Mutability vs FA - Or 2 ele j-pole vs Flight aware aerial?


#1

In my loft I have a Pi, A Flight Aware receiver and a homebrew 2 element j-pole.

I’ve just put a second unit up there consisting of a Pi, a Flight Aware receiver and a Flight Aware aerial.

The original is running the ADS-B Receiver Project script with 1090-mutability.

The new one is running a brand spanky new FA 3.5 image, downloaded just a few days ago.

Despite running gain optimisation on the new setup, the original is is receiving a LOT more.

Is this a 1090-mutability vs 1090-FA thing or is my little j-pole working significantly better than the Flight Aware aerial?

I’ve set up some port forwarding so people can see the difference.

Receiver with homebrew two element j-pole aerial ** link removed **

Receiver with commercial Flight Aware aerial ** link removed **

Both receivers are identical. There is no coaxial cable involved.

My original plan was to have the new unit mounted outside at the top of my amateur radio mast, 10m up in the air and clear of all the local rooftops.


Which Antenna for best Results?
#2

dump1090-fa and dump1090-mutability use essentially the same demodulator. Unsurprisingly, having different antennas gives you different results.


#3

I would also expect a difference between different aerials.

I’d expect a forty five quid commercial aerial to work better than a homebrew two element j-pole colinear.

Or is that naive of me?

Do we know what’s inside the Flight Aware aerial?


#4

The Cantenna performed better than FA antenna, till I added a Filter. Adding filter reversed the performance, and the FA antenna +Filter performed better than Cantenna. Reason is FA antenna is high gain, and, although I am not sure, but guess that possibly it is tuned at multiple frequencies, and picks much more RF noise than low gain DIY Cantenna.


#5

I’m running the FA receiver with the amp/filter built in.


#6

Is your FA receiver a:
(a) Pro Stick Plus, blue (a dvb-t with integral amp and filter)
OR
(b) FlightFeeder (an FA supplied box containing RPi, Filter, ProStick which has integral amp).

In case of (a), an external Filter between FA antenna and ProStick Plus often improves performance. If you have an external filter, just give it a try.


#7

It’s a)

I don’t have a spare filter so it’s not a quick job just to slap one in. Is the FA manufactured inline filter any different to the one built into the Pro Stick Plus?


#8

Look at the clipped/strong signals. I had to lower my gain because the planes near-by where too “loud” for the total gain and where discarded.
Internal_IP/dump1090-fa/data/stats.json


#9

Yes, these are different types. The integral filter is a SAW Filter chip, while the external filter is an LC ladder filter.

However, the reason for improvement is not the type of filter, rather it is due to location of filter. The integral filter is located between amp and tuner, and does not block interfering rf signals to reach amplifier, causing overload and cross a modulation. The external filter is placed between antenna and amplifier, and prevents interfering rf signals reaching the amplifier.

Graphs below show the results of the tests I conducted last year.

FA Antenna


#10

Does the default FlightAware image show those stats? When I substitute my internal ip on that link, I get a page of text.

I didn’t realise the filter wasn’t before the amp. That’s an odd design.


#11

The stats page from the FlightAware is just text, sorry.

The design is not strange, the preamp should be more “immune” to the overloading than the first stage inside the RF chip. Theoretically, to save as much as possible signal/noise ratio, the preamp should not have a filter in front.
But in my experience, even from planes far away, I have +6…10dB margin, so adding a filter in front (-3dB) doesn’t make enough “damage”. Personally I am limited by the horizon, not by the gain/signal level.
In your case you can pick up planes from 220 miles… so you have plenty gain.


#12

By default no, but if you want these graphs, you have to install “Web Portal” by Joe Prochazka’s scripts on Piaware image.

ADS-B Receiver Project Setup Scripts.

NOTE: The script will offer to upgrade piaware data feeder and dump1090-fa. It will also offer you to install or upgrade FR24 feeder, Planefinder feeder, and Adsbexchange feeder. Say NO to everything (except system update) , till you reach the stage where you are asked about web portal installation. Say OK to web portal installation, but NO to “advanced options”.


#13

Thanks - My original installation was using the ads-b receiver project image so I’ve missed out on how to add a few tweaks to a standard installation.

I’ve updated my second link in the original post so although it still goes to the map, it’s possible to get the performance graphs as well - I’ll monitor this for a couple of days and see how it compares to the other receiver.


#14

I’ve added a FA bandpass filter and re-run the gain script. Here’s the results:

===Totals===
Gain, Messages, Positions, Aircraft
49.6 372389 20172 459
48.0 370450 20488 459
44.5 368386 20364 452
42.1 364032 20094 453
40.2 368717 19884 444
38.6 366253 19864 457
36.4 363700 19339 452

I’ve gone with 48.0

Although the number of aircraft, messages and reports have gone up since adding the filter, this setup is still a long way behind the homebrew two element j-pole aerial.

Again for reference.

Two element j-pole performance graphs ** link removed **
Flight aware vertical performance graphs ** link removed **

These two receivers are quite literally next to each other in the loft.

I added the filter around 10:50 today and then you can see the gap where I ran the gain script.

My plan is to put the new receiver with the Flight Aware vertical outside, about 10m up in the air but I’m just concerned by the apparent lack of performance compared to what I’ve already got.

Would appreciate any more thoughts on the above.

Thanks.


#15

The 2 element, 1/2 lambda, j-pole might get better results. It’s difficult to build correctly, so most of users will not attempt.
As for the cost… If you would build your antenna for sale, you wouldn’t sell it just for material cost, add the labor cost (let’s say $90/hour if you are a firm that pays taxes, insurance, etc) and see where you end up with the cost.

PS: leave some distance between antennas, they can influence each other.


#16

Hi Keith, looking at the 2 RPIs you have, as you say, quite a difference between the two. I can’t think why they are different - sorry.
Can you post the dimensions of your j pole and I’ll have a go at building it and compare with the FA antenna I have.


#17

This is the aerial I built - Image shamelessly stolen from another thread on these forums.

I then adjusted it using my Mini VNA Tiny to be resonant at the correct place so it’s spot on. I didn’t put the FA aerial on the VNA.


#18

Ah yes, I think that is a design by aldoir. I did try it but not successfully. I’ll have another go


#19

I tried the longer version and apart from the stability, I just couldn’t get it to tune.

I wonder how much of the difference may be down to the (effective) radiation pattern of the FA aerial compared to the colinear. Without knowing what’s actually in the FA stick, there’s no way to know what it’s doing.

It was all built and boxed properly ready to go outside but I’ve had to bodge it a bit to get the bandpass filter in. I might still see if I can get it outside tomorrow anyway, even though it is bodged at the moment. I know that in the loft I’m quite heavily blocked in one direction.


#20

Even if one knows what is inside, it is hard to find what it is doing unless tested by sophisticated test equipment.

Anyway, here is FlightAware 26 inch Aerial from inside.

Photo was originally posted by David Baker somewhere in forum. I have stolen it from there :slight_smile: