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March 26, 2026

The LOS Link Budget: What Really Determines a Datalink Modem's Range?

The “150 KM” or “200 KM” range figure printed on a datalink modem’s datasheet might look like a single fixed number, but it’s actually the engineering outcome of several variables combined. In this article, we break down the core components of the link budget calculation that determines range in line-of-sight (LOS) communication.

A link budget is the sum of all gains and losses a signal experiences from transmitter to receiver, determining whether it remains readable (demodulable) upon arrival. Simplified:

Received Signal Power = Transmit Power + Transmit Antenna Gain − Free Space Path Loss + Receive Antenna Gain − Cable/Connector Losses

This sum must stay above the minimum threshold the receiver needs to correctly decode the signal at that modulation and data rate — the receiver sensitivity. The gap between the two is called the “link margin,” providing a safety buffer against unpredictable losses like weather conditions or multipath effects.

Free Space Path Loss and Its Relationship to Frequency

Free space path loss increases with the square of distance — doubling the distance adds roughly 6 dB of loss. A less widely known effect, however, is that this loss also increases with frequency: at the same distance, a system operating in a higher GHz band experiences more path loss than one operating in a lower band.

This is the answer to “why do some long-range datalink systems favor lower GHz bands (e.g., L-band, S-band)?” A lower frequency makes it possible to achieve longer range with the same transmit power and antenna gain — but this typically comes at the cost of narrower bandwidth (and therefore lower maximum data rate).

The Role of Antenna Gain

Antenna gain is one of the “cheapest” dB’s to gain in a link budget — rather than increasing transmit power (which is subject to regulation and power consumption limits), using a directional antenna can significantly extend range. But directional antennas narrow the coverage angle, which creates a need for an auto-tracking antenna system that maintains continuous alignment with a moving platform such as a UAV.

Striking the Balance in Practice

In real-world datalink design, these parameters are interdependent:

  • Want longer range? Lower the frequency, increase antenna gain, or trade off data rate.
  • Want higher data rate? Usually requires wider bandwidth (and therefore higher frequency), which limits range.
  • Platform weight constrained? Higher transmit power may be preferred over directional/high-gain antennas (a weight-vs-power trade-off).

This is why there’s no single “best” datalink modem — the right answer is an engineering choice optimized for the operational scenario (platform weight, required data rate, target range). A modem family offering range classes from 10 KM to 200 KM exists precisely to cover this diversity from a single supplier.

Conclusion

Next time you see a range figure on a datalink modem, remember that behind that number is a balance struck between frequency, transmit power, antenna gain, and data rate — and knowing that helps you ask the right questions when choosing the right product.