Saturday, 14 April 2018

ADS-B Groundplane antenna - version 2

I finally had chance to analyze the return loss and bandwidth of the wire groundplane ADS-B antenna recently, and found that it was actually tuned to 1300MHz! As this is higher in frequency than the required 1090MHz, this meant that the antenna elements were too short. Had the frequency been low, then it would have been a simple case of pruning the elements a millimeter or so at a time to frequency, but being too short means they need extending, which is all but impossible due to the enamel insulation.

So I decided that I would build a new antenna, this time deliberately too low in frequency, to allow me to prune it. I also opted to use bare 1.5mm2 copper wire, which will allow me to add length if required.

One of the most difficult aspects of the original antenna was the mounting disk for the ground plane elements. On the original this is made from PCB stock, but I wanted something that was truly circular for this one. As luck would have it, a visit to the National Coal Mining Museum at Caphouse Colliery supplied me with the perfect starting material - a brass pit check!

Caphouse Colliery pit check

 Finding the center of the disk as somewhat tricky, and even then, the hole isnt perfectly centered, but it also is a little large, so I was able to adjust the BNC socket position on the disk to be more accurately centered later.

The wire im using here is 1.5mm2 domestic Twin and Earth mains cable, stripped of its insulation. The pre-stamped small hole in the pit check, intended for hanging it on the banksmans board in the lamp room of the pit head, was perfect for allowing a small woodscrew to hold the disk in place, while ordinary masking tape held the wire elements steady ready for soldering. The elements here are cut to 85mm, allowing 5mm for soldering.

Cardinal point elements in place

With the first four elements in place, the next four were soldered in. Being a solid brass disk, and with the aid of my new 150W soldering iron, the joints were made quickly and cleanly.

Remaining elements fitted

Once cool, the assembled ground plane was removed from the wooden board, and the vertical element added. This was cut initially to 100mm, then reduced down to 90mm once soldered and straightened.
First Test

 I have yet to analyze the properties of this build, and will adjust the ground plane element angles down to achieve the correct impedance at that time. For now, a quick test of it on the ADS-B receiver showed that it is working, and seems to be working as well as the original antenna.

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