Coaxial Cables with Your PiAware

What is coax cable?

Your ADS-B antenna picks up aircraft transponder signals that need to be sent to a computer (such as a Raspberry Pi) where they can be interpreted. One approach is to have the electronics as close as possible to the antenna so the signal can be digitized as soon as possible. However, in this case the PiAware or FlightFeeder electronics would need to be outside with the antenna. Instead, we recommend that you use a high quality coaxial cable (or “coax” cable for short) to get the signal from the antenna to the PiAware or FlightFeeder system with the least amount of distortion and attenuation. This allows you to install the PiAware or FlightFeeder device in a secure indoor location.

A good cable has a few key attributes:

• The outside protective jacket is rated for weather (i.e., it is resistant to snow, rain and sun).  
• The cable has a protective shielding layer. Shielding prevents outside radio interference from affecting the signal traveling through the cable. Good cables have 80% shielding coverage and greater than 40 dB of effective shielding.
• The longer the cable, the more signal is lost just traveling down the wire. The amount of lost signal (attenuation) varies based on the type of cable and the frequency of the signal being measured. The attenuation is typically rated per 100 feet (~30 meters) for specific frequency increments. If you use a 10-foot cable then the attenuation will be 1/10 of the stated rating for that type of coax cable at the stated frequency. Because of cable attenuation, you should always use the shortest length of coax cable that is feasible for your installation.
• The cable impedance is 50 ohms. All components of the system should have the same impedance to prevent signal distortion. The parts we recommend or provide all have 50 ohms impedance. The impedance is controlled by the thickness of the dielectric.

If you have a FlightFeeder kit then you already have an ideal cable for your installation. If you are looking for better cable to upgrade your PiAware, the two types that we recommend are LMR 200 and LMR 400. You can find datasheets for them below:

• https://www.timesmicrowave.com/documents/resources/LMR-200.pdf
• https://www.timesmicrowave.com/documents/resources/LMR-400.pdf

You will also notice that the datasheets specify the impedance, attenuation vs. frequency, shielding and other characteristics of each cable type.

When connecting your cable to your antenna outside, be sure the connection is watertight. Any water entry into the connection or cable will be very detrimental to the performance of your receiver setup. We recommend sealing the connection between the antenna and the cable with self-amalgamating tape or a moldable plastic seal designed for sealing cable connections.