continuous rotation high gain antenna

[typist]

I had this stupid idea last night, mounting a high gain parabolic reflector on an old ceiling fan (mounted upside down, possibly with some gearbox), and let it rotate continuously.

A long mode S squitter is 8+112 bits if I am not mistaken, thus 120us (microseconds). Am I wrong if I assume I could pick up most squitters if I rotate the antenna fast enough?

Good coax rotary joints are expensive, so I would just mount the entire radarbox and a netbook on the ceiling fan contraption. USB is relayed over a local wifi loop to the workstation were the radarbox software runs (usb over wifi works perfectly and amazingly reliably for me already). To test it the idea, netbook battery power is more than enough. For a more permanent installation, power could be supplied to the netbook via electrical slip rings (the kind which is used in windmills, 30-40$). The whole contraption would then ideally be weatherproofed with a fibreglass dome (or a pvc bucket...).

I was thinking about sth like this antenna:

http://www.moonraker.eu/Scanning-Receiving-and-Avionics/Avionics/Avionic-Antennas-And-Accessories/RADAR-SKYSEARCHER-1090MHz-DIRECTIONAL-ANTENNA

Yes, I know, this is a particularly clumsy and useless setup...one of the advantages of a ADS-B is that it doesn't need expensive rotary constructions like primary radar, so making it rotate is a bit of a joke. Still... I'm wondering if anyone with more experience than me can tell me if a setup like this could possibly ever catch most squitters? what kind of rpm should I be aiming for?

thank you in advance for your thoughts :)

[DaveReid]

I think you're missing the point.

A rotating primary radar only detects aircraft because it's transmitting and receiving all the time, i.e. any time it's pointing at an aircraft it knows it's always going to get a reflected signal.

OTOH a rotating Mode S receive-only antenna, assuming it's directional, will by definition have a limited beam width and will only pick up signals if an aircraft happens to be transmitting a squitter or interrogation response during the period that the beam is pointing at the aircraft.

Say, for example, your antenna has a 20º beam width - that only gives you a 1 in 18 (about 6%) chance of picking up any given aircraft transmission, with the remaining 94% of transmissons happening while the antenna is pointing in the wrong direction.  And that ratio will hold whatever rate you rotate the antenna at.

The answer, of course, is to have a much wider beam so that you can pick up most, if not all transmissions - 360º would be good :-)

[typist]

thanks :) this is kind of what I assumed was halfbaked...with regards to my idea. the standard antenna which comes with radarbox seems to receive roughly 2-3 messages per second (or less) per aircraft. so...with 20 deg useful beamwidth, could I theoretically cover every sector for roughly a second, if I let the antenna rotate the entire 360 deg in 18 seconds (roughly 3 rpm), and adjust software timeouts to say 1 minute? yes, this way I would miss most of the transmissions...and a plane at 900 kmh will have travelled 7.5 kilometers already if I am lucky enough to pick it up twice per minute...and I probably dont need high gain anymore because it will either be closer already, or below the horizon.

yes...I do realize a better omni antenna is the solution, and a lot less hassle :)

[AirNav Development]

Very interesting topic. Even if limited to receive on a few degrees, as anyone tested this antenna in comparison with common Mode-s antennas?

[DaveReid]

Very interesting topic. Even if limited to receive on a few degrees, as anyone tested this antenna in comparison with common Mode-s antennas?

As it happens, I've had a (non-rotating) Skysearcher antenna hooked up to my Brand X Mode S receiver for the last 3 months, pointing at Heathrow.  Reception is, as you would expect, better than the omnidirectional magmount in the direction it's pointing, and still not too bad in other directions.

I haven't tried it with RadarBox, but that's only because the Skysearcher came with a downlead that terminates in a BNC.

[AirNav Development]

>Reception is, as you would expect, better than the omnidirectional magmount in the direction it's pointing

How better is it? How do you measure it? Range?

[dxman]

if the openings angle of this antenna is about 90 degrees or more, you can put up 4 antennas in each direction (N,S,E,W) en connect them via 1/4 wave coupler to the main coax.
So you have an omnidirectional antenna with more gain.

[DaveReid]

How better is it? How do you measure it? Range?

No, I'm not interested in range.  It may well be better than the omni in the direction that it's pointing, I haven't measured that.

Where I wanted (and got) an improvment is the ability to track aircraft further down the glideslope on approach, and catch them earlier on climbout, both of which add value to my EGLL movements website.

Incidentally, the arrival algorithm also now identifies inbound aircraft while they are still in the holding stacks, so users get more warning of anything interesting that's coming in.

www.civilaircraftregisters.org/Mode_S_Resources/LogReport/EGLLADSB.asp

[bratters]

Very interesting topic. Even if limited to receive on a few degrees, as anyone tested this antenna in comparison with common Mode-s antennas?

As it happens, I've had a (non-rotating) Skysearcher antenna hooked up to my Brand X Mode S receiver for the last 3 months, pointing at Heathrow.  Reception is, as you would expect, better than the omnidirectional magmount in the direction it's pointing, and still not too bad in other directions.

I haven't tried it with RadarBox, but that's only because the Skysearcher came with a downlead that terminates in a BNC.

Morning Dave - I'm very interested in the Skysearcher and as you are the only guy who seems to have one I wonder if I could ask you a couple of questions?

Firstly the beam width - the traffic I'm interested in would be contained comfortably in a 45 degree angle. Am I right in assuming there's a narrow angle "hotspot" of X degrees which reduces in effectiveness as the angle widens?

Range we don't know but would you have any reason to suppose that the range in the preferred direction would be less than that of a "standard" aerial?

And finally, have you any knowledge of a radio controlled low-loss switch that could be used to run the Skysearcher and my existing aerial jointly into the one box,  thus allowing switching between them as the occasion demanded?

Thanks  John.

[DaveReid]

Morning Dave - I'm very interested in the Skysearcher and as you are the only guy who seems to have one I wonder if I could ask you a couple of questions?

Firstly the beam width - the traffic I'm interested in would be contained comfortably in a 45 degree angle. Am I right in assuming there's a narrow angle "hotspot" of X degrees which reduces in effectiveness as the angle widens?

I would assume so - I did in fact ask Moonraker if they could provide a a polar diagram, but they said no.

Range we don't know but would you have any reason to suppose that the range in the preferred direction would be less than that of a "standard" aerial?

If the range in the direction it's pointing was less than the omni then there wouldn't be much point in having it :-)  But, as I said, I haven't done any range measurements.

And finally, have you any knowledge of a radio controlled low-loss switch that could be used to run the Skysearcher and my existing aerial jointly into the one box,  thus allowing switching between them as the occasion demanded?

Nope, that's definitely out of my comfort zone :-)

[bratters]

Thanks Dave - the idea of "homing in" to stuff coming into my local airport appeals. Although only crowflight 14 miles distant, reception from that direction is not the best.

RB tucked up in the loft means any aerial switch mechanism will have to be radio controlled, which should be a nice little challenge.

[typist]

And finally, have you any knowledge of a radio controlled low-loss switch that could be used to run the Skysearcher and my existing aerial jointly into the one box,  thus allowing switching between them as the occasion demanded?

this looks like an ok switch:

http://www.ssejim.co.uk/44-remotecontrolledantennaswitch.new.20.htm

not radio controlled though, you can at something like this, possibly something with a better range:

http://bestofferbuy.com/Wireless-Digital-Remote-Control-Switch-for-Home-Appliance-(Model:T-923B)-White-p-25305.html?currency=GBP&utm_source=gbase&utm_medium=cse&utm_campaign=gbase

[AirNav Development]

Dave why don't you add departure airport information to the EGLL arrivals page?

[DaveReid]

Dave why don't you add departure airport information to the EGLL arrivals page?

Because it's an arrivals page :-)

Actually it's on my to-do list, thanks for the reminder.  Departures are already on the search page, in fact.

[AirNav Development]

Tks. The addition of the origin airport will be something very interesting. When do you plan to implement it? Also where is the search page you mention?

Also and browsing other files on the same folder, which applications are these?
http://www.civilaircraftregisters.org/Mode_S_Resources/AirNav%20NavData%20Update%201.jpg
http://www.civilaircraftregisters.org/Mode_S_Resources/AirNav%20NavData%20Update%202.jpg

Have you developed some addons for RadarBox? Would it be possible to share them with other users?