A Primer on RealAntennas

Written for RP-1 v4.3.0.0.

See the RealAntennas Wiki for more information.

General Advice

• Match the band of your peer antenna/station.
  • Higher frequencies (S, X) are generally better for dish-to-dish communication at long ranges, since both sides of the connection can benefit from the higher gain of high-frequency dishes.
  • Lower frequencies (VHF) are generally better for omni-to-omni communication at short ranges, since the path loss is lower for lower frequencies, and the gain is the same.
  • For omni-to-dish or vice versa, the two factors cancel out and other factors determine the "best" frequency choice.
• Omni antennas have the same dBi gain for all bands.
• Dish antennas get better dBi gain for higher frequency bands, but have decreasing beamwidth.
  • Beamwidth and gain both represent how well the antenna can focus radiated energy.
  • You cannot increase one without decreasing the other.
• The Power (Active) field tells you how much power you need to transmit at the maximum possible data rate.
  • If nothing is being transmitted, it draws the idle power (Power (Idle)).
  • Otherwise, it linearly interpolates between the two according to the portion of the maximum possible data rate being used.
• The DSN (Madrid, Canberra, Goldstone) is always visible above 30Mm.
• Only TL2+ antennas can relay.
• Higher tech levels decrease power/bit (but may increase idle power).
• Higher tech levels upgrade your ground stations, which increases their effective range.
• Every 3dB of extra gain/power gets 2x data rate for the same range.
• Every 6dB of extra gain/power gets 2x range for the same data rate.
• If the range doubles but the gain/power stays the same, the data rate becomes 1/4 of what it was.

Link Details

• For a link to be made, the transmission rate must be non-zero in both directions!
• Link strength is determined by the sum of gains minus sum of losses, as compared to the minimum strength needed to overcome noise, plus some margin.
  • Gains: TxPower + TxGain + RxGain
  • Losses: Path Loss + Pointing Loss
  • Min Link Noise: log(Atmosphere Temp + Body Temp + Receiver Temp) + log(Minimum Bandwidth) + Margin
  • Min Link Eb/N0 = Gains - Losses - Min Link Noise
  • Every 3dB of extra received power is used to double the data rate, up to a cap dependent on the maximum bandwidth of the Frequency Band.
• Gains are entirely dependent on your antenna dBi + dBm, and the gain of the receiver.
• Losses are dependent on distance and antenna pointing.
  • Path Loss increases by 6dB per doubling in distance. It also increases by 6dB per doubling in frequency.
  • Pointing Loss is relevant for dish antennas (see below).
• Min Link Noise is mostly dependent on tech. Better tech antennas have lower receiver temperatures and lower required margins.
  • Atmosphere Temp and Body Temp are usually situational, and represent the atmosphere/other celestial bodies introducing noise.
  • Receiver Temp is purely dependent on tech for vessel antennas and Tracking Station level for ground-station antennas.

Band Choice

• Higher frequency bands have the following benefits:
  • High bandwidth, which increases the maximum attainable data rate you can get.
  • Higher gain for the same dish, which means you can use less power transmitting.
  • Higher gain at the receiver, which also means you can use less power transmitting.
• Higher frequency bands have the following downsides:
  • S, X, Ku and Ka bands have less stations on Earth, so you can only use them consistently past GEO.
  • Higher path loss (6dB more path loss per doubling of frequency)
  • Slightly higher atmospheric attentuation (?). This usually doesn't matter.
  • Lower beamwidth for the same dish, so if your intended targets are spread out you can't talk to all of them at once.
  • Higher minimum data rate to connect, which increases the minimum dB you need to get any connection if you don't care about data rate.

Dishes

• Dish gain increases with frequency (2x frequency -> +6dBi gain).
• Dish gain increases with dish radius (2x radius -> +6dBi gain).
• Dishes have a 3dB cone (half beamwidth from centre) and a 10dB cone (full beamwidth from centre).
  • If your target is inside the bright magenta cone, you get at most 3dB of pointing losses.
  • If your target is inside the dark purple cone, you get at most 10dB of pointing losses.
  • Outside of that, there is no connection whatsoever. (-200dB of pointing loss)
• Dishes can be targeted via the Antenna Targeting menu independent of their physical facing.