Airband Antenna – 118 to 136 MHz

Like many in the amateur radio hobby, I have an interest in listening to my local airport air traffic and en route air traffic operating between Sydney – Canberra – Melbourne centres.

I am fortunate that my QTH is located in a RF line-of-sight path to Canberra International Airport (13 km east) and an air navigation repeater site 35 km south-east at 1730 m ASL. My Rx only equipment includes a RTL-SDR, an Icom IC-R20 and a Uniden 92XLT receiver, each fitted with a BNC mount 1/4 wave telescopic whip antenna. Other options include my Yaesu FT-817 or FT-857D as a VHF airband receiver.

For the most part air traffic and control tower signal strengths are good at S5 to S8, however there are one or two regional channels where signals are quite weak, which I describe as ‘in-the-noise’. I don’t need an external mounted antenna such as a wideband Discone, a Collinear or a half-wave Coaxial vertical, however a lightweight half-wave dipole would do the trick.

Local and regional air traffic frequencies (MHz):

  • Canberra Tower – 118.700
  • Canberra Ground – 121.700
  • Canberra Approach – 124.500
  • Canberra Departures – 125.900
  • Canberra Terminal Weather – 127.450
  • Wagga Wagga Tower – 118.200
  • Albury Tower – 123.250
  • Area 21 – 124.100
  • En route to Melbourne – 132.200
  • En route to Sydney – 128.400
  • Sydney Approach (south) – 128.300

This aim of this post is to share the construction and assembly of an indoor airband half-wave dipole antenna. I am using the same design as I did for my 2m 145 MHz half-wave dipole. In this case the antenna design is centered on 125 MHz, therefore each element is 560 mm long. This aim of this post is to share the construction and assembly of an indoor airband half-wave dipole antenna. I am using the same design as I did for my 2m 145 MHz half-wave dipole. In this case the antenna design is centered on 125 MHz, therefore each element is 560 mm long.

Antenna Dimensions

Frequency (MHz)1/2 wave * 0.951/4 wave * 0.95
1181207 mm603 mm
1251120 mm560 mm
1361047 mm523 mm

Diagram

125 MHz half-wave dipole antenna

Antenna layout

Materials

  • 2 * telescopic antennas, 7 sections 750 mm (28 inches) long with M2.5 holes (see pics). The mounting hole will determine your selection of screw sizes.
  • 1 * section of kitchen cutting board 30 mm x 55 mm
  • 1 * BNC panel socket with M2.5 mm holes
  • 4 * M2.5 12 mm long machine screws
  • 2 * M2.5 lock nuts
  • 1 * T50-17 mix (Blue/Yellow) 12.8 mm toroid. 17 mix: 20 to 200 MHz
  • 2 * 300 mm (12 inch) lengths of 20 AWG enamelled copper wire
  • 3 * solder tabs

Toroid Choke – T50-17

Toroid choke 8 turns of 20 AWG copper wire (not to scale)
Airband antenna
Top view – Assembled components, less the toroid choke
BNC panel mount
Airband antenna toroid choke
T50-17 Toroid assembled
Toroid to BNC connector
Toroid winding soldered to the BNC center pin
Top view
Airband antenna VSWR
VSWR sweep centered on 125 MHz
Assembled airband antenna
Airband antenna complete, now extend the elements to 560 mm (22 inches)

Hang the antenna in your shack from a wall mount, in the roof space or by some other method you find appropriate. šŸ™‚ For Rx purposes, I will use a short length of 50 ohm RG-174 coax to the receiver.

This antenna works equally well on 2m 146 MHz, adjust each element length to 485 mm. For low power QRP portable field/SOTA work you can mount the antenna on a telescopic pole or hang it from a tree branch. Each element can be rotated in the horizontal or vertical plane to match the receiving station antenna polarisation.

Last update: 4 December 2019