1. The antenna that had to hear a whisper

Carl Sagan’s Contact returns as the recalled work because the radio telescopes in that novel are instruments designed to detect the faintest signals from the greatest distances. The engineering problem is the same for a LEO desktop in reverse: instead of receiving a whisper from light-years away, it must transmit a whisper to a ground station hundreds or thousands of kilometers below, and sometimes do both at once. The link budget is the accounting system that decides whether the whisper is loud enough.

This entry reads about RF link budgets and antennas for LEO.

A link budget adds and subtracts decibels to determine whether a signal can be decoded at the receiver. In its simplest form:

Received power = transmit power + transmit antenna gain + receive antenna gain − path loss − other losses

The received power must exceed the receiver sensitivity by enough margin to overcome fading, pointing errors, atmospheric attenuation, and interference. This excess is called the link margin. A margin of 3 dB means the received power is twice the minimum required; 6 dB means four times.

For LEO, the path loss changes continuously because the distance and elevation angle change. At low elevation the signal passes through more atmosphere and the distance is greater. At zenith the path is shortest and the atmosphere is thinnest. The link budget must close at the worst-case elevation the mission requires.

3. Antennas for small spacecraft

The antenna is where the link budget is won or lost. Options for small spacecraft include:

  • Monopole and dipole: simple, omnidirectional in one plane, common for VHF/UHF telemetry. They are reliable but have low gain.
  • Patch antenna: flat, low profile, can be body-mounted or phased into arrays. Common for S-band and higher.
  • Helical antenna: provides higher gain and circular polarization, useful for higher frequencies.
  • Phased array: electronic beam steering without moving parts, but more complex and power-hungry.

The choice depends on frequency, required data rate, allowable mass, pointing capability, and whether the spacecraft can aim an antenna. A body-mounted patch on a nadir-pointing spacecraft gives a broad beam toward Earth. A gimballed high-gain antenna gives a narrow beam with more gain but requires pointing.

4. Frequency bands

LEO small spacecraft commonly use:

  • VHF/UHF: simple hardware, global amateur and commercial allocations, low data rates, crowded spectrum.
  • S-band: higher data rates, widely allocated, moderate hardware complexity.
  • X-band: even higher data rates, more directional antennas, more power and thermal load.
  • Commercial relay bands: links through systems like Starlink, Iridium, or Kuiper, using their frequencies and protocols.

Each band has different propagation characteristics, licensing requirements, and available hardware. The desktop’s communications architecture may use multiple bands: VHF/UHF for command and telemetry, S-band or relay for payload data.

5. What this changes

  • The link budget is the fundamental calculation that determines whether communications work.
  • Antenna choice trades simplicity, gain, pointing, mass, and power.
  • Frequency band choice trades data rate, spectrum availability, licensing, and hardware maturity.
  • The next entry will read about ground stations, relays, modulation, and coding.