DIY ADSB Coaxial Collinear Antenna Project

[Grahame Hutchison]

I am currently running an AirNavSystems RadarBox ADSB Receiver connected to a 7 Segment Coaxial Collinear Antenna manufactured by DPD Productions in California (this is normally a 9 segment antenna, however Dave built a 7 segment version for me to fit US Postal Service requirements). A 10 meter run of LMR400 coax cable connects the two, and the antenna is mounted on a 3 metre mast on the arch of our roof. This current setup delivers 250nm consistently on the eastern side of the compass, and down to Canberra and out to Orange on the western side (reception on the western side is limited by the house next door). During periods of Tropospheric Ducting, this setup has received transmissions from as far out as 1,200 km (600 nm plus).

The project over the past couple of day was to try and build my own ADSB Coaxial Collinear Antenna using 2 metres of left over LMR400 coax cable from the above installation. The aim of the project was to provide a Northern Beaches ADSB site using a Kinetics box, in order to enhance the PlanePlotter MLAT environment for Sydney (my existing AirNavSystems RadarBox has very good reception but does not provide suitable data for MLAT). Inspiration for the project came from this article http://www.balarad.net/ and this YouTube video is also useful from a construction point of view.

Calculations for this project are:

The wave length of the signal is

Lambda = C/F

where C is velocity of the electromagnetic signal in a vacuum, i.e. c = 300 million meters per sec, and F is frequency of the ADS-B signal, i.e. 1090 MHz,

this gives a wave length Lambda of 275 mm.

The length L of each antenna element is half the wave length reduced by the velocity factor of the coaxial cable L=0.5*275mm*0.85=116 mm (I checked the velocity of propagation factor for my LMR400 and it was 0.85).

There is no clear guidance on how many segments to use, more than 12 apparently does not improve reception, and more than 9 starts to get too long antenna wise.

I cut 9 LMR400 coax sections 116mm long, plus 20mm at either end for the connection overlap (the exposed braid section in photograph two). The shielding braid and foil were removed to leave the centre core exposed. The segments are interlaced together as laid out in the second photograph, the core of one being connected to the shielding of the next. Because the core of the LMR400 is so thick, I had to file a bevelled end on the core so it would slip under black outer case of the next cable segment, and in contact with the shielding braid. It is important to check the electrical continuity of each segment join with a multimeter, as any shorts are difficult to find after the antenna is fully assembled. Each segment join was then covered with heat shrink for protection.

Once the 9 segments were all connected together, I soldered 15 metres of RG58 coax to the first segment, and inserted the antenna into a 1.15 metre length of 25mm conduit. Each end of the conduit was then sealed with 25mm round rubber leg tips which fitted perfectly.

I mounted the antenna on my existing mast, offset to one side and completely below the existing antenna. This was done to reduce any interference between the antennas - I am not exactly sure whether they will interfere with each other, however we have plenty of data from the existing antenna, to notice any drop in reception.

The other end of the RG58 has a SMA connector directly onto the SBS-1 box, which is connected to a laptop running BaseStation and PlanePlotter.

Compared to the small mobile antenna I was using, the reception has now improved at least five fold, tracking aircraft out to 200nm to the north and east.

The only way to directly compare the AirNavSystems Radarbox and Kinetics SBS-1 boxes, and the DPD Productions antenna with my home built antenna, would be to run another LMR400 coax instead of the RG58 on the Kinetics SBS-1 box - a project for another day.

Some of the tools and hardware needed for the project


Shown here are 3 coaxial segment similar to the 9 in the finished antenna


The existing DPD Productions antenna is located at the top of the mast, and my homebuilt half way up