LiveATC.net provides audio streams of the various air traffic control (ATC) feeds from around the world. Since I find this system to be very beneficial in terms of education, tracking, and general geekdom!
I currently run a small Linux server to provide the audio feeds from my location to the LiveATC servers. However, I thought I would attempt to improve the antenna being used to receive the airband communications.
In researching the various antenna designs, I though I would build a J-Pole antenna, since this provides a small amount of gain (somewhere around 2.5dB) compared to a discone antenna which is unity gain.
For the design of the antenna, a simple search on the web provided various descriptions on how to calculate the dimensions of the antenna.
1 x 10ft length of 1/2″ copper pipe
1 x 1/2″ T-union
1 x 1/2″ 90 degree
2 x 1/2″ end cap
1 x 1/2″ NPT thread coupler
Solder, flux paste, blow torch
1 x 1/2″ NPT thread to 1″ pipe
10″ (approx) length of 1″ pipe, bell end (to save using a coupler)
1.5″ fence post
4″ Antenna Mast Mount (RadioShack Model: 15-0883 Catalog #: 15-883)
(I did use some 1/4″ bolts instead of the U-bolts that came with this mount to allow for the 1.5″ fence post – the supplied U-bolts only accommodate a 1.25″ pipe).
Left over can of Krylon paint.
Hose clamps (for attaching the antenna to the fence pole)
some left over RG-58 coax cable
Several websites simply show a calculator with no explanation, so I tried to research the wavelengths for the various elements, here’s what I came up with:
Speed of light in a vacuum = 299,792,458 m/s = c
Velocity of electricity in copper = 95% speed of light
Velocity of electricity in copper = c x 0.95 = 284,802,835 m/s = cCu
Center Frequency of antenna = 128 MHz = f
(picked since it’s roughly the middle of the airband of 118-136 MHz)
Wavelength of 128MHz in copper = cCu / f = λ = 2.23m (7.30ft)
A = 0.75 λ = 1.65m (5.47ft)
B = 0.25 λ = 0.56m (1.82ft)
C = 0.025 λ = 0.06m (0.18ft)
D = 0.025 λ = 0.06m (0.18ft)
Here’s a picture of the antenna installed on to the rear of my hangar.