Tuning the DVB Antenna for 1090MHz Specific reception

[Vic]

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



The exposed coaxial centre is, in itself, an antenna of around 19mm in length.


 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


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!

[trapdoor]

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.

[Keithvinning]

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
 

[Deker]

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  )


The cut length is measured from the point where the screen will be cut back to.


Strip back the outer sheath and braid to leave ~69mm of inner.


Slip over a couple of layers of heat shrink to "stiffen it up a bit (oooher)


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.


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

[exfirepro]

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)

[Deker]

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

Deker.

[exfirepro]

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)

[Deker]

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.

ATB
Deker

[exfirepro]

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

[GrahamB]

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.

[JCurtis]

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.

[Vic]

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....



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...



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..



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

[exfirepro]

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.

[GarethHorne]

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.

[exfirepro]

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