This time on Hak5 we continue our SDR series by building our very own ADS-B antenna. All that and more, this time on Hak5!
Building an ADS-B Antenna
Today we’re building our first Antenna from scratch!
- Not all Antennas are made the same
- Different antennas are tuned to different frequencies
- Small antenna included with RTL-SDR not great for all frequencies
- Better antenna = better range and clarity
- As a real world example we’ll be building an antenna to pick up ADS-B beacons
- This way we can easily compare our antenna with that of the stock RTL-SDR
But first we should understand the…
Elements of an Antenna
driven element – driver
parasitic element -pickup energy from driven element and re-radiate it
Types of Antennas
Today we’ll be building an ADS-B Antenna of the type…
Coaxial Collinear Antenna
Mad Props to Dusan Balara from balarad.net for this design
- Omnidirectional Radiation Pattern
- Good for seeing what’s on the horizon (like airplanes)
- Easy to make
Collinear in Geometry basically means laying in a row
So our Coaxial Collinear Antenna is essentially an array of simple dipole antennas made out of Coax cable strung together in a straight line.
In theory when we double the amount of elements (in phase) we’ll double the gain by a factor of about 3 dB. In practice? Well, there are going to be losses
– but it’ll still be pretty good.
And we’ll be making it out of…
For our Antenna construction we’ll use
75 Ohm RG-6 Coaxial Cable – typically used in Satellite TV installation. Cheap. Easy.
Ohm (?) = electrical resistance. R = V/I (Resistance = Voltage / Current)
But first we’ll need to do some math…
We’re building a half wavelength antenna so for that we’ll need to know the wavelength.
We know ADS-B is at 1090 MHz and the speed of light (in a vacuum) hasn’t changed*
So wavelength = c/f or wavelength = 299,792 km/sec / 1,090,000,000 Hertz
Which means wavelength = 275 mm
Which would mean a half wavelength is 137.5 mm, right?
We need to take into account the velocity factor (VF). What’s that?
Sometimes called Wave Propagation Speed or Velocity of Propagation
It’s the speed the wave travels through our material, relative to the speed of light.
Remember when we said it’s cool that the speed of light doesn’t change?
In the vacuum of space radio signals travel at the speed of light.
Therefore the Velocity Factor is 100%
That’s the ratio. The percentage of the speed of light.
Here on Earth we must take into account the characteristics of our materials.
The insulation around our coaxial cable is called a dielectric material
That’s because it can be polarized by an electric field.
It essentially has a slowing effect on the electrical signal
Thus our (VF) won’t be 100% of the speed of light.
This is where the rubber meets the road.
The (VF) is going to vary depending on transmission lines, aka cables.
A closed-cell foam dielectric may have a (VF) of 90%
A Teflon dielectric may have a (VF) of 70%
And that needs to work into the length in which we cut our antenna elements
So what’s the Velocity Factor of our RG-6 coax cable?
In order to figure that out we would need to do a complex calculation
Velocity Factor = 1 / Square Root of K
K = the Relative Permittivity
Also known as the Dielectric Constant
Essentially the ratio of the amount of electric energy a material can store
relative to that stored in a vacuum.
For example if the Vacuum = 1, then Teflon = 2.1 and Paper = 3.85
It gets worse: Air = 1.0005
Wouldn’t it be easier if we all just lived in a vacuum?
Cut to the chase…
Google it. The cable manufacturer lists the VF or VOP for Velocity of Propagation
And the VF of our 75 Ohm RG-6 Coax cable? It’s 85%
Antenna Element Length
So now that we’ve figured out the wavelength and the velocity factor, we can find out how long to cut our antenna elements.
Wavelength / 2 (Remember we’re going with a half wavelength antenna) * VF
So 275mm / 2 = 137. 137 * 0.85 = 116.
So our elements should be 116 mm, or 11.6 mm.
Wait, did you just round down in your math?
Sure – because this is the real world. That 85% figure is a guideline.
We don’t have the precision equipment to test the cable, nor cut it to exact specifications
Also we’re glossing over a ton of other factors which I’m sure we’ll get comments about
Let’s just roll with it.