#6773 Calculations for 1090 MHz Slim Jim directional antenna
My self made 1090 MHz Slim Jim directional antenna with help of an online calculator.
The original calculator website no longer exists but I found this website called Slim Jim and J Pole calculator.
The calculations (only in millimetre):
Frequency = 1090 MHz
A) Overall length = 200.9174 mm
B) Half wave radiator section - λ/2 = 132.1100 mm
C) Quarter ware matching section - λ/4 = 66.05504 mm
D) 50Ω feed point - adjust for 1:1 SWR = 13.76146 mm
E) Gap - A-B-C = 2.752293 mm
F) Spacing - not critical = 5.986238 mm
Why did I made this antenna? Well for receiving 1090 MHz Mode S and ADS-B data from aircraft.
Automatic Dependent Surveillance–Broadcast (ADS–B) is a surveillance technology in which an aircraft determines its position via satellite navigation
or other sensors and periodically broadcasts it, enabling it to be tracked. The information can be received by air traffic control ground stations as a replacement for secondary surveillance radar, as no interrogation signal is needed from the ground. It can also be received by other aircraft to provide situational awareness and allow self-separation. ADS–B is "automatic" in that it requires no pilot or external input. It is "dependent" in that it depends on data from the aircraft's navigation system. ADS-B-equipped aircraft and vehicles exchange information on one of two frequencies: 978 MHz or 1090 MHz. Mode A/C and S transponders, as well as Traffic Collision and Avoidance Systems (TCAS), use 1090 MHz. ADS-B extends the message elements of Mode S, adding information about the aircraft and its position. This extended squitter is known as 1090ES. An international technical advisory committee chose 1090ES as the worldwide standard for ADS-B.