Quick Answer
RFdesign makes four telemetry modem families. The 868x and 900x are the original full-size models (30 x 57mm, RP-SMA connectors, 14g). The 868ux and 900ux are the updated compact models (25 x 33mm, u.FL connectors, 8g) with the same RF output but lower power draw and a smaller PCB. Pick 868MHz for the EU and UK, 900MHz for the US, Australia, and rest of world. All four run the same SiK firmware and are cross-compatible with each other.
Which Frequency Band: 868MHz or 900MHz?
The first choice is not about model generation. It is about which frequency band your country permits.
| Feature | 868MHz (868x / 868ux) | 900MHz (900x / 900ux) |
|---|---|---|
| Frequency Range | 868-870 MHz | 902-928 MHz |
| Primary Region | EU / UK | US / Canada / Australia / ROW |
| EU Duty Cycle | 10% at 500mW (high power) | N/A (not EU band) |
| Max Transmit Power | 1W (unlocked) / 500mW (EU) | 1W |
| Typical Range (stock antennas) | ~15km | ~15km+ |
| Max Range (directional) | 40km+ | 40km+ |
The 900MHz band has no duty cycle restrictions, which gives it a slight edge in range and throughput. EU-certified 868MHz modems are limited to 10% transmit duty cycle at full power (500mW) or 100% at reduced power (5mW). This is a regulatory requirement, not a hardware limitation. The key is using the correct band for your region.
x Series vs ux Series
Once you have chosen your frequency band, the next choice is between the x series and the ux series. Both share the same radio output (1W / +30dBm), AES hardware encryption, MAVLink compatibility, and SiK firmware. The differences are physical size, weight, connectors, and power draw.
| Specification | x Series (868x / 900x) | ux Series (868ux / 900ux) |
|---|---|---|
| Size | 30 x 57 x 12.8mm | 25 x 33 x 10.4mm |
| Weight | 14g (7g bare) | 8g (3.5g SMT) |
| RF Connectors | 2x RP-SMA | 2x u.FL |
| Peak Current Draw | ~1.2A | ~1A |
| GPIO Pins | 6 (PWM/SBUS capable) | Fewer |
When to Pick the x Series
The RFD868x and RFD900x suit pilots who want straightforward antenna swaps. RP-SMA connectors mean no adapters or pigtail soldering. The six GPIO pins help with custom integrations. If you already own x-series modems, there is no need to replace them since they work interchangeably with ux models.
When to Pick the ux Series
The RFD868ux is the better pick for new builds where weight matters. At 8g it is 43% lighter than the 868x. The u.FL connectors suit permanent installations where you solder a pigtail once and forget about it. Power consumption sits around 1A peak versus 1.2A, which leaves more battery capacity for flight time.
The RFD900ux shares these advantages on the 900MHz band and holds DoD Blue UAS approval, making it the standard choice for US government and defence work requiring NDAA Section 848 compliance. For more detail, see our NDAA compliance guide.
Bundles
Telemetry needs a matched pair: one for the aircraft and one for the ground station. Bundles save time and include factory-paired firmware.
- RFD868ux EU Bundle: two 868ux modems, pre-paired. Best for EU pilots starting from scratch.
- RFD868x EU Bundle: same idea with the full-size x-series and RP-SMA connectors.
- RFD900x US Bundle: FCC-approved 900MHz pair for US and Canadian pilots.
Browse the full RFdesign telemetry range for individual modems, cables, and accessories.
Compatibility
All four modem families run the same SiK firmware and communicate over MAVLink. An 868ux on the aircraft will talk to an 868x on the ground station with zero configuration changes. The only rule is matching frequency bands: 868MHz and 900MHz modems cannot talk to each other. None of these modems work with the older RFdesign "a" series or "+" series. For help getting started, read our setup guide for ArduPilot and PX4.
Frequently Asked Questions
Is the 868ux worth buying over the 868x?
For new purchases, yes. The 868ux is smaller, lighter, draws less current, and costs roughly the same. Buy the 868x only if you specifically need RP-SMA connectors for easy antenna swapping or the extra GPIO pins. If you already have working 868x modems, there is no reason to switch since both generations are fully compatible.
Can I mix 868MHz and 900MHz modems?
No. The two frequency bands cannot communicate with each other. Both ends of a telemetry link must use the same band. You can, however, run 868MHz telemetry alongside 900MHz RC control (or vice versa) since they are independent radio links.
What range can I expect?
With the stock dipole antennas supplied in bundles, expect roughly 15km of reliable telemetry. With directional antennas on the ground station, 40km or more is achievable. Actual range depends on terrain, antenna quality, and data rate settings. Lower data rates extend range at the cost of update frequency.