Dipoles

[PaulSS]

What is going to give me the best set up, assuming the antennas end up mounted in the same place inside a fabric aircraft; either an end-fed dipole (like wot Rostetta has) plugged directly into the ADSB dongle (with USB to PAW box) or centre-fed dipole (aka the PAW shop), with coax to the same ADSB dongle and a very short USB to PAW box. I need the USB lead because the ADSB dongle is basically too big to allow the GPS mouse to plug comfortably into the USB slots.

My shortest post I think 

[exfirepro]

Hi Paul,

ADSB side isn’t a major issue as the signals are so (relatively) strong. For remote siting in a fabric aircraft, I would go for the PAW internal dipole and keep the dongle on the PAW.

With the P3i side, you need to keep losses to a minimum (and can’t of course move the Bridge). I have tried both running the standard Rosetta PAW end-fed dipole on a coax extension to site it away from the PAW itself and also using the standard PAW internal dipole with its built-in cable with similar results (though the internal dipole option should in theory be better as this setup will have one less potentially lossy ‘cable joint’).

I have also tested using PAW externals under the pod on low loss cables for both P3i and 1090, but have stopped using the external for 1090 due to swamping from the nearby transponder antenna and have reverted the 1090 side to the supplied end-fed antenna mounted directly on the unit.

So my current setup is PAW External 869.5 on a low-loss coax cable for P3i, with the standard end-fed antenna fitted directly on the Bridge for 1090 reception.

Best Regards

Peter

[PaulSS]

Thank you for your reply, Peter 

I've got an external Flarm antenna for the P3i side of things but the PAW itself will be mounted underneath and to the rear of my seats, close to the aluminium box, that is the luggage bay. In order to remove the ADSB antenna from the shielding effects of all that metal I thought I would remote mount it above and behind the box, so that it has almost 360 deg coverage. If the end-fed dipole is going to be as good as a centre-fed bit of kit then that is great, as I already have the bits. I'll just attach the antenna to the Flight Aware dongle, mount that combination away from the aluminium box and then run a 3m male-to-male USB cable to the PAW.

I've got a spare 'Ted' antenna and would like to mount it externally for ADSB in but that would have to be relatively close to my transponder antenna and, as you point out, that's not a good thing. I may be able to squeeze about 80cm between the two but I don't know if that would be enough.

[exfirepro]

My external PAW 1090 was about a meter from my existing TED transponder antenna and I was getting weird ghost Mode-C alerts, which we could only attribute to the high power transponder signals ‘swamping’ the ‘front end’ of the SDR Receiver. Since swapping back to the on-board PAW 1090 antenna inside the pod a further 50cm away, these ghost alerts have gone.

As you are already using a USB lead to separate the SDR from the PAW, I would go with your proposal to use the standard 1090 antenna on your SDR and see how you go.

Regards

Peter

[PaulSS]

Thanks again, Peter. If and when I am allowed to escape from Japan and actually get to my machine I will let you know how it all goes 

(If it's not the Corona Virus that maroons me in Nippon it will be the longest 787 course known to mankind  )

[Kurt37]

And if I may use the thread for antenna position.

In the PAW Mode S 3D YT Video it shows a C152 with optimised PAW antennas. https://youtu.be/J1imxhjG8ds, minute 12:37. 
It states that two PAW/P3i antennas are used, each located at the top corners of the wind shield.

Two questions:

1. Is the optimum position for PAW antenna in e.g. a Cessna as high as possible, so the obstruction from the engine is the least? Or in other words, the target is to have as most coverage of the ground as possible for atom station reception? Are there limitations as how near the antennas are allowed to be near Metal?

2. How did the use 2 antennas of one outlet? Any splitters you can recommend?

Kind regards
Patrick

[exfirepro]

Hi Patrick,

The original article was I think referring to the use of 2 x PAW 'horn dipoles' as they used to be called - one for PAW P3i on 869.5MHz and the second for 1090 MHz.

Using a splitter to combine 2 'aerials' for enough only is relatively straightforward as impedance match isn't critical, but combining 2 antennas for transmit requires a proper matching splitter/combiner if you don't want to lose signal and potentially damage your transmitter.

Regards

Peter

[Kurt37]

Ok so the yt set-up describes one P3I and one ADSB antenna, each in on top corner of the windshield.

Is for example the top right (or top left) hand corner a preferred position for the P3I antenna?

Kind regards,
Patrick

[exfirepro]

Hi Patrick,

Where a choice has to be made - such as when limited to internal antennas in a metal-bodied aircraft - the principle is to maximise signal and reception to the front of the aircraft due to the highest closing speed with other aircraft being from that direction.

Positioning the antennas as near vertical as possible in the top corners of the windshield is likely to maximise forward view while minimising obscuration by the engine and bodywork - such as window pillars - and minimising intrusion into visual scan areas. This advice obviously applies equally to P3i and 1090 antennas, however where there is conflict, it makes sense to prioritise the positioning of the P3i antenna - bearing in mind the much lower transmit (and receive) power compared to (most) 1090MHz devices.
 
Hope that helps

Peter

[Kurt37]

Okay, understood. My objective apart from direct air2air coverage is also air2ground coverage for mode S/3D. I suspect line of sight is needed between P3I antenna and ground station with no metal obscurance.
So vertically and as high as possible in a top hand corner could be a good spot for a C172. In a GFK airframe I guess position is not so critical.

Kind regards,
Patrick