Transmit power and range

[ianfallon]

Just been up for a short 15 minute flight to do some air - ground testing. I ran:

- One unit on the ground, in the back of my estate car in open space on the airfield. 20AWG cable + 2 amp cig lighter + GPS dongle.

- 2nd unit in the aircraft, big charge pack power + 20AWG cable + Skydemon for GPS input. Aircraft is metal and PAW was on the floor of the a/c.

Both units have the baro chips and both also the soldered SMA connector on the ARF.

Performance of the ARF side was very disappointing  I lost the ground unit about 900ft in the climb out. A pass over the airfield at 1300ft AGL clearly in direct line of sight of the car showed nothing. Short final - nothing. Taxying in, picked up the ground unit again.

A bit stumped as to what to do now - was hoping the soldered SMA would give great results 

I'll try some master-slave diagnostics later on the ground with both units being the master in turns - does anyone have good figures for the power expected for a given distance between units ?

[The Westmorland Flyer]

Was the ground unit inside the car? If so then I'm surprised you got it at all! Your airborne unit on the floor was stuck deep inside one Faraday cage and if the ground unit was inside another Faraday cage, i.e. your car, then I would have said all bets were off.

800MHz is quite good at going through walls, foliage, etc. but won't go through metalwork however much power we run. Really, the antennas have to be in the clear to have a chance of working. If you put the ground unit on the car roof and the airborne unit on the instrument coaming then I think you'll get better results.

[ianfallon]

Thanks, yes it was. The thing is, that's exactly the setup 2 metal aircraft with PAWs will often have. A lot can't / won't fit antennas to the outside, esp CofA aircraft. I don't really have instrument combing to speak of in the RV4 either to put it on - it's cozy in there! Hmmm - I didn't realise it was going to be that bad 

The ADS-B freq seems fine but I guess the transmission power is much higher as well.

I'll see what I can do about positioning it higher up at canopy level next time and hope to do some air-air tests soon which should be a bit more realistic.

[The Westmorland Flyer]

You're exactly right on that Ian, which is why I raised the point in that long missive a couple of pages back in this thread. PAW is really only running very low power. Not sure how much but my guess would be no more than 200-500mW peak power. Compare that with the 200W peak output that even fairly basic Mode A/C/S transponders are running, to an external antenna at that, and you can see that were dealing with several orders of magnitude difference.

The solution is going to lie in innovative antenna design/placement. One thing that would certainly be worth investigating is stick-on antennas that can be (temporarily) attached to a cockpit window. Like the stick-on rear window car radio aerials of yesteryear. As the antennas are so small, they would be barely noticeable. Biggest issue is getting a reliable connection to them.

Fl*** (I gather we dare not speak its name here) has the same limitations but pretty well all gliders are composite, so relatively transparent to 800MHz wirelessry. Power Fl*** is a completely different beast and does require external antennas for exactly the reasons we are discussing here.

We have some good RF engineers amongst our number... a solution will be found!

[ianfallon]

 

btw. Do you think the "10s of kms" range claim of 2 ARFs is in any way realistic ? (assuming in free air and line of sight) either with the stock pigtails+antennas or with something else ?

[The Westmorland Flyer]

Free space and line of sight? Certainly. Probably even more. The challenge is getting both of those characteristics in all directions at the same time

Years ago I remember playing with some early solid state kit on 1300MHz. Probably only running 50mW or so we were able to establish comms between two line of sight hilltop locations over 20 miles apart without too much difficulty.

The little wireless wiggles don't get worn out too quickly in free space but they don't like banging their heads on lumps of metal!

[rg]

This is a massive flaw. Frankly the PAW needs to be capable of transmitting through the metal skin of a CofA a/c...

Stupid question but the ARF card is high power? So why don't we just bump up the power?

Hopefully Lee can add something to this discussion

[ianfallon]

A wouldn't say that exactly. Don't panic yet. There's also a bit of a question mark over the soldered SMAs.

[stephenmelody]

DON'T PANIC!! DON'T PANIC!! DON'T PANIC!! DON'T PANIC!! DON'T PANIC!!

[rg]

Ok. Ok.  I'll chill.  I'll fly over mig29's field on Thursday and check out air/ground reception.

[The Westmorland Flyer]

This is a massive flaw. Frankly the PAW needs to be capable of transmitting through the metal skin of a CofA a/c...

I do hope you are joking?

No radio transmitter, no matter how powerful, will transmit through the metal skin of anything.That's why all your aircraft's antennas are on the outside. It's how a kilowatt of high energy microwaves can cook your food without cooking you at the same time.

No flaw... just the laws of physics at work. Fortunately there are solutions.

[JCurtis]

The only "real" tests are air to air, which is after all the intended environment.

Recently I was picking up a flyby on my ground test unit and based on the lat/long reported the transmitter was about 3.2nm (5k) away at 1750ft. 
I think I had the power turned down on my unit at the time as I was tinkering with some stuff so I wasn't detected.

[trapdoor]

Stupid question but the ARF card is high power? So why don't we just bump up the power?

It's only 'high power' in the sphere of Licence-free ISM transmitters. 500mW (or half a watt) isn't exactly high-power but compared to most hand-held or integrated transmitters used for telemetry signalling in the ISM band, it is.

Theoretically, the ARF at the settings configured by the PAW transmits at the maximum allowable for that frequency, but whether all of the licensable power is actually getting to the antenna is a moot point and really needs some specialist test kit to check it out accurately. Also could do with checking how well the transmitter locks to the set frequency and whether there is any sort of drift or variance.

[rg]

No I'm not.

The low cost portable nature and ‘bring your luggage with you’ concept does imply this needs to be possible.

If it can't penetrate one layer then surely if an antenna is mounted underneath an a/c the anyone above and behind will not see the signal will they as that will mean penetrating at least 2 layers??

[rg]

3.2nm (5k) away at 1750ft.  .

I'd be MORE that happy with those numbers providing I can have the antenna in the cockpit.