The Raspberry Pi 2B based ADS-B receiver system has now been operating succesfully for over 24h, feeding data into the 360radar servers, and so I am now an official contributor and have my log-in's for access to the system.
So the next task is to work to rebuild the receiver system into a self contained system that can be mounted in its entirety at the mast-head, in order to gain the clearest view of the sky, for the furthest range, and at the lowest possible signal loss.
There are a number of difficulties with this, but principly they come down to power feed and data feed. WiFi could be used, and indeed I have ordered a Raspberry Pi ZeroW to play with this option. The Pi zeroW is a smaller board than the Pi 2/2B or 3, but with less physical connectivity, it only has one USB micro port, unlike the 2B with its four standard USB sockets, and it has no Ethernet port. But it does have on-board WiFi and Bluetooth.
The power feed is the other issue. If using WiFi, then just a DC supply is needed. But the cost of suitably rated DC cable is no cheaper than exterior grade CAT5, and Ethernet would be a more reliable data connection than WiFi (and more secure). This brings up the possibility of using PoE (Power over Ethernet) as a convenient means of supplying the system with power.
I did briefly toy with an alternative board, the Orange Pi, as this is smaller than the R-Pi and could apparently do PoE 'natively'. Sounded good, but the chipsets are different which would have meant much difficulty in compiling the code. This idea ive abandoned once I discovered that the 'native' PoE was simply LAN pins 4 and 5 being available at solder pads. No buck converter or low drop-out 5V regulator is provided, meaning that to provide any meaningful PoE capability I would have to add a Buck Converter anyway! So its easier to stay with the R-Pi and use PoE injectors/splitters.
A colleague has sent me another R-Pi, not sure which model as yet, in order that I can release Sams 2B back to him, rather than embed it in the system. Many thanks Mike! Im looking forward to receiving this unit and seeing what modifications are needed, if any, to my plan.
So, on then to the plan! The plan is cunning... and simplistic. The R-Pi, RTL dongle, Buck Converter, and Antenna, will all be enclosed within a suitable weather radome. Lets look at the total system, from air-side to data link side -
The antenna, which is built on the body of a chassis mount BNC socket, is connected via a BNC patch lead to a BNC to MCX pigtail cable. To reduce losses in future I may replace the patch lead with a BNC-BNC adaptor, or change the BNC to MCX pigtail, which is a chassis BNC socket, to one with a BNC male flying plug.
The MCX connector plugs into the RTL SDR dongles antenna socket. The dongle will be removed from its plastic case and mounted bare, with its chip being fitted with a heatsink. So much for the RF side. Now the fun starts...
The RTL dongle is power hungry, ive measured its current draw at 280mA. This is more than really sensible for the R-Pi's USB ports. So, a Y-splitter cable, with two female and one male USB connector, will be used to seperate the R-Pi's USB port and allow a higher current power feed. The RTL dongle will plug into one of the female connectors, and the male connector into the R-Pi. The remaining female connector will have a USB break out module inserted, which allows me to make direct connections. To this will be connected a short microUSB plug cable, Vbus and Gnd only, this is the power feed to the R-Pi. Two more slightly heavier gauge wires will connect Vbus and Gnd of this break-out board to the output of a Buck Converter module. There should be no need to isolate the Buck Converters supply from that of the R-Pi's USB port, but I may do so anyway, this should be just a case of lifting the appropriate polyfuse on the board, or otherwise isolating the boards 5v rail from one of the USB sockets.
So thats the RF and local system power sorted. Now for the data link and PoE.
A low cost PoE splitter will connect the incoming CAT5 cable to the R-Pi's Ethernet port. The 2.1mm DC socket of this splitter will then connect to the input of the Buck Converter. This I 'might' do with a proper 2.1mm DC jack, if I can be bothered, or I may cut the connector off and hard wire it to the module. It probably doesnt matter. Its probably sensible to add a fuse at this point though.
And that, apart from a suitable waterproof cable gland, a sturdy radome and some pole mounting hardware, is the head end system in its entirety!
All that remains then is the 'interface' end - where the system connects to the LAN and hence on to the internet. This is really very simple. The other half of the PoE pair - the Injector - will connect between the CAT5 cable run and the router. A suitably rated DC power supply then feeds into the injector, which feeds this up the unused 4,5/7,8 cores of the CAT5 cable. Here we have to account for the resistive losses of the run length, and the current maximum ratings of the cable, which is 577mA per core.
At the head end, we want to be able to provide a minimum of 10W. As this is a 5V system, that means 2A. Clearly this cant be done within the cable rating even ignoring resistance loss. This of course is why we have the Buck Converter!
As we have two cores to carry supply, we have a maximum current capacity of just over 1.1A. But i'd rather derate the cable to no more than 500mA per core. Using a 12V supply we can have 416mA per core, even better is to go up to 24V or even, if possible 48V, where we would have little over 100mA per core, for a 10W power total. So anywhere between 24-48V will be ideal, depending on what power packs I can get my paws on. Ideally, a mod to the Router will be made to allow the router and PoE all to run on just one efficient PSU.
So all the necessary parts are either in stock or on order, other than a PSU, and a suitable housing. I suspect however that will end up being PVC pipe, as I can get suitable end caps etc, and it will all seal nicely.
In the meantime, while I await delivery of the parts, I have a 1:72 scale Sopwith Camel to build!
Sorry, no pictures with this post! But hey, have £50 on me -
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