Coax, Connectors, and Feedline
Cable selection, RF connectors, and feedline loss minimization for LoRa installations.
Coax Cable Selection Guide
Coax Cable Selection Guide
The coaxial cable connecting your LoRa radio to its antenna is a critical component that directly subtracts from your link budget. Every decibel of cable loss is a decibel less of received signal and, equivalently, a decibel less of radiated power. Understanding the tradeoffs between cable types helps you make smart choices for your deployment.
Understanding Cable Loss
Coaxial cable loss is caused by two primary mechanisms:
- Conductor (ohmic) loss: Resistance of the inner and outer conductors dissipates RF energy as heat. Increases with frequency (skin effect drives current to conductor surface, effectively reducing conductor area).
- Dielectric loss: The insulating material between conductors absorbs some RF energy. Also increases with frequency.
Both losses increase with frequency, which is why a cable that seems acceptable at VHF (150 MHz) can be disastrously lossy at 915 MHz. Always check specifications at or near your operating frequency.
Cable Loss Comparison at 915 MHz
| Cable Type | Outer Diam. | Loss (dB/100 ft) @ 915 MHz | Loss per 10 ft | Impedance | Flexibility |
|---|---|---|---|---|---|
| RG-174 | 2.8 mm | ~23 dB | ~2.3 dB | 50 Ω | Very flexible; pigtails only |
| RG-58/U | 5 mm | ~12.5 dB | ~1.25 dB | 50 Ω | Flexible; common |
| RG-8X (mini 8) | 6.1 mm | ~8.5 dB | ~0.85 dB | 50 Ω | Semi-flex; good budget cable |
| RG-213/U | 10.3 mm | ~5.5 dB | ~0.55 dB | 50 Ω | Stiff; older mil-spec |
| LMR-100A | 2.79 mm | ~15.5 dB | ~1.55 dB | 50 Ω | Very flexible; pigtails/jumpers |
| LMR-200 | 5.4 mm | ~6.8 dB | ~0.68 dB | 50 Ω | Semi-flexible; good midrange |
| LMR-400 | 10.3 mm | ~3.0 dB | ~0.30 dB | 50 Ω | Semi-rigid; best low-loss practical |
| LMR-600 | 15.8 mm | ~2.0 dB | ~0.20 dB | 50 Ω | Rigid; tower/commercial use |
| Andrew FSJ1-50A (1/4" Superflex) | 7.1 mm | ~4.4 dB | ~0.44 dB | 50 Ω | Flexible hardline; pro installations |
Practical Loss Examples
To illustrate the real-world impact, consider a typical outdoor node installation with 20 ft (6 m) of cable between the radio and antenna:
| Cable Choice | Loss for 20 ft | Equivalent TX Power Reduction | Range Penalty |
|---|---|---|---|
| RG-58 | 2.5 dB | 17 dBm → 14.5 dBm (effective) | ~16% shorter range |
| LMR-200 | 1.4 dB | 17 dBm → 15.6 dBm (effective) | ~8% shorter range |
| LMR-400 | 0.6 dB | 17 dBm → 16.4 dBm (effective) | ~4% shorter range |
Cable Selection Recommendations
Short runs (under 3 ft / 1 m) - pigtails and jumpers
Use LMR-100A or RG-174. These are flexible enough to route in tight spaces and the short length keeps absolute loss acceptable (under 0.5 dB). This is the correct cable for the factory pigtail from the LoRa radio to the connector panel.
Medium runs (3 - 20 ft / 1 - 6 m)
LMR-200 is the best choice: meaningful loss improvement over RG-58, flexible enough to route around obstacles, and connectors are readily available. This is the correct choice for most outdoor node installations where the radio is inside an enclosure and the antenna is a few feet above.
Long runs (20 - 100 ft / 6 - 30 m)
LMR-400 is strongly recommended. The loss reduction over LMR-200 is significant at these lengths. For runs over 50 ft, consider whether you are better served by moving the radio closer to the antenna (POE-powered remote radio, for example).
When to upgrade your cable
Upgrade cable when feedline loss exceeds 3 dB. At 3 dB loss, you are throwing away half your transmit power before it even reaches the antenna, and your receive sensitivity is degraded by 3 dB - the equivalent of halving your effective radiated power in both directions simultaneously. No antenna upgrade will compensate for this.
Avoiding Common Coax Mistakes
- Never kink or crush coax. A kink in RG-58 at 915 MHz can add 1 - 3 dB of loss at that point. LMR-400 has a minimum bend radius of about 1 inch; exceeding this damages the shield and dielectric.
- Waterproof all outdoor connectors. Water ingress between the connector and cable will corrode the connection and introduce significant loss within weeks. Use self-amalgamating tape over all outdoor connections.
- Do not daisy-chain adapters. Each adapter adds 0.1 - 0.3 dB of loss and a potential failure point. If you need an N to SMA connection, use a single pigtail, not an N-to-PL259 + PL259-to-BNC + BNC-to-SMA chain.
- Store connectors facing down outdoors. Connector faces should point downward or be shielded from direct rainfall to prevent standing water in the connector mating face.
RF Connectors for LoRa Hardware
RF Connectors for LoRa Hardware
RF connector incompatibility is one of the most common and frustrating problems when assembling LoRa mesh hardware. Knowing which connectors are standard on which hardware and understanding adapter losses will save hours of troubleshooting and return shipping.
The Principal Connector Families
SMA (SubMiniature version A)
SMA connectors are the workhorses of small-form RF hardware. They are threaded (10-32 thread), rated to 18 GHz in standard form, and handle up to 500 W continuous at low frequencies. Two variants cause constant confusion:
| Type | Center Pin on Male | Center Pin on Female | Notes |
|---|---|---|---|
| SMA (standard) | Pin protrudes | Socket (receptacle) | Used on most professional RF equipment and high-quality antennas |
| RP-SMA (Reverse Polarity) | Socket (receptacle) | Pin protrudes | FCC-mandated on consumer WiFi devices to prevent non-certified antenna attachment; extremely common on consumer LoRa hardware |
Critical: Standard SMA and RP-SMA are physically intermateable - the threads engage and the connector tightens - but they do NOT make electrical contact. You will have a physically connected but RF-dead assembly. Always verify polarity before tightening.
Which LoRa Hardware Uses Which?
| Hardware | Connector |
|---|---|
| RAK WisBlock (RAK4631, RAK19007) | RP-SMA female on enclosure; U.FL on module |
| Lilygo T-Beam (most versions) | SMA female (standard) |
| Heltec WiFi LoRa 32 v2/v3 | U.FL / IPEX on PCB; optional SMA adapter |
| Meshtastic T-Echo (SoftRF) | U.FL on PCB |
| Seeed WIO-E5 module | U.FL on module |
| Dragino LPS8 gateway | N-female (standard) |
| RAK Wisgate Edge (commercial gateway) | N-female (standard) |
| TTGO LoRa32 v2 | U.FL with bundled SMA pigtail |
| Adafruit Feather M0 RFM95W | U.FL; use an SMA edge-launch or U.FL pigtail |
Note: Connector types can vary by hardware revision. Always verify on the actual unit or current product page before ordering cables and adapters.
N-Type Connector
The N-type is a larger, weatherproof threaded connector rated to 11 GHz (standard) or 18 GHz (precision). It is the connector of choice for any serious outdoor installation - towers, rooftop gateways, commercial deployments. N-type connectors have excellent weatherproofing when properly assembled, low contact resistance, and are designed for repeated mating cycles.
- Used on: Commercial gateways (Dragino, RAK Wisgate, Kerlink, MultiTech), tower-mount antennas, LMR-400 and larger cable installations
- Loss: Typically 0.05 - 0.1 dB per connector pair at 915 MHz
- Availability: Widely available; both solder and crimp versions for all major coax types
U.FL / IPEX / MHF1 Connector
U.FL (the Hirose trade name) or IPEX/MHF1 (equivalent generic and Amphenol variants) are ultra-miniature snap-lock coaxial connectors used on PCBs to connect the RF IC to an external antenna pigtail. They are rated to about 500 mating cycles.
- Used on: Almost all LoRa and GPS modules mounted on PCBs - Heltec, RAK module cores, TTGO, and most other SoC-level boards
- Important: Extremely fragile; do not repeatedly disconnect/reconnect. Lock in place and leave. If you need a permanent connection, solder a small bead of hot glue after mating to prevent accidental disconnection.
- Pigtails: Use only U.FL-to-SMA (or RP-SMA) pigtails made with RG-178 or similar micro-coax. The connector at the board end is U.FL female (socket on pigtail). The connector at the panel end should match your application (SMA, N, etc.)
- Loss: U.FL connector pair itself adds approximately 0.2 - 0.5 dB at 915 MHz, plus cable loss of the pigtail
Adapter Losses
Each adapter in the RF path adds loss and a potential failure point. Typical losses at 915 MHz:
| Adapter Type | Typical Loss at 915 MHz | Notes |
|---|---|---|
| SMA(M) to SMA(F) barrel | 0.1 - 0.2 dB | Use only when necessary; prefer direct cable |
| SMA to N-type | 0.1 - 0.3 dB | Acceptable for indoor patch panels; not preferred outdoors |
| RP-SMA to SMA | 0.1 - 0.2 dB | Common necessity when mixing hardware |
| U.FL to SMA pigtail | 0.2 - 0.5 dB | U.FL connector + cable loss; unavoidable for PCB boards |
| PL-259/SO-239 (UHF) | 0.3 - 0.8 dB | Not designed for 915 MHz; avoid entirely |
Quality Matters
A cheap SMA connector or adapter purchased in a $3 bag of 20 pieces is not equivalent to a $5 Amphenol or TE Connectivity connector. Differences include:
- Contact resistance: quality connectors use silver or gold plating; cheap ones use brass or tin that oxidizes
- Dimensional tolerances: loose tolerances cause intermittent contact at vibration or thermal cycling
- Dielectric quality: cheap connectors use low-grade PTFE substitutes that absorb moisture
- Thread quality: soft aluminum threads strip after a few matings
For outdoor permanent installations, spend the money on proper connectors. For bench development, economy connectors are acceptable. Never use economy connectors in a deployed outdoor node.
Minimizing Feedline Loss
Minimizing Feedline Loss
Feedline loss is the silent enemy of RF system performance. Unlike antenna gain (which you buy) or transmit power (which you set), feedline loss just silently destroys the performance you already have. This page provides the tools to quantify, minimize, and budget feedline loss in your LoRa mesh installations.
The Link Budget Impact of Feedline Loss
Feedline loss hits you twice - once on transmit and once on receive. On transmit, every dB of cable loss reduces your effective radiated power by 1 dB. On receive, cable loss before the receiver's low-noise amplifier (LNA) degrades the noise figure of the entire receive chain by 1 dB per 1 dB of cable loss.
Example: 20 dBm TX, 5 dB cable loss, 5 dBi antenna
EIRP = 20 dBm + 5 dBi − 5 dB = 20 dBm
Example: Same cable with a 2 dBi antenna
EIRP = 20 dBm + 2 dBi − 5 dB = 17 dBm
Conclusion: 5 dB of cable loss eliminated all benefit of the better antenna.
The 5 dBi antenna with 5 dB of cable loss performs WORSE than a 2 dBi antenna with 5 dB of cable loss.Cable Length Math
To calculate cable loss for a given run, use the loss per 100 ft specification from cable data sheets:
Loss (dB) = (Loss per 100 ft at 915 MHz) × (Run length in feet) ÷ 100
Examples for a 15 ft run:
LMR-100A: 15.5 dB/100ft × 15/100 = 2.33 dB
LMR-200: 6.8 dB/100ft × 15/100 = 1.02 dB
LMR-400: 3.0 dB/100ft × 15/100 = 0.45 dBFor metric calculations (loss per 100 m):
Loss (dB) = (Loss per 100 m at 915 MHz) × (Run length in meters) ÷ 100The Full System Loss Budget
Account for every component in the RF path between radio and antenna:
| Component | Typical Loss | Notes |
|---|---|---|
| U.FL connector (at PCB) | 0.2 - 0.5 dB | Present on most PCB-based LoRa boards |
| U.FL-to-SMA pigtail (6") | 0.3 - 0.5 dB | RG-178 pigtail from PCB to enclosure panel |
| SMA to N-type adapter | 0.1 - 0.2 dB | If converting at the enclosure panel |
| Main feedline (LMR-200, 10 ft) | 0.68 dB | From enclosure to antenna base |
| N-type connector at antenna | 0.1 dB | Quality N-type connector |
| Lightning arrestor | 0.1 - 0.3 dB | If inline gas discharge tube used |
| Total example | ~1.6 - 2.3 dB |
In this example, a real system with 10 ft of LMR-200 would have about 2 dB of total system feedline loss. This is acceptable. If you replace the LMR-200 with RG-58, the main cable alone adds 1.25 dB extra, pushing total loss above 3 dB - where you start losing meaningful range.
Inline Connectors Double Loss
Every barrel connector, adapter, or splice in the cable run adds loss and a potential water ingress point. For outdoor installations:
- Plan your cable routing so you can run a single unbroken cable from the enclosure to the antenna
- If you must make a field splice, use a waterproof N-type barrel connector (not SMA) and seal with self-amalgamating tape
- Adapters at the radio or antenna end are sometimes unavoidable; minimize them everywhere else
When Cable Loss Is Unavoidable: Remote Radio Head
For installations requiring very long cable runs (tower top, building rooftop with equipment room far from the rooftop), consider placing the radio module in a weatherproof enclosure directly at the antenna mounting point. Power is delivered via a long DC cable, and data is retrieved via Ethernet or WiFi (or just on-board storage). This approach reduces feedline loss to the short U.FL pigtail and short jumper, typically under 1 dB total.
Checking Your Cable with SWR
A cable that looks fine externally can have significant internal damage (crushed, kinked, or water-damaged dielectric). A quick SWR check with a NanoVNA or antenna analyzer can reveal the problem. Connect the analyzer to one end with the other end open or shorted. A healthy cable will show predictable impedance; a damaged cable will show irregular spikes or elevated VSWR at unexpected frequencies due to impedance discontinuities at the damage point.