Min range for ADSB


#1

I am curious to know if there is a minimum range to pick up ADBS signals?

Reason I ask is, I track planes coming in on final approach almost directly over my house. Once they get with in about 2-3 miles of my receiver, Skyview stops tracking it showing no more messages. Same for departures and once they are about 5 miles or so away, it pops up on Skyview. Interestingly I have line of sight to the A/C during this time but seems they are too close receive messages or something. Generally these planes are passing by between 1500-2500 AGL and my antenna has a clear shot of them coming in or taking off.

Any thoughts?


#2

This is due to the dynamic range of the receiver.
Look at the june 2017 newsletter on dynamic range.
https://flightaware.com/adsb/faq#previousnewsletters

The default gain is maximum gain with AGC on . Gain settings is set to -10.
You can also set the gain from 0 to 50.

There are some scripts floating around the forums that will go through all the gain settings and find the maximum message rate for your site. It is quicker to just set the setting to about 35-40 since you are close to an airport. If you are actually on the airport a gain of about 20 will work best.


#3

Thanks for that. I didn’t see the article but just read it. I previously did some testing with those scripts (with slight modification) and found the highest average message counts were a gain setting of 48 for me. But that said it is likely the planes that are close are blinding me. So I may have to find a new sweet spot and not try for the those 200+ mile planes.

I put everything in a xls and this is what I came up with running 5 tests for each of the listed values for 90 second duration’s.


#4

As first step, set your gain to 25 and watch if you start seeing planes on runway, landing & taking off. If that test is sucessful, try other adjuscent values like 20 and 30.


#5

I see planes on occasion, but once airborne, I pretty much loose everything with in 5-7 miles of me. I even listened to one plane that was cleared for take off, it flew right by me, made left traffic and I didnt pick him up on Skyview until he was about 8-9 miles out. I had the gain set to 1.4 and still could not see him in Skyview. I tried other random gain settings and only noticed the distant planes I was loosing from my feed.

I think the best course of action here is to just go with what gives me the most messages. I just wont get to see the planes in close which I appears to be a disadvantage living to close to an airport.


#6

@deltaleader71:
If reducing gain does not solve the problem, then most likely it is the antenna’s radiation pattern which is causing this.

Normally a high gain antenna has two horizontal lobes like infinity symbol ∞ . However if antenna is not properly tuned, the two lobes may either get pulled up or pulled down. In your case the lobes seem to be pulled up, causing nearby planes to disappear.

This can be tested by installing the antenna upside down, making upwards lobes downwards.

If it is hard to install antenna upside down, then other easy alternative to test is to make a 1/4 wave antenna like “Quick Spider” or “Cantenna”. Both have horizontal lobes and dont need any tuning/trimming.

Most likely you will be able to see planes on runway/landing/take off with these 1/4 wave groundplane antennas. These are very easy to make and cost almost nothing. No harm in trying.

How to Make CANTENNA

How to Make QUICK SPIDER


#7

This is interesting. I could probably test this out once the weather improves or even try making a quick one. I am just using the FA Antenna. This is what it looks like on my house looking towards the north. Thanks for this information. Learning something new all the time here!


#8

@deltaleader71
Better dont disturb your Flightaware antenna, leave it as is for now. This radiation loop pulled-up problem is very rare with commercial antennas, because these are well designed, and after assembly are tested, trimmed and tuned at factory using appropriate intruments and jigs.

This problem is very commomn with DIY Collinear antennas like CoCo (coaxial collinear). This is due to

  1. Use of inaccurate Velocity Factor
  2. Errors in cutting and assembling pieces
  3. The hobbyist has absolutely NIL test equipment to test/trim/tune.

For you the best option to TEST is to make a DIY Cantenna or QuickSpider. You dont need to mount it on roof, just place it indoors near a large window or outer wall. Alternatively place it outside in your yard few feet above ground, on a wooden/plastic stick/pipe driven in ground (like a plant supporting stake).

Cantenna supported on a wooden stake . Spider supported on a plastic pipe in yard

EDIT - Additional info:

  1. Both Cantenna and Spider are 1/4 wavelength antennas, and are naturally tuned and do NOT require test equipment to tune/trim

  2. Radiation Pattern of both these antennas support picking signals from low flying planes as well as Planes on ground.

Radiation Curve of Cantenna
Cantenna Radiation Pattern

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Radiation Curve of Spider
Spider Radiation Pattern


#9

Interesting problem.

What is the signal level (RSSI) reported by Skyview when the plane firsts appears? I have helicopters flying at about 1000 feet nearby and they disappear when the RSSI drops to about -1.1dB

I SSH into the Pi and issue a command to change the gain and another to restart dump1090 which takes about 10 seconds and I can immediately see the outcome without having to run the scripts and then analyse the outcome.

The transmitter on the plane is reasonably high power and being so close I would imagine there is signal everywhere.

Have you tried running the small antenna that may have come with your receiver inside your house?

Did it change what you saw in Skyview?

If you don’t have such an antenna try putting about 100mm of wire in the antenna socket.

Again, if you can’t see the aircraft until they are 5 miles away, try reducing the gain to single digits.

Don’t forget to restart dump1090 each time you change the gain.

S.


#10

@SweetPea11
@deltaleader71

There are many ways to make a temporary indoor antenna. Photos of some temporary antennas I have made are given below:

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whip_on_a_can.

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whip_on_plate

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#11

Yes, there are many different antenna designs, some are simple and fool proof such as a λ/4 mag base sitting on a biscuit tin lid. These can be made to work reasonably well with nothing more than a measuring tape.

Others are complex and require both an understanding of how they work and some facility to ensure they are actually functioning as designed. A good example is the coaxial collinear as described in the discussion forum which sometimes seems to work brilliantly and sometimes it just sits there.

In this case I was simply suggesting that deltaleader71 was probably getting too much signal because he was so close to the transmitter and using a gain antenna on the roof was possibly making it worse causing an overload of the front end of the receiver.

I was actually suggesting a poor, indoor antenna as a way of establishing overload as his problem.

A finger on the centre conductor of the antenna socket would probably provide sufficient attenuation to prove a point.

S


#12

Thanks guys. Lots of good information to consider here. I suspect at this point I seeing what David referenced in the 2017 newsletter. That said, I will have to try one of the antenna ideas mentioned above.

Typically I see planes coming in from long distances with varying RSSI. They tend to drop off when within roughly 10 miles or less. Their RSSI in Skyview is usually less than -1.8db from what I can tell spot checking those.

So I tested during a busy time the other day, manually changing the gain and restarting dump1090. Doing this as planes passed close by. I did not notice any changes such as planes showing up or on the ground with single digit gains. Only noticeable changes being the messages/second were a lot lower and obviously not seeing planes at longer distances.

I think my next steps will be to try a different antenna as suggested. I have something that might work and I can use to test during high traffic periods and see how this effects the planes close in. Let me work on that over the next day or so and I will report back on my findings.

Thank you all for the input!


#13

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Very true!

@SweetPea11: & @deltaleader71: Please see below the photos of optimized whip I regularly use with one of my Pi.

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Dimensions of Optimized Whip

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Optimized Whip Placed Indoors Near a Window, on A Metal Can

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Reception By Above Optimized Whip (Jan 07, 2018)


#14

Slightly puzzled, reading the newsletter it says the default setting is -10 for maximum gain but then suggests trying a setting of 40 and then going higher or lower.

I have very little idea about the theory of radio signals so am curious, why would you not adjust it slightly from the default setting and gradually decrease gain rather than change completely to 40 then adjust from there?

Geffers


#15

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The gain -10 setting does NOT set the value of gain to -10 or maximum. The gain setting -10 is a special value which switches the dongle to AGC mode (Automatic Gain Control mode). In this mode the dongle dynamically adjusts gain value between minimum (0.0) and maximum (49.6)

Supported gain values (29) are: 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6


#16

Eight Months ago, very close to Toronto Pearson Airport CYYZ…

With my Android Phone + OTG cable + DVB-T Dongle (generic) + dongle’s whip antenna on a tin can + Flightaware’s “Flaght Scanner” App on my Android Phone.

An this is what I got:

Screenshot_2017-05-18-18-06-55


#17

On-ground, taking-off, and landing aircraft at CYYZ.
From my apartment about 10 miles from CYYZ, using Optimized Stock Whip + Filter + ProStick


#18

I expressed an interest in the app when I saw it mentioned on the forum but didn’t get (or missed) a reply.

Which app is it? The flightaware app that shows seems just to show general data received and doesn’t mention use with one’s own ADS-B receiver.

Geffers


#19

Thank you, that has clarified for me.

Geffers


#20

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The App you see now for Android/iPhone/iPad is “FlightAware Flight Tracker”, which has nothing to do with user’s receiver (DVB-T/ProStick).

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The App I use is “Flightaware Flight Scanner” which is no more available on Google Play, but its source code is available at Github

http://flightaware.com/adsb/android/

FA Flight Scanner icon-100x100