutions
Possible fixes:
Use an adapter
SL16 → SO-239 adapter.
Replace the connector
Install a true SO-239 jack.
Use the proper SL16 plug
Less common and harder to source.
9. Practical Advice
When buying RF equipment from overseas vendors:
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confirm 5/8-24 UNEF threads
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verify it specifically states SO-239
If the listing says SL16, M16, or simply “UHF type”, it may be the metric variant.