CHECKING IMPEDANCE, POLARIZATION, AND PLACEMENT...
STATUS: READY
LORA ANTENNAS :: THE MOST IMPORTANT RF COMPONENT
> A good radio with a poor antenna is still a poor radio system.
WHY THE ANTENNA MATTERS
The antenna is part of the link budget. Its efficiency, impedance match, radiation pattern, polarization, height, and surroundings directly affect both transmission and reception. A decorative antenna with the wrong frequency can perform worse than a simple correctly sized wire.
868 MHz REFERENCE DIMENSIONS
At 868 MHz in free space:
Full wavelength λ ≈ 34.5 cm
Half wavelength λ/2 ≈ 17.3 cm
Quarter wavelength λ/4 ≈ 8.6 cm
These are theoretical starting lengths, not final cut dimensions.Real elements are affected by conductor diameter, insulation velocity factor, loading coils, the enclosure, PCB ground plane, nearby cables, and the operator’s body. Final tuning should be measured on the finished installation.
COMMON ANTENNA TYPES
Simple, efficient, and compact. It normally needs a ground plane or the device body to provide the missing half of the antenna system.
Two opposite elements, less dependent on a ground plane. Useful for testing and fixed installations.
Outdoor vertical antenna with gain concentrated toward the horizon. Useful when mounted high, but excessive gain can create elevation-pattern nulls.
Compact internal antenna. Performance depends strongly on the enclosure, PCB layout, cable routing, battery, and nearby metal.
Short helical or loaded whip. Convenient, but quality varies widely and very small antennas are usually less efficient.
Yagi or panel antennas concentrate energy in one direction. Useful for a fixed point-to-point link, not for an omnidirectional mesh node.
50-OHM SYSTEM
Most LoRa modules, coaxial cables, connectors, and antennas are designed around 50 ohms. A mismatch causes part of the transmitted energy to be reflected instead of radiated. The matching network on the PCB and the antenna installation must work together.
More power reaches the antenna, receive efficiency improves, and the transmitter sees a safer load.
Reduced range, unstable measurements, higher reflected power, and performance that changes when the device is touched.
CONNECTORS
SMA Standard threaded RF connector
RP-SMA Reverse-polarity center contact; visually similar but not interchangeable
U.FL/IPEX Tiny internal snap connector; fragile and intended for limited mating cycles
N-type Rugged outdoor connector, often used on base antennas
Always verify the connector body AND center contact before buying an adapter.POLARIZATION
A vertical whip produces mainly vertical polarization. For the best link, the receiving antenna should use the same polarization. Turning one node horizontally can create a large polarization loss.
Portable / mesh use: keep antennas vertical
Fixed point-to-point link: align both antennas the same way
Do not hide a vertical antenna flat under a metal shelfPLACEMENT RULES
- Height is usually more valuable than a small increase in antenna gain.
- Keep the antenna away from metal, reinforced concrete, large batteries, and noisy electronics.
- Do not place the antenna directly behind a solar panel or inside a metal enclosure.
- Keep the active element clear of walls when possible.
- For an outdoor node, put the radio near the antenna rather than using a long thin coaxial cable.
- Use a drip loop and weatherproof the connector without trapping water inside it.
COAXIAL CABLE LOSS
Coax loss increases with frequency and cable length. Thin cables such as the short pigtails used inside devices are convenient but become lossy over long runs. A high-gain antenna at the end of a poor cable may deliver less signal than a modest antenna connected by a short low-loss cable.
GAIN AND EIRP
Antenna gain redistributes energy; it does not create power. A vertical high-gain antenna narrows the elevation pattern and sends more energy toward the horizon. The legal radiated-power limit includes transmitter power, cable loss, and antenna gain.
EIRP ≈ transmitter output
- cable and connector loss
+ antenna gainMEASURING WITH A VNA
A vector network analyzer can measure return loss and VSWR around the operating frequency. Calibrate the VNA at the end of the measurement cable, then test the antenna in its real enclosure and intended position.
Practical measurement workflow:
1. Calibrate open / short / load at the cable end.
2. Sweep around the target band, for example 850–890 MHz.
3. Install the antenna on the real device or ground plane.
4. Keep hands and metal objects away during the reading.
5. Adjust length or matching only in small steps.
6. Re-test after closing the enclosure.PRACTICAL CHOICES
Use the integrated antenna as designed. Keep the device upright and away from the body when maximum range matters.
Use a known 868 MHz antenna on the correct connector, with the board and antenna near a window or outside in a protected enclosure.
Use a weather-rated vertical 868 MHz antenna, short low-loss coax, proper strain relief, and a high clear mounting point.
ANTENNA CHECKLIST
[ ] Correct frequency: EU 868 MHz, not 915 MHz
[ ] Correct connector: SMA / RP-SMA / U.FL identified
[ ] Antenna connected before transmission
[ ] Both antennas use the same polarization
[ ] Antenna clear of metal and major obstacles
[ ] Coax kept short or low-loss
[ ] Outdoor connector weatherproofed
[ ] EIRP remains within regional rules
[ ] VNA measurement performed on the final installation when possible
[ ] Real field test completed