Directional Antennas

Directional Antennas 
 Directional antennas concentrate RF energy in a specific direction rather than radiating omnidirectionally. They are used for point-to-point backbone links between fixed sites where maximum range is needed in a known direction. 

 ALFA 12 dBi Yagi - $50+ 
 A Yagi-Uda directional antenna with 12 dBi gain at 915 MHz. A 12 dBi Yagi has a half-power beam width of approximately 30 - 40 degrees and must be aimed precisely at the target node. Used for connecting distant nodes or bridging a gap in mesh coverage across a valley or open terrain. 
 
 Gain: 12 dBi 
 Pattern: Directional (Yagi) 
 Use case: Point-to-point links, extending mesh over long distances in one direction 
 

 When to Use a Directional Antenna 
 
 You need to extend the mesh over a specific long-distance path (e.g., across a lake or through a valley cut) 
 You have two sites that need reliable high-margin connectivity but no intermediate repeaters 
 You want to add gain without affecting nearby nodes in other directions 
 

 When NOT to Use a Directional Antenna 
 
 For a general community repeater that should cover all directions - a Yagi will be deaf and blind to nodes not in its beam 
 When nodes are located in multiple directions from the installation point 
 On handheld portable devices - you would have to point the device at the target node at all times 
 

 Aiming a Yagi 
 A 12 dBi Yagi has a half-power beam width of roughly 35 degrees. Aiming must be reasonably accurate: 
 
 Use a compass bearing to the target node. 
 Tilt slightly toward the target if it is at a higher or lower elevation. 
 Use the MeshCore or Meshtastic RSSI/SNR values from the target node to fine-tune aim while rotating the antenna. 
 Lock the mount when signal is maximised. Mark the final orientation so you can verify it has not shifted after a windstorm. 
 

 Link Budget for a Directional Link 
 Example: Two Station G2 nodes (36.5 dBm TX, - 130 dBm sensitivity) with 12 dBi Yagi antennas, 20km apart, flat terrain: 
 
 TX power: 36.5 dBm 
 TX antenna gain: +12 dBi 
 EIRP: 48.5 dBm (check your regional EIRP limits before deploying - FCC limits EIRP for point-to-point at this power level) 
 Free space path loss at 20km, 915 MHz: ~117 dB 
 RX antenna gain: +12 dBi 
 Received power: 48.5 - 117 + 12 = - 56.5 dBm 
 RX sensitivity: - 130 dBm 
 Link margin: 73.5 dB - more than adequate 
 
 In practice, real-world obstructions and multipath reduce this margin. 20 dB of link margin is considered comfortable; 73 dB provides very high reliability.