OGN-R antenna solutions

[PaulRuskin]

I've been doing some work on a couple of antenna solutions for OGN-R stations.

One (from Keith) is a 5 dBi antenna similar to but shorter than the 9 dBi Chinese antenna.  This is much easier to use than building pieces of plastic pipe.  The second is a single antenna solution using an RF splitter and a single 9 dBi Chinese antenna.  Theoretically, best results should come from the two antenna solution.  The single antenna solution should have a ~30% reduced range for the OGN receiver, but normal range for the PAW one.  However, given that we have good OGN coverage in a lot of areas, this is probably a good tradeoff given the extra functionality.  Cost is pretty much the same.

Given the relatively high output power of the PAW radio (+27 dBm) it's a concern as to whether the input to the SDR radio will be damaged if too much power is coupled.  So I've just done some measurements on the Orwell setup where there is a 9dBi antenna about 50cm away from a 9dBi antenna, and another 9 dBi antenna about 7m away.

The two close antennas have about a -20 dB gain between them.  That implies about 5mW peak power coupled into the OGN radio, which at a duty cycle of not more than 10% will hopefully be OK.  Interestingly, the splitter also gives a 20dB isolation between the radios, so the result is about the same.

The distant antennas have in excess of -40 dB gain between them, so if you can, keeping the two antennas a decent distance apart could be useful.

I'll shortly be running two OGN-R stations at Orwell - one single antenna and one dual antenna.  It should be interesting to compare the results.

Paul

[Admin]

Hi Paul

A single antenna solution if possible, would definitely be an advantage, for me this type of solution is skirting the realms of physics in my neanderthal brain I'm afraid 
So I can only bow to your better knowledge regarding any potential degraded performance for TX/RX of PAW and RX of OGN

It would seem comparing RX for both PAW & OGN should be relatively easy given your setup, in fact what I could do is run an OGN Client - which I do right now to gather stats on which OGN-R Stations are up and running.
It would be simple to filter this data to compare what is being captured by each station, so for example if we had

StationX - Combined Antenna
StationY - Dual Antenna

I can collect the stats for the OGN and PAW aircraft for each, then simply diff the outputs, we would probably expect differences at the extremities of range, but a large overlap elsewhere

The TX performance, is a little more difficult (but not impossible) to define, it would basically need aircraft in the air circling the station at increasing radius - this is something Keith and myself did in the early days. PilotAware will log all received G/S information, so it is easy to post-process afterwards.

Thx
Lee

[PaulRuskin]

Hi Lee

In the first instance I suspect we can get a view just by looking at recent traffic from the Flarm range tool (one thing to beware - I've noted that the two OGN-R stations are interacting with each other - one is broadcasting Flarm traffic which is being picked up as PAW traffic by the other).

The theoretical performance figures are fairly easy to do - the splitter loses 3 dB for each signal.  For the PAW signal (both ways) that's not an issue, since we're using a higher gain antenna.  In fact we probably gain a dB or two compared with the standard setup.  For the OGN one, we'd already be using the high gain antenna, so we lose a bit of signal, and thus probably range.  About 30% IIRC.

But it's Orwell and Orwell2 if you want to play.

Paul

[Paul_Sengupta]

Have you measured the VSWR on the PAW transmitter when the single antenna is used?

[PaulRuskin]

No, it was a quick measurement, and we were mostly concerned to make sure we weren't about to blow up the OGN radio.

But we've looked at the matching of the splitter with three different Chinese antennas now, and it seems quite good.

Paul

[exfirepro]

Paul,

I thought about this some time back and it is certainly feasible. Of course if we simply connect a 9dBi antenna to the PAW Bridge and transmit through it, we can easily exceed the maximum permitted ERP of 500mW, which the PAW Bridge is designed to achieve using the standard ‘sleeve’ (end-fed) dipole (depending of course on coax / connector losses) - hence Keith’s diy plastic tube solution to improve antenna height whilst NOT increasing gain.

I have tried a 5dBi antenna (same as Keith’s) on my PAW transmit side and we can get away with using it on most twin antenna setups as the power loss in the coax and connectors pretty much balances out the additional gain, so doesn’t exceed the max permissible ERP, but we need to assess each installation individually if we are to stay ‘legal’ in all cases.

WRT the use of a single 9dBi, the additional 3dB loss in the splitter would probably still keep things legal unless very short lengths of coax are used. Are you building your own splitters or have you sourced commercial ones? It will be interesting to see the results.

Best Regards

Peter

[PaulRuskin]

I've sourced commercial splitters.  These https://www.instockwireless.com/power_divider_pd1020.htm

If we're rolling out a good number of these (and I plan to), then I want to do as little hand building of hardware as possible.

Also, I have a build of the software with remote management capability baked in.

Paul

[Kevin W]

Interested in how you are getting on Paul?  Looking at onglide.com Orwell seems to be doing better than Orwell2, but there may be other things at play (as Orwell2 looks to have been down for a while by Grafana).

Cheers
Kev