PilotAware
British Forum => General Discussion => Topic started by: Vic on May 28, 2016, 04:45:25 pm
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During my unsuccessful test flight with my PAW (ADSB only currently) due to connection issues with my Nexus 7, I realised that the DVB antenna was too long to comfortably mount on the coaming of our 172 under the rake of the windscreen. It ended up laying on its side at the base of the screen.
The DVB Antenna which comes with these USB TV tuners is a fairly basic device, It has a quarter wave tuned length to pick up broadcast stations in the 450MHz to 700 MHz band roughly. The screw off monopole top is around 12cm long. The ideal quarter wave resonant length at 1090Mhz is calculated at 68.9mm
I have a small telescopic antenna that can screw onto the DVB-T antenna base. It varies from 60mm up to around 200mm. I was using this on the PAW to find I was getting the best results with it at it's shortest length. While I was experimenting with the PAW in the garden and the antenna up on a fence post, I realised that even without the aerial present, just the base, I was still picking up aircraft around 50NM away! This alone is actually good enough for PAW use!
Antenna base with monopole removed
(http://i65.tinypic.com/2vukew3.jpg)
The exposed coaxial centre is, in itself, an antenna of around 19mm in length.
(http://i63.tinypic.com/fe05fm.jpg)
So, to complete the construction of a properly tuned ADSB antenna, I cut down the top whip to 50mm in length, gently twisting off the end protector and gluing it back on the new (very sharp) end.
Results of ADSB reception is improved over the original antenna length, seeing targets out to 175Km in my fairly overlooked garden.
Tuned antenna
(http://i66.tinypic.com/ibetzs.jpg)
I know this sort of sensitivity is way over the top in terms of PAW usage but having an antenna tuned to the correct specific frequency is a good start in eliminating other unwanted frequencies, specifically in the natural case of this DVB antenna in its unmodified state, strong TV broadcasts and phone masts!
I now have a much shorter 1090MHz specific antenna which can sit easily on top of the pilotaware unit itself under the windscreen . I also took the opportunity to shorten the cable from the MCX plug as its easy from the antenna end of the cable. The braiding isn't clamped under the metal disk anymore, but soldered to it aiding with strain releif.
So, my tip is to unscrew the top whip of the DVB antenna and cut it to a total of 50mm in length. Hope someone finds this useful!
(http://i68.tinypic.com/2h2i3hz.jpg)
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Yes, we did this in beta testing. Though there were a couple of 'experts' who doubted the validity of cutting the whip to the correct length/frequency ::)
However, I found I really didn't want to see traffic as far away as Liverpool (from Bristol) so I bought a Siretta 'cellular' stubby 1/4 wave in SMA and mounted this via a pigtail on the case and it gave enough attenuation to get traffic within a reasonable radius. I will be doing exactly the same with the 'new' classic when it arrives as the supplied antenna a) has a magnet in and b) is pretty poor quality.
I tested a fair few specialist and tuned antennae both on the ARF and the SDR dongle, and because of limited clearance and space on the coaming of my Pup, I'll not be using the supplied antennae.
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Hi Vic
You are absolutely correct. Just cutting the antenna in half gives an improvement and if you tune it properly and put it outside the aircraft the reception is great. However you will then limit the data you receive by setting the height in SkyDemon etc so do use the gain the you have just gained as it were.
However us engineers like to get it right so I have an optimised external antenna.
Thanks for this post.
Keith
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Inspired by Vic, and wanting a tidier antenna setup, I decided to try my own whip antenna.
I purchased a SMA to MCX pig tail and cut 1/4 wave length from the MCX end. (The SMA spare end will come in handy even if I never use it ;D )
(http://i1052.photobucket.com/albums/s443/pumpu9/20160603_1941131_zpsw7xiubyi.jpg)
The cut length is measured from the point where the screen will be cut back to.
(http://i1052.photobucket.com/albums/s443/pumpu9/20160603_1948551_zpsgrmodkkd.jpg)
Strip back the outer sheath and braid to leave ~69mm of inner.
(http://i1052.photobucket.com/albums/s443/pumpu9/20160603_1952201_zps5d9x8uoa.jpg)
Slip over a couple of layers of heat shrink to "stiffen it up a bit (oooher)
(http://i1052.photobucket.com/albums/s443/pumpu9/20160603_2006221_zpsro1nqkty.jpg)
I also found on the Bay a short 90degee mini USB- 90degree USB to tidy up the power cabling. I'll velcro the battery pack to the PAW and the whole lot can sit velcroed to the top of the dash as one nice compact unit.
(http://i1052.photobucket.com/albums/s443/pumpu9/20160603_2007411_zpsefmgimg2.jpg)
A quick initial test indicated that the RX range using the nuew whip is about 1/2 to 3/4 of the standard untuned whip 80km vs 50km (antenna was 0.5m AGL). But even so, it will easily 'see' aircraft off the page when the display scale in Sky demon is set to 500mil or less.
I'll need to do a bit more testing with my second 'none bridge' PAW to test the standard whip against my DIY effort, simultaneously.
Only concern is the ADSB warning calibration. I'll need to mentally 'calibrate' that to judge the warning level vs approx distance.
ATB
Deker
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Inspired by Vic, and wanting a tidier antenna setup, I decided to try my own whip antenna.
Nice job Deker. This antenna design (or one very similar) has been published before on the forum, but you have certainly gone into it thoroughly, and the pics are a great help for anyone else trying to copy it. Like the nice neat 90 degree mini USB power lead. I just hope it is 'heavy enough' cable - though at such a short length, you shouldn't have any problems. Can you post a link to where you ordered it from please and I will have a look. All in all a nice tidy package. Well done!
Only concern is the ADSB warning calibration. I'll need to mentally 'calibrate' that to judge the warning level vs approx distance.
A bit confused by this bit. The warnings for ADSB/P3i contacts are calculated by comparing the other aircraft's GPS-based transmitted position in relation to your own, so your aerial mod won't have affected this at all. Mode-S alerts on the other hand are triggered by strength of received signal. but I doubt if your mod will have had any significant effect, bearing in mind the relatively high strength of Mode-S transmissions.
Regards
Peter
(Mode S Development Tester)
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A bit confused by this bit. The warnings for ADSB/P3i contacts are calculated by comparing the other aircraft's GPS-based transmitted position in relation to your own, so your aerial mod won't have affected this at all. Mode-S alerts on the other hand are triggered by strength of received signal. but I doubt if your mod will have had any significant effect, bearing in mind the relatively high strength of Mode-S transmissions.
Regards
Peter
(Mode S Development Tester)
Hi Peter,
I should have said, mode S calibration and the warning levels. Not sure if the reduced sensitivity would significantly effect the received mode S signal strength and hence the trigger levels.
By what you say, not significantly.
I'll try to set up both my PAWs with the two types of antennas and get a grab of the traffic screens for comparison.
Here is the link to the cable.
http://www.ebay.co.uk/itm/221712730771?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT (http://www.ebay.co.uk/itm/221712730771?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT)
Deker.
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Hi Again Deker,
I wouldn't get too worried about the mode-S side, we (Lee and his Mode S testers) have provided user selectable 'Mode -S Detection Range' within the [Configure] menu to allow for operation in high signal strength environments (i.e. close to Commercial Air Traffic) or in sparsely populated (in aviation terms) environments, so you can simply pre-select the range you find most appropriate for your own flying environment. I take it you know you can also set Mode-S Separation Altitudes in [Configure]. IIRC, the current 20160511 software version still has the 'older' trigger levels, but Lee will be releasing a software update with (amongst other things) the revised trigger levels very soon which will improve this.
Added:
I have had a look at the on-line photos of the cable on the link you gave me. Pleasantly surprised to see a shot of a 'dissected' cable, which gives me some confidence. It certainly looks reasonably substantial, so voltage drop shouldn't be a problem over such a short length, but if you do have any problems with WiFi or PAW dropping out, this would be the first area I would check.
Regards
Peter
(Mode S Development Tester)
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Hello Peter,
I'm using 20160511 and have found the mode s and detect settings.
Planning for a trip to Sandown tomorrow with flying buddy, where I'll be able to fully test and play when I'm flying the "passenger" leg.
Also just tested the short cable vs standard supplied cable. Voltage at spare Raspberry Pi USB are within a few mV of each other.
I'm assuming the 5v on the USB sockets aren't regulated.
(http://i1052.photobucket.com/albums/s443/pumpu9/Cable%20comparison_zpsscix03x9.jpg)
ATB
Deker
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Thanks Deker,
Valuable information and looks like you should have no problems. I have now ordered a couple of these short leads to tidy up my demo kit. Just have to wait for the slow boat from Hong Kong.
Enjoy your trip to Sandown tomorrow. I'm hoping to head up the west coast from East Fortune to Oban as they are having a Fly-in and possibly on to Mull depending on who else is about at the club.
Fly safe
Best Regards
Peter
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On the basis that I am completely unschooled in the dark arts of radio engineering I hope you won't mind me asking a question.
Having read the thread I was wondering if the improvements that tuning the 1090MHz antenna may not have a detrimental effect on the system operation overall? If the PAW processor see even more "targets" doesn't that mean that it will have to spend more time processing them all and filtering out those you don't want to see? I take the point that the plus of the increased sensitivity can be balanced by the filters in the PAW unit but that costs processor cycles, which in turn means power draw (as I believe the Pi only does what it has too rather than everything all the time).
A shorter aerial may be of use in terms of installation but if the longer aerial actually provides a viable level of sensitivity and reduction in the level of irrelevant data then it may be more a question of balancing what we can see against what we need to see.
Be nice I did say that I am not trained in these matters.
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On the basis that I am completely unschooled in the dark arts of radio engineering I hope you won't mind me asking a question.
Having read the thread I was wondering if the improvements that tuning the 1090MHz antenna may not have a detrimental effect on the system operation overall? If the PAW processor see even more "targets" doesn't that mean that it will have to spend more time processing them all and filtering out those you don't want to see? I take the point that the plus of the increased sensitivity can be balanced by the filters in the PAW unit but that costs processor cycles, which in turn means power draw (as I believe the Pi only does what it has too rather than everything all the time).
A shorter aerial may be of use in terms of installation but if the longer aerial actually provides a viable level of sensitivity and reduction in the level of irrelevant data then it may be more a question of balancing what we can see against what we need to see.
Be nice I did say that I am not trained in these matters.
Personally I'd always go for the correct antenna, as it should in theory enable the 'right' signal to have the most power fed into the receiver and assist in rejecting unwanted signals. The filtering is done with in your navigation software, PilotAware (I believe) sends everything over to the display device that decides what to display based on the filters you set.
I doubt the Pi is getting into trouble CPU wise, they pack quite a bit of power in there.
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Still not happy with the D-TV antenna flapping about, especially weightless with the magnet removed, I tried for a neater solution and I think it's worked exceptionally well.
I found that a standard Wifi router antenna (£3 on Ebay) pulls apart so you can get to the monopole inside. I cut this to only just over 17mm in length which is 1/16 lambda at 1090Mhz. I then cut down the top cover of the antenna and chamfered back a spigot so it would refit into the base....
(http://s33.postimg.org/3mt0769x7/PAW004.jpg) (http://postimg.org/image/3mt0769x7/)
I now have a stubby antenna of less than 3 inches in length connected to a RP-SMA to MCX pigtail (RP= Reverse polarity, handy so it can't be mixed up with the PAW antenna). So, Where to mount it? I thought maybe it could fit somewhere on the, case but I haven't received my Bridge yet so didn't want to commit. Searched around for something everyone should have around the house, then it struck me that there's already a free stable mounting point on the Pi...
(http://s33.postimg.org/oyh96oeyj/PAW005.jpg) (http://postimg.org/image/oyh96oeyj/)
Finally, sourced a pigtail with a right angle MCX (bit more brass too so aids cooling of the hot running ADSB dongle!). I think the final solution is fairly neat, even more so when I get some black tyraps! ;) and sturdy..
(http://s33.postimg.org/9usdhvbgb/PAW008.jpg) (http://postimg.org/image/9usdhvbgb/)
I thought the 1/16 wave antenna would also have the benefit of reducing the reception range of the ADSB but I noticed in the aircraft the other day, It was still picking up targets over 200km away!
Vic
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Hi Vic,
Another fine 'homebrew' mod. I'm all in favour of 'tuning' the ADSB antenna to its correct (shorter) length or replacing it with a short, lightweight alternative such as Yourself and Deker have both shown earlier in this thread, which are more practical for use on top of cockpit coamings, etc., but you do need to be very careful in your drive for 'miniaturisation', that you don't attenuate 1090MHz signals so much that it starts to have a negative effect on the reliability of the Mode S alerts.
You say you are still getting (presumably ADSB) signals from 200 Km out. This is not surprising as they are transmitted at up to 500 Watts at the antenna and can generally be received on the proverbial 'piece of wet string'. Mode A/C/S signals are however substantially weaker and due to the vagaries of fit in different aircraft much more variable. In testing for example we found that they can be significantly reduced by the antenna position in relation to your own receive antenna, being easily blocked by aircraft metal bodywork, engines or even people.
Rather than cut antenna length to a level which may significantly attenuate received signal strength, it's much better to keep the antenna just a bit longer and utilise the altitude filters in your 'Nav' software to eliminate unwanted CAT above the height of your choice and let the antenna provide sufficient signal to ensure your 'Mode S Detection' continues to warn you of nearby danger.
Not a criticism in any way - merely a note of caution. Keep up the good work!
Fly safe.
Best Regards
Peter
(Mode S Development Tester)
p.s. Please keep us posted on any Mode S issues you might find.
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Since I've got a couple of the original aerials I've just trimmed one of them down as suggested in Vic's original post. With the PilotAware box in the garden I can swap between the two and the tuned version definitely brings in more contacts with all the filters turned off, even got a couple of bearingless mode S ones! Hopefully it will make it easier to mount under the canopy in the aircraft too.
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Hi Gareth,
Yes, it's perfectly acceptable to 'tune' the ADSB aerial by shortening it, but it should be done scientifically, - not merely chopped off to whatever length fits in the aircraft. My worry is that if people randomly shorten it to the point where the received signal becomes seriously attenuated, the signal strength won't get to the Mode S trigger levels before the other aircraft is right up your 'adsb' - so to speak.
Regards
Peter
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Thanks Peter, yes, it was cut to the 69mm total length (19mm base + 50mm top) as Vic suggested so should be ok. I'm just waiting for my tuned dipole antenna for the bridge to try and work out an acceptable method of mounting them both with a clear view forward in a metal aeroplane without a permanent external aerial.
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Interestingly (and very timely) these two videos have appeared on Youtube looking at the supplied DVB-T antennas..
https://www.youtube.com/watch?v=Rb_GzhvVTQ4 ..for the techies amongst us. Good though there is resonance at around 540MHz so also at 1090! the cable probably is quite lossy then though.
https://www.youtube.com/watch?v=3RsazRx4cxE ..shows a teardown of the antennas much as I did earlier in the thread.
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He doesn't rate them much!!!!
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So, my tip is to unscrew the top whip of the DVB antenna and cut it to a total of 50mm in length. Hope someone finds this useful!
(http://i68.tinypic.com/2h2i3hz.jpg)
Morning Vic, just to make sure I have understood the total length of the top whip antenna, when cutting this down to 50mm, is this including the thread section that screws into the base, or is it just the pole section only. I hope I have made myself clear ;D ;D ;D
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Hi Alan,
We met yesterday on the stand at the show. Not sure if Vic is about at the moment. The 'optimum' length overall IIRC is 69mm from point where the inner coax connects to the radiating element inside the bottom of the antenna. As this is a receive only antenna, however, and we are working with fairly strong transmitters at this 'end' of the unit, the exact length is not critical. If anything though, beware of cutting it too short as this might reduce the strength of received signals, which has a direct knock-on effect on triggering of the alert systems.
Regards
Peter
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(https://s3.amazonaws.com/feather-client-files-aviary-prod-us-east-1/2016-12-05/9f26ded2a8644b4b808bcd05c83bb722.jpg)
This is what I have done, but I'm not sure if this is correct 8) 8)
Any assistance on this would be much appreciated, I've also shortened the length of the cable and re-soldered, even the earth plate, as recommended by Peter after seeing him at the show ;) ;)
Regards
Allan
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Good Morning Alan,
I have just re-read the whole of this thread and now see where the confusion arises. In his original post, Vic mentioned that the short section of exposed coax inner inside the base ..'is itself an antenna of about 19mm', but it's not clear whether he then included this as part of his 69mm total antenna length. If so, the '50mm' vertical part would need to include the vertical section inside the base cone from the point where the coax is soldered on, though that would make the antenna much shorter overall than it looks in Vic's pictures. I suspect therefore that when he says 'cut the top whip to a total of 50mm', he means from the bottom of the screw connector on the left in your photo. This would make the overall length of the vertical part (excluding the horizontal coax, but including the vertical part inside the base) about right. I may however be wrong, so have dropped Vic a PM to ask him to clarify.
As I have said before, this antenna is 'receive only' so its length is far from critical - as long as you don't make it so short that you start to attenuate the received signals as this will have a directly negative effect on the triggering of warnings for 'bearingless' Mode C/S targets.
Watch this space
Regards
Peter
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Thanks Peter,
The overall length of this antenna including the thread section is about 65mm and this is picking up aircraft almost 300km away when in flight, so I'll probably leave as is :) :), note this does not include the base section in this measurement :) ;)
Like you said best to leave as is rather than making it to short ;D ;D
Regards
Allan
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Hi Allan, sorry for the delay in getting back.
Yes, the whole antenna starts from where the braiding of the Coaxial cable stops covering the inner conductor. Everything from there onwards forms part of the resonant length of the antenna. Think of it as blowing across the top of a bottle and adding water to get a note you want only we are doing it with radio waves.
However, the perfect antenna relies on a clear area of space around it in all directions and we'll never get that anyway inside a light aircraft. You will perfectly fine with what you have there and may, from experience, even have to adjust the sensitivity down in the PAW settings.
Probably more important than all this is to solder the braiding down to the base disc of the antenna. ;)
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Thanks Vic,
Hopefully we have now cleared that point up and Alan now has a more effective antenna installation (with the braid now soldered to the base disc as well).
Regards to all
Peter
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Thanks Vic & Peter for your assistance on this.
This is what mine looks like ;D ;D ;D
(http://i38.photobucket.com/albums/e142/allanbirt/2016-12-052014.44.59_zpspsna7fve.jpg)
All wires soldered, even the earth plate 8) 8) 8)
Regards
Allan
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Well done Allan,
Have fun and fly safe.
Regards
Peter
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made the same mod to a pigtail, added a couple of layers of heat shrink - works a treat!
(http://www.mmcpix.com/Gallery/antenna.jpg)
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Nice Mod,
Should work well, though you might find it starts to slip round on the MRX plug eventually away from the vertical if the plug ever starts to work loose on the dongle. A tiny dab of silicone or hot glue (carefully applied) will prevent this.
Regards
Peter
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Nice Mod,
Should work well, though you might find it starts to slip round on the MRX plug eventually away from the vertical if the plug ever starts to work loose on the dongle. A tiny dab of silicone or hot glue (carefully applied) will prevent this.
Regards
Peter
aha good thought :)
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Hold on - 69mm is a little too long!
There needs to be a correction for the diameter of the antenna element.
This is called 'end factor correction' K.
65 mm will resonate at 1090MHz.
If you want the explanation read on.
Using V = f x λ we get:
wavelength = Velocity / frequency
wavelength = 300,000,000 / 1090,000,000 = 0.275 metres or 275 mm.
¼ wavelength whip antenna will be 275mm / 4 = 68.8 mm, all correct so far. So what am I beefing about.
As I said 68.8 mm will not be resonant at 1090MHz, it will be 'overtuned', it needs an end factor correction to bring it into resonance.
End factor correction is dependent on the ratio of the half wavelength to the thickness of the antenna element.
In our case 137.6 mm to 1.5 mm or 92:1
By looking this ratio up in any good antenna theory book the antenna length should be 0.96 times its calculated size.
68.8 x 0.96 = 65.4 mm.
So there you have it :)
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Chris,
You have a valid point, but to be accurate, the speed of light in air is nearer 299,700,000 m/sec, which when applied to your calculations makes the antenna length 65.989mm.
But remember, this is a "receive only' antenna, working with very strong signals compared to the P3i side, (or the likes of FLARM). For most users, the reason for trimming the antenna is to ease fitting on top of an aircraft coaming against a sloping windscreen, rather than for accurate frequency response.
In practice, the 'actual' length isn't critical - anywhere between about 65 - 75mm measured from the base will work fine. It's more critical in fact not to cut it too short as this could have a detrimental effect on the measured strength of the signal passed to the software, which is then used to trigger the bearingless Mode C or S alerts.
Regards
Peter
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A quarter wave whip should also have a ground plane.
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True Paul,
But again, much less of an issue for receive only.
P
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Also note that the antenna and screw doesn't go right to the bottom of the base...
But there's a bit of metal and a magnet which might affect things.
And transponders are high powered.
I use a "pepper" GSM antenna. Receives out to 40-60 miles which is good enough for traffic purposes.
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I think a bit of thought needs to go into the ADSB antenna choice.
It's all very well saying that you get aircraft out to 40 - 50 miles, however if they are high level CAT, then you could be hiding a problem it's lower GA aircraft. CATs positioning and high power assures a strong signal is received, but GA which may be below and the transponder on the underside will be at a big disadvantage.
My preference would be to fit a tuned, omnidirectional antenna to get the strongest signal possible then restrict volume of hits in the PAW software by having a bubble around your aircraft of say 40km
Can the radio experts tell me if a mag mounted antenna is using the metal sheet it is connected to as a ground plain, or as a director/reflector. I don't get the fact that it is not electrical bonded to the base of the antenna, yet it seem to be vital the screen is bonded to the magnet. Can electricity jump through paint⚡️😳
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Yes, there will be capacitance between the internal plate and the metal it's stuck to.
I may only get 15 miles on GA transponders...but I think that's ok.
The problem will come when things like the SkyEcho are to be received, then I guess one will need a good ADS-B antenna.
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Can the radio experts tell me if a mag mounted antenna is using the metal sheet it is connected to as a ground plain, or as a director/reflector. I don't get the fact that it is not electrical bonded to the base of the antenna, yet it seem to be vital the screen is bonded to the magnet. Can electricity jump through paint⚡️😳
Ian,
I wouldn't call myself an 'expert', but the simple answer is 'YES', the ground plane of a magnetically mounted antenna, fitted on a steel car roof for example, is capacitively coupled to the 'earth' braid of the cable feeding the antenna whip or 'monopole' and forms an integral part of the system. For this reason, magnetic antennas (or any other monopole whips) should be mounted as near as possible to the centre of the ground plane. Failure to do so causes the transmission pattern to be 'skewed' with maximum transmission effectively across the longest length of ground plane away from the antenna itself.
Without a ground plane, a high proportion of the energy from a monopole transmitter will be radiated at a very high angle and horizontal propagation will be limited to a relatively short distance. The antenna is also unlikely to prove a good electrical match to the coaxial feed line - setting up standing waves which can wastefully (and in some cases destructively) reflect power back into the transmitter. Unless capacitively connected, it is therefore essential that the antenna ground plane of a transmitting antenna is electrically connected to the outer braid of the coax at the feed point.
A good ground plane both reduces the angle of maximum transmission (effectively providing 'gain' to the antenna) and improves 'impedance matching', lowering standing wave ratio (SWR) and ensuring that maximum power is transferred from the transmitter feed line into the antenna. With a 'perfect' ground plane extending infinitely in all directions around the monopole however, antenna impedance will be about 35 Ohms, which is not a very good match to coaxial cable. For transmit antennas, the impedance of a monopole antenna is often increased to match 'standard' 50 ohm coax by replacing the horizontal ground plane with a set of 'radials' angled downwards at approximately 60 degrees (see attachment).
With receive only antennas, the same principles apply, but matching is generally less critical, though the closer to the ideal we can achieve, the stronger the signal passed to the receiver itself is likely to be.
Lots of reading out there if you want to get technical.
Regards
Peter
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Hi Peter
Thanks for making me look at that calculation again.
I had made a mistake. I must check my working more carefully. ???
My calculation should have been 300,000,000 / 1090,000,000 = 275 mm and 275mm / 4 = 68.8 mm, then 68.8 x 0.96 is 66.055mm
I didn't think that using 299,700,000 would make that much difference, only being 1 part in a 1000, and it doesn't, you got 65.989mm
But as you say this is less critical on receive.
Thanks for the wake up. :D
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Hi Chris,
With PAW's 1090 MHz side being 'receive only', things aren't so critical, especially for ADSB targets, which are generally strong signals from high power transmitters and present us with accurate GPS derived position data. The main concern with 1090 MHz, is that 'bearingless' Mode C and S alerts are triggered by the strength level of the received signal - and dictated by the detection 'Range' you have selected in the PAW 'Configure' Screen. To be effective the alert triggers rely on a well sited antenna to ensure we receive the maximum signal strength possible from the other aircraft. Tuning the antenna obviously helps, but we have found that an antenna which is too long is generally far less critical than one which is too short, which is why I always warn users against over shortening the antenna, but am not too worried about leaving it slightly long.
Of more importance is antenna positioning. Although the antenna in theory receives signals equally from all round the aircraft, it is very difficult to position it in such a way as to completely avoid screening by aircraft metalwork, engines or occupants. In this respect, we generally advise users to position the antenna where it will get the best clear view to the front of the aircraft as this is the direction from where we normally get the least warning of approaching aircraft due to the combined effect of closing speeds.
For metal bodied aircraft, an alternative would be to fit 'twin' 1/4 wave monopoles externally mounted in the clear above and below the fuselage, and use the body of the aircraft as their common ground plane, but this is technically fairly difficult to achieve a good impedance match to the receiver. It might be worth a bit of experimentation however, when I can find the time.
Best Regards
Peter
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.
Interesting topic, that needs to be highlighted once again.
One question: If you tune the aerial to 70mm, will this make the aerial/dongle combination more efficient? ie, draw less electrical power, because it is only looking at specific frequencies?
We are running the PAW from battery, and any conservation would be appreciated. Judging by the heat it puts out, the ADSB dongle is the biggest consumer, and lessening that would be helpful. It seems odd that the receive-only ADSB aerial would draw more current than the PAW transmit aerial....
Thanks,
R
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Hi Rog,
Tuning the antenna shouldn't make much difference in practice to the electrical power draw as the desired receive frequency is set in software, so the SDR will simply ignore 'off frequency' signals. Using an antenna tuned to the correct frequency however improves the signal strength (and therefore reliability) of the received signals on the chosen frequency. As I have said several times, however, (despite the precise calculation discussions above), exact frequency tuning is far less critical for the relatively high power 1090 MHz ADSB, Mode C and Mode S signals which the SDR Dongle is looking at, compared to the much lower power P3i signals which the other (Bridge) part of the PAW is looking for.
Best Regards
Peter
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As I have said several times, however, (despite the precise calculation discussions above), exact frequency tuning is far less critical for the relatively high power 1090 MHz ADSB, Mode C and Mode S signals which the SDR Dongle is looking at.
Indeed. We broke our aerial, and still got 100k range on the ADSB.
But theoretically, if the ADSB aerial was inclined against the canopy by 25º, should we lengthen the optimum to 84mm? Or do radio waves not behave like that?
Cheers,
Ralph
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No they don't behave like that. They are tuning forks for radios. The length gives the tone, no matter what angle you hold it.
Given the power of the transponders 25 degrees probrobly make little or no difference to the range. More important not to have blind spots rather than extreme range. Anything more than 50km is sufficient.
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I have shortened my antenna to 69 mm (19mm base plus 50mm stick), in line with the comments here. Making a 1/4 wave antenna.
However while doing so I noticed that the original antenna stick was 115mm long, plus rhe 19mm base, which makes a 134mm antenna. Is that not close enough to the 138mm 1/2 wavelength, to be an even more effectove antenna?
If the antenna was 138mm, would it be even more sensitive?
Thanks,
R
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In theory a half-wave antenna would be a better performer for more distant signals, given its radiation pattern, although that is more important for transmitting than receiving and given the strength of ADSB signals, the difference would probably be negligible.
Half-wave antennas also less reliant on ground, although we shouldn't go as far as to declare them to be ground independent, as some manufacturers claim.
One other significant difference though is the feedpoint impedance. The quarter wave is a relatively low impedance feedpoint device, hence our ability to fed it happily through 50 ohm coax cable. Not so the half-wave. It is a high impedance feedpoint device and should really be connected to the coax via an impedance matching circuit, such as a parallel combination of a coil and a capacitor, tuned to the frequency of operation. That tuned circuit, often called a tank, has a low impedance at the bottom of the pair but a high impedance at its top. That characteristic of a parallel tuned circuit makes it easy to connect the antenna to the top and the 50 ohm coax to the bottom.
in terms of tuning the antenna to resonance, 4mm at 1090MHz is a significant difference in length.
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Hi Bill,
Long time no speak! How are things out in Oz? Nice to hear from you again. I had read Rog’s post yesterday morning but was just about to head up to Perth (UK not WA) to try to help sort out some PAW probs in a couple of RVs, so had to leave it.
I knew there was a technical reason to do with voltage and impedance at feed points and was racking my brain on the drive up but couldn’t dig out the details, so thank you for a very clear and accurate explanation.
Best Regards as always
Peter
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G'day Peter
I still monitor the Pilot Aware scene, even though I can't use it here in Oz. Speaking of which, I would not have been confused about your heading up to Perth in Scotland, rather than Perth in Western Australia. I might though have been confused in to thinking that you were maybe heading to Perth in Tasmania, which is older settlement that that young upstart village over in the west. In that case, since it is only 11 kms up the highway from here, I would have invited you to drop in for coffee and even invited you to use my strip :)
That said, you would probably have written that you were heading down to Perth, given our inverse orientation.
Cheers
Bill
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that young upstart village over in the west.
HEY, some of us live in that 'village' ;D
Mind you, if this village has 680 times the population of the Perth in 'ladies parts' I don't know what that conurbation would be called....a 'hovel'? ;)
Is that your strip I can see in Powranna (no village), close to the highway? That looks a great place for an aircraft :)
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Yes Paul, that would most likely be my strip, aligned 31/ 13 and right next to the highway. There are two other strips in Powranna, one on Powranna Road but aligned more east/west and an old one on Symmons Plains property just to my north and on the other side of the highway, behind the motor racetrack. The latter is aligned 32/14, which shares my advantage of nicely suiting our prevailing north westerly winds. It is used very occasionally though, except for model aircraft that are flown there most Sunday mornings.
The local crop dusting company likes to use our strip when dropping fertiliser granules etc on the neighbouring croplands. We had a Thrush operating off the strip a month ago. Nothing like the sound of a 800shp turbo running up in your own front yard!
The only things to watch out for here are the A.C. Mains (3 phase) feedlines that run to the house from the pole across the highway on the southern approach and the what might be politely termed "close proximity" of Class D airspace for Launceston ( and for the non-Australian resident readers, we are not talking of the one in Cornwall). Luckily, the Launceston Airport isn't what you could really consider as busy.
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The point of cutting down the antenna isn't really to tune it, but rather to make it fit in a tighter location while still retaining a fairly good receive ability. The impedance mismatch shouldn't matter too much for receive.
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Hi all,
just to add a little more info on this subject, I've just done a 'before and after' check by using a shortened DVB-T 'whip' aerial as suggested in the original post. The screenshot show the increased detection range achieved by shortening the aerial to 50mm.
For info, my reason for doing so was mainly to aid installation into a pa28 rather than increased detection but it's a nice 'bonus' to achieve as well as making the system more compact for installation.
Thanks,
Sam
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Hi Sam,
An antenna carefully tuned to the correct frequency will always be more efficient than one of random length, hence the effects you are seeing. Good to see ‘physical evidence’ to support the theory though and the shorter length is definitely a bonus when it comes to achieving an effective fit. Well done.
Best Regards
Peter
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Having read the posts with interest and having shortened the cable, can I please clarify the point at which the aerial mast will be cut. I have the 19mm exposed coax and inner aerial post and therefore need to cut the mast to 50mm.
Therefore, do I understand correctly, if I unscrew the aerial and lay it down, that when cut the entire bit left will be 50mm? This will obviously include the screw-on post section so tip to tip 50mm, with the screw-on section being 10mm of that, leaving 40mm of wire?
Or, is it 50mm of the thin “wire” aerial on top of the screw on post?
Have seen the earlier confusion with where to cut.
Thanks
Derek
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Hi Derek,
Overall length needs to be 69mm, so you are correct - cut the whip to 50mm from bottom of the female coupler - I find a good strong pair of ‘side-cutters’ work well, but watch out for the tip heading off at speed (I usually cover it with a cloth while cutting to catch the cut end). The plastic end cap can be gently persuaded off the waste bit with a bit of gentle heat from a heat gun or similar, then fixed back on the new tip with a spot of superglue (hot glue also works).
As you will have read, the overall length is fairly ‘non-critical’ by the way as the antenna has already been working as receive only.
Regards
Peter
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Many thanks Peter, understood.
I was just a bit nervous as the posted pictures in this discussion were looking a bit longer than what mine was going to look like...must just be the perpsective of the pics. Right....50mm from the base of the screw end it is.
Regards
Derek
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Most times I’m cutting them down, it’s to fit the PAW on a coaming against a windscreen, so as the length isn’t critical, I often just cut the whip ‘in half’ by eye - it still works fine for traffic awareness, bearing in mind we can only see aircraft a couple of miles away at best.
Regards
Peter