Antenna Location & GPS Input

[Paul_Sengupta]

The calculation for wavelength is to take the speed of light and divide it by the frequency.

So 299 800 000 (m/s) / 868 000 000 (Hz) = 0.345 metres, or about 35cm.

However, it's not as simple as that as the transponder transmits at a very high power, so it would be advisable to put any antenna as far away as you can from the transponder antenna, otherwise any receiver using that antenna will be swamped by the transponder's transmission.

[PaulSS]

Thanks, Paul, even though it has thrown another spanner in the (future) works 

I was thinking the external antenna(s) could be mounted basically under the seats, in a suitable place for the ground plane. However, I have just managed to find out the transponder antenna is mounted about in line with the wing strut attachment and slightly offset to the right side of the aircraft. Given your thoughts above, I think that might be a bit close to the PAW antenna(s). I might have to see if they can be mounted on top of the fuselage, behind the turtledeck (because that gets removed). Unless you have a better idea of course.........

You'll notice that I keep writing antenna(s) and that's because I still haven't really established if I need to mount the P3I AND the ADSB antennas externally or just the P3I. I'm thinking that I might be able to get away with the P3I antenna external and the ADSB antenna that comes with the PAW kit inside the rear fuselage (fabric). Any thoughts on this?

Thank you again for your expertise and I'm pleased to say that I came up with the same sort of distances between antennas but I'd used the 1090 of the transponder so had a shorter distance. Of course I should have used the lower frequency of the separation would be too little. Can I just confirm that the minimum spacing, assuming they are just receiving, would be 1 wavelength or wouldn't it matter?

My head just exploded 

[exfirepro]

Paul,

Are you aware that you can cut the 1090MHz (ADSB) antenna down to about half its length without any detrimental effect? In fact at that length it is 'tuned' to 1090MHz and virtually disappears on top of the coaming, with great forward view apart from downwards, though this is not normally an issue.

Regards

Peter

[PaulSS]

Hi Peter,

Yes, I had read that in another part of the forum but as I'm considering a permanent installation and designing that from scratch this would offer me the opportunity to not have to 'Heath Robinson' it and have things on coamings etc. That's the reasonable part of the argument, then there's my OCD which has already been seen with not really wanting even a GPS matchbox on top if it :-)  My absolute ideal would be to have all antennas inside the fabric fuselage but I totally understand the reasons put forth here for not doing so and that's why I'm trying to seek the Nirvana of external antennas.....for however many I don't actually know 

[Paul_Sengupta]

I would have thought that using a cut down to a quarter wave with a ground plane inside the fuselage would be ok for the ADS-B antenna, but you'd have to try it. Alternatively you could buy/make a 1090 MHz dipole so you wouldn't need the ground plane. Easy enough to make.

As for the P3i antenna, well, it might be a case of wait and see where you think may be the best place by actually looking at various locations, both internal and external.

As for receive antennas and spacing, still not quite that simple, you can have them closer than 1 wavelength, but there will be a certain interaction in the radiation (reception) pattern putting any sort of antenna anywhere near anything metal, which includes another antenna.

On my PAW, I've got my two antennas separated by approximately the length of the Raspberry Pi and ADS-B dongle. Seems to work ok.

[PaulSS]

Thank you for all the replies and advice. I think I'm probably getting too far ahead of myself and will have to wait, see and experiment, as suggested by Paul. Having said all that, the posts on here have been invaluable in increasing my knowledge of the radio wave theory and what can and probably can't be done, so no time wasted 

Sounds like an external P3I tuned monopole on a ground plane and maybe a cut down ADSB antenna in the fuselage might, possibly, maybe the way to go.

Cheers 

[Ian Melville]

For the P3i I would always go for a dipole where possible. A monopole is not able to see much behind the ground-plane. A belly-mounted monopole on a ground-plane will less able, or not able to see traffic that is above the host aircraft. Locations for dipoles are tricky as well due to the aircraft structure. I would locate it so that it gets the best view of where you threats are coming from. End fed dipole(as supplied with Classic PAW) below the aircraft would be a good starting point.

[PaulSS]

Good stuff, Ian    So option 316 could be an end-fed dipole mounted in the fuselage or externally and the ADSB antenna cut down and inside the fuselage (fabric). With coax as short as possible of course and dipole away from aircraft structure.

[GeoffreyC]

Here's some photos of my PAW installation that I've recently completed in my Flexwing Microlight (Pegasus Quantum).

There's not a massive amount of spare space inside the cockpit and I didn't really want to start making holes in the pod unless I absolutely had to,  so I started off with installing PAW underneath the base tube,  and to my great surprise it works absolutely fine.   
I was a bit worried that the GPS would need to be in the front of the pod to get a clear view of the sky, but where it is it sees 7 satellites quite happily and works fine.
I did move around which was plugged into which USB socket,  but that was only so I could see the GPS light when sitting in the pod,  and also had access to plug a USB stick in for upgrades.

The ADSB small antennae cable was shortened by prising the bottom off the antennae and stripping back the cable length.  I soldered the outer braid to the bottom tab of the antennae base (previously it was just resting on the base), and reduced the antennae height as advised on this forum.   Finally Velcro'd the ADSB antennae to the top of the base tube,  again works fine, picking up planes from 50+ km away.

For the P3I I decided to cable tie it vertically to the lower section of the front strut.   Although this is therefore behind the fibreglass of the pod I figured this was least worst option and is clear of engine, me, etc.

Only thing I have had any difficulty with is the power supply.  Originally tried a car USB plug but loads of noise through the headset.  Then tried an Anker twin USB converter and that worked fine but when I connected PAW audio into my Microavionics unit I was getting feedback that varied with engine power.   No audio and it was fine but I wanted the audio.
So today I tried my son's Power Bank (model #B30224) and this works perfect.  No feedback and I can hear PAW audio all the time.

Geoffrey

[exfirepro]

Geoffrey,

Great Pics and thanks for posting. This is very similar to the installation I did on our CFI's trike, except that I put it on top of the base tube where the battery used to be (long story). The main unit is velcroed and cable tied in place with the P3i antenna mounted upright in its normal place (no coax loss) and the shortened 1090MHz antenna with significantly shortened coax (same as you did) sitting on top of the PAW on a piece of double sided tape. Seems to work great, with no problems so far. This is very similar to what I do as a 'demo' kit which can be slipped down inside the windshield and run off a small battery pack to let people try PAW out for themselves.

Remarkable how well it works with a bit of ingenuity.

p.s. Re your interference through the audio lead when connected to aircraft power, it would be worth trying a ground loop isolator such as: -

https://www.amazon.co.uk/gp/product/B01LYYRT7M/ref=oh_aui_detailpage_o05_s00?ie=UTF8&psc=1 ,or

https://www.amazon.co.uk/gp/product/B01IETQQQK/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

...in line with the audio lead. Several PAW users have used one of these (or similar) successfully to cure this problem.

Regards

Peter

[Paul_Sengupta]

You've got the P3i antenna right up against a metal pole. You really want it out on its own away from any metal.

[PaulSS]

You've got the P3i antenna right up against a metal pole. You really want it out on its own away from any metal.

Haha, I spotted that, so I'm learning. It's not true what they say about old dogs.

Another thing I thought of; is it possible to use one of those splitter boxes and share the transponder aerial, on an aircraft equipped with one, for the ADSB input to PAW? I believe the frequencies are the same so was wondering about doing away with duplication of antennas.

[GeoffreyC]

You've got the P3i antenna right up against a metal pole. You really want it out on its own away from any metal.

Hi Paul

Yes I did think about this.  Following the advice that the P3I should be vertical, the alternative would have been to Velcro/sticky pad it to the inside of the pod,  but that would have been less secure and I thought I'd first see whether it worked first on the front strut,  which it did.

My fuse box is on the top of the base bar otherwise I'd have probably gone for something like Peter did.   I pass on my experience as being one implementation that is working OK for me,  ultimately there's going to be some compromises in fitting the antenna as far as possible clear of metal, engine, pilot and any other obstructions.   

Cheers, Geoffrey

[exfirepro]

You've got the P3i antenna right up against a metal pole. You really want it out on its own away from any metal.

Another thing I thought of; is it possible to use one of those splitter boxes and share the transponder aerial, on an aircraft equipped with one, for the ADSB input to PAW? I believe the frequencies are the same so was wondering about doing away with duplication of antennas.

PaulSS,

DEFINITELY NOT I'm afraid. Splitters are OK in certain circumstances - such as for 'stacking' antenna arrays to achieve higher directional gain (I've used them for this in other fields), but in this case, even with a well designed splitter, you would run the very high risk of squirting the high power output from your transponder straight into your PAW with disastrous consequences - probably to both!! You need to do exactly the opposite - i.e. provide as much separation as is reasonably practical to prevent the high power transponder/ADSB signal from effectively shouting in PAW's ear and deafening it.

Regards

Peter

[exfirepro]

You've got the P3i antenna right up against a metal pole. You really want it out on its own away from any metal.

Hi Paul

Yes I did think about this.  Following the advice that the P3I should be vertical, the alternative would have been to Velcro/sticky pad it to the inside of the pod,  but that would have been less secure and I thought I'd first see whether it worked first on the front strut,  which it did.

My fuse box is on the top of the base bar otherwise I'd have probably gone for something like Peter did.   I pass on my experience as being one implementation that is working OK for me,  ultimately there's going to be some compromises in fitting the antenna as far as possible clear of metal, engine, pilot and any other obstructions.   

Cheers, Geoffrey

Geoffrey,

Try 'hanging' the antenna 'upside down' by attaching it to the plastic pitot pipe to the left of its present location with a cable tie (doesn't need to be pulled too tight). That will move it far enough away from the front strut to be effectively 'out in the clear' and will give much less screening to the right. The antenna will work fine upside down and that location should not cause any problems this far forward in the pod.

Regards

Peter