Antenna Location & GPS Input

[PaulSS]

Hello everyone,

I'm in the absolutely rubbish position of not owning any sort of PAW system, nor even an aircraft. However, I plan to remedy that situation and have been spending far too much time looking at goodies for my potential craft and asking far too many questions of long-suffering but patient people, such as the excellent support at MGL (sorry Paul). My plan would be to install a permanent PAW system and I have read all the the documents on the main site regarding that AND antenna location, so I'm not just going for the easy 'answer my questions' without having done some homework 

My potential steed will be fabric covered (Stitts Polyfibre) and I'd really like to know if mounting one of the tuned low profile antennas in the tail would be a decent set up. Clearly it wouldn't have too much metal around it apart from the tube fuselage and I understand it would not be a good idea to actually mount the antenna to one of those (vertical) tubes. I THINK it would be acceptable for the ADSB antenna because of the incoming signal strength but it would be good to know about the P3I reception from back there. I suppose I am in the enviable stage of being able to think about 'extras' before the order is even in and may be able to plan ahead for a couple of ground plates to be mounted in the tail before it gets covered but, if possible, I'd rather keep things nice and protected inside the fabric......and fewer bits sticking out. So, has anyone done this in a fabric covered aircraft yet and can report on their findings?

The second absolutely trivial thing I was thinking about was reducing the number of GPS antennas I would (potentially) have. At the moment my plan is to run two MGL iEFIS units and both will have separate GPS antennas. If I permanent-mount the PAW then could I use a GPS output from one of those EFIS units instead of having a third antenna on top of the glareshield? I'll be using EFIS GPS position for input to the transponder for ADSB but even if I only run one GPS 'out' from each unit then the PAW can be fed the data. As I said, trivia and only a means to have two matchboxes instead of three on the glare shield.

I'll leave my meanderings there for now as I have so many thoughts about power supplies and filters for MGL EFIS that I would bore everyone to death and that's before I've even got a toy to play with.

Thank you for your thoughts and any information on these vital questions 

[exfirepro]

Paul,

Immediate thought is that mounting the P3i antenna in the tail would probably mean a long co-ax run, which isn't a great idea when you are only playing with 500mW out and correspondingly weak received signals. Depends of course on where you will be putting your main PAW unit, but again, you don't want this too far away from your display tablet as the connection is via WiFi. Also with the 1090MHz antenna - although the transmitted signals are much stronger, you still don't want to hide the antenna behind metalwork if you can avoid doing so.

If planning from a 'blank sheet' the best option would be to mount your antennas on a suitable ground plane where they are reasonable close to the main PAW unit and have a good view forwards (things hit you much faster from that direction). In a fabric aircraft, you could mount them internally, though you will inevitably get some screening from the metal frame structure. Personally, I'd go for externally mounted antennas sited so as to have their best view forwards e.g. on the underside of the fuselage around the wing root area.

Re your second query, I have some experience of using external GPS (from FlarmMouse) to feed PAW, but have to say it was considerably less effective than PAW's own fitted GPS - which may work fine in a fabric covered aircraft without the need to use a remote GPS Mouse. The other potential issue you would have is that PAW may not be configured to accept the GPS data in the form your MGLs provide. Sorry I can't be more specific.

Hope this helps

Regards

Peter

[Ian Melville]

I was about to type much of the same comments as Pete, so he saved me some tablet taping 

I will add that an antenna in a presumably wooden or glass fin, would not be accessible for maintenance? Though Europas have the VHF dipole in the fin/rudder gap.

[Admin]

Immediate thought is that mounting the P3i antenna in the tail would probably mean a long co-ax run, which isn't a great idea when you are only playing with 500mW out and correspondingly weak received signals.

Hi All,
We have experimented with RG8X (Keith Vinning please correct me if I am wrong)
This has very low loss, something like 4.5db/100ft.
So the benefit of getting the antenna in the clear, far outweighs the loss in the coax.
We have had excellent results from this type, ensuring you get good quality connectors
and make a good job of crimping.

Thx
Lee

[PaulSS]

Thanks, guys, for all your replies. I was just about to reply to Peter and Ian and thought I'd got it straight but now I've been thrown a tantalising morsel by Lee 

My plan would be to mount the unit behind the panel both for ease of power supply and connecting to the EFIS units. I don't know if the PAW installed GPS would be effective from that position (glareshield above, bits of radio etc either side and the panel's metal and engine to the back and front) but that's not a huge deal; I was simply trying to get away from having another GPS antenna on top of the panel if I could. They're only tiny, so it's wouldn't affect the price of fish but it's my OCD playing up again.

The aircraft I am considering is the Eurofox (lovely looking little things) and I would imagine the antennas, if mounted inside the fabric, would be halfway down the fuselage (ish). I was quite happy to accept Peter's advice about the cable length but it would be good to find out if Lee's proposal may alleviate the problem. If not, then would the area beneath or just to the rear of the seats (below the baggage area) be the sort of place where you'd mount external antennas. I was looking at the tuned monopole whip in the shop. Would you use one of those for P3I and one for ADSB or would it just be one for P3I and the normal ADSB aerial could live inside?

If you think this level of questioning is bad, can you imagine what it will be like when I actually get an aircraft and my PAW kit 

[exfirepro]

Paul,

The under body mounting I suggested was based on Keith Vinning's Sportcruiser fit, which uses (I think) standard PAW 3 metre coax extensions to feed 1/4 wave monopole antennas and I know gave great results in testing (and continues to do so by all accounts). RG8X co-ax with good quality SMA connectors would be even better as it is (as per Lee's post) lower loss at PAW frequencies, though slightly heavier.

If you think this level of questioning is bad, can you imagine what it will be like when I actually get an aircraft and my PAW kit 

No worries - if we can help we will do so. Lots of 'daft' questions answered so far - probably by equally daft answers. Just thought I'd say it before anyone else does - hopefully not so daft answers really 

Regards

Peter

[JCurtis]

Well RG8X is about 11dB @ 800MHz / 100 feet, to get down to 4.5dB the frequency is more like 400MHz-450MHz (check the table in the link below).  To get down to ~4dB  / 100 feet you need something like 9913 or RG213 - all of which are really too big and a pain to terminate in a GA environment.

That said, keep the cable short and it should be OK. If you work out your cable run then you can roughly calculate the cable loss.  Previously I've used nylon rope (to get a similar diameter to the cable) to physically work out an optional route - it's cheap and easier to handle.  Note that you should not put really tight bends onto coax cable, for RG-8X Belden recommend a bend radius of no more than 2.5 inches (http://www.belden.com/techdatas/english/9258.pdf).

To get an approx loss based on the cable length...

   ((your length in feet) / 100 feet) * dbLoss per 100 feet

So if your installing 10 feet of cable, ((10/100) * 11dB) = 1.1dB which should be no problem at all with decent crimped connectors.

For coax and connectors, try and avoid cheap clone cable (read stupid cheap cable from China on eBay), you are really only going to get what you pay for.

[PaulSS]

Thank you everyone for your help. I can see that I'll have a lot of figurin' to do once I get my craft but it'll be worth it 

[tnowak]

I am not suggesting this company has the coax cable suitable for PAW use, but they do a range of coax cables and will make up coax cable assemblies at reasonable cost.
https://www.gigatronix.co.uk/cabulator/coaxial/cable-type

I used them to make up a low loss cable for my Trig TT21 transponder.
Excellent quality and quick turnaround.

Tony

[Paul_Sengupta]

One thing about the antenna, for a 1/4 wave monopole you'll need a ground plane, but not for a dipole.

[PaulSS]

So....just looking at the kit available in the shop: I could possibly have an 'endfeed dipole' mounted somehow (vertically) inside the fuselage using RG8X cable extension. Perhaps the 'tuned low profile antenna' mounted somehow (vertically) inside the fuselage with an RG8X cable extension or, option 3, the 'tuned monopole whip' mounted on an aluminium ground plane and poking through the fabric with the same extension cable. I thinks that's the gist of it or am I wrong, again?

Also, the ADSB antenna can just stay as it is, inside the fuselage, with the RG8X cable extension?

And there's me having dumped all that attenuation theory etc when I'd done the ATPL exams 

[Keithvinning]

Well RG8X is about 11dB @ 800MHz / 100 feet, to get down to 4.5dB the frequency is more like 400MHz-450MHz (check the table in the link below).  To get down to ~4dB  / 100 feet you need something like 9913 or RG213 - all of which are really too big and a pain to terminate in a GA environment.

That said, keep the cable short and it should be OK. If you work out your cable run then you can roughly calculate the cable loss.  Previously I've used nylon rope (to get a similar diameter to the cable) to physically work out an optional route - it's cheap and easier to handle.  Note that you should not put really tight bends onto coax cable, for RG-8X Belden recommend a bend radius of no more than 2.5 inches (http://www.belden.com/techdatas/english/9258.pdf).

To get an approx loss based on the cable length...



   ((your length in feet) / 100 feet) * dbLoss per 100 feet

So if your installing 10 feet of cable, ((10/100) * 11dB) = 1.1dB which should be no problem at all with decent crimped connectors.

For coax and connectors, try and avoid cheap clone cable (read stupid cheap cable from China on eBay), you are really only going to get what you pay for.

Hi Jeremy

Thanks as ever for sharing your expertise its really appreciated.
I have searched for something better than RG8x to no avail. As you say there is better stuff but one also has to consider that not only must the cable be easily available (RG8x is very available overnight from Rocket Radio on eBay) but also the SMA connectors. RG8x is a funny size (7mm OD I think) and I have to get the connectors from the states.
 
Interestingly enough I am still using the 3 metre extension cables available from pilotawarehardware.com which are less efficient than RG8x but still give a fantastic range on the sportscruiser. I am going to change the cables over to RG8x to see how much the range is improved but as it is over 40Kms air to air at the moment I guess its academic.

Interestingly I have been playing around with ground based antennas using a standard PilotAware dipole (This must be used to meet the PilotAware ETSi submission if transmitting). We have one located about 25-30ft above the ground at Wellesbourne with 10metres of RG8x cable between it and the PilotAware unit in the tower and we can see PilotAware users of unknown installations at over 50Km. We contacted one guy to check out if he had a special installation and it was just a standard unit on the coming of his Jabiru 400. So the RG8x can't be that bad.


PaulSS

Great choice the EuroFox. I wouldn't put it on the tail. Even though this is the best theoretical place to put it access will be a problem.
Speak to Roger at Eurofox he is very helpful. My initial thoughts would be to get the guys at the factory to put (possibly rivet)  a ground plane across the chromo frame on the bottom rear of the seats where you can put the antennas. They may have other ideas as they need ground planes for radios and transponders and Eurofox are a very mature company.

This will be a good position as it is a compromise between length of CoAx and antenna location. Will you build a trike or a tail dragger? The main undercarriage struts are fibreglass I think so no problem there.

Anyway let us know how you get on great project. The Eurofox is a good choice.

 
   

[PaulSS]

Thanks for your advice, Keith. I too think the Eurofox looks a great little aircraft and I hope to be ordering one soon when we visit the UK, from Oz, in June. No, it won't be going back there but we'll be coming back to the frozen north 

My choice will include the little wheel at the back and your thoughts on the location of the external antenna are, I am pleased to say, in line with what I was thinking. I don't think that will affect the radio, as that's atop the fuselage towards the tail but I will be careful to ensure the transponder (on the underside of course) and the PAW antenna are separated. I'm too dim to work the minimum separation distance but I assume it would need to be > 1 wavelength and I shall attempt to work out what that is when I've finished typing. Of course, if anyone wants to chime in with the answer, then please do.....just so I can check my workings you understand 

I'm just editing this to add: you mentioned antennas mounted to ground planes under the seats. Am I to take it that the ADSB antenna should also be external and would this use the same tuned monopole antenna as the P3I? I told you I was dim but I'm just wondering if those same aerials can be used given the differing frequencies between the P3I and ADSB. Now, what would the separation on those two be....and separating from the transponder aerial? I'm confused again 

I think I'd better give Roger a bit of a rest from my questions for the time being. Between him and Paul at MGL I don't know which one will strangle me first for interrupting their peace with another 'what if'.

[JCurtis]

I have searched for something better than RG8x to no avail. As you say there is better stuff but one also has to consider that not only must the cable be easily available (RG8x is very available overnight from Rocket Radio on eBay) but also the SMA connectors. RG8x is a funny size (7mm OD I think) and I have to get the connectors from the states.

In that size of cable I think RG8X is pretty much as good as you are going to get.  So much depends on noise in the system, receiver sensitivity, good connections, etc.

[Keithvinning]

Thanks Jeremy

Great to have a second opinion. I'll let you know how I get on.