Results from FlightAware 1090 MHz ADS-B Antenna - 26 in

I actually have exactly that (a whip-cone) for other purposes but while cones are really good for wide/multi band use, they’re not so good for narrow band use. No to mention, they’re hard to tune and they can collect snow. Here’s some preliminary info to demonstrate why I’m concerned about bandwidth…

This is what I have to deal with:
image

The FA antenna connected to an unfiltered SDR will find absolutely NO aircraft at any gain setting. Right now I have the amp gain set to 20 as a test. Still no aircraft. The SDR is just completely overloaded. My antenna, with an amp gain of 20 sees 35 aircraft with NO filter. This is a simultaneous test, 2 radios of the same model with the same settings, two cables, made and swept by me, etc. if I add a filter to the FA antenna, I’ll get about 5 less aircraft than my antenna. Granted I could increase the gain on the FA’s SDR to compensate for the filter insertion loss but that wouldn’t be fair. This is one of the advantages of constructing and tuning your own antennas. You can fit it to your own specific environment. Now, would I actually run without filters? Probably not but I’ll have to do more tests. My “production” system is capturing about 215 aircraft but that’s with filters, antennas on the roof and a gain of about 40.

Again, this isn’t a knock on the FA antenna. You simply can’t mass produce antennas that work “the best” in every environment.

Oh, here’s what my plot looks like. The range rings are at 50mn intervals. The pesky Rocky Mountains get into the way in the west. :slight_smile:

1 Like

I don’t think that the ground plane has to be tuned that precisely like the active element you are describing.
Based on those high power antennas that I see in the picture, I would guess you need filters there. I have a LNA filter after my antenna.

The gain of the RTL-SDR Blog ADS-B LNA is 27 dB’s at 1090 MHz, and out of band signals are reduced by at least 60 - 80 dB’s. Attenuation in the broadcast FM band and below 800 MHz is actually closer to over 100 dB’s. In the LNA signal path there is first a low insertion loss high pass filter that reduces the strength of any broadcast FM, TV, pager or other similar signals that are usually extremely strong.

 

Exactly my situation, except that you have cell phone/communication antennas at hill top, and I have these on roof tops of my building and adjuscent buildings.

 

EXPAND TO SEE THE GRAPHS

1 Like

You’re right, it doesn’t have to be but 5 mm of movement up or down on the tips of the radials can alter the impedance by a few ohms or more and that in turn can have a big effect on the “notch” in S11/VSWR. The deeper the notch the more you’ll reject outside your chosen target. It might be a moot point since I’ll probably use filters anyway but saving the insertion loss could mean reducing the gain on the LNA which in turn will result in less heat being generated.

I’m going to do a test without filters and see what levels I get from the TV/FM stations up on Lookout Mountain.

I’m kinda “lucky” in the cell phone respect. I get crap reception at my house from all the carriers. :slight_smile:

You can do a scan and see the levels:

The LNA that I linked has FM and TV traps besides the usual SAW filters.

Results of tests I conducted in November 2016:

Please note the drop in performance of ProStick Plus (which you call “Blue Flightaware Dongle”) when External Filter was removed.

FA-Antenna+Filter

Yeah I’ve done that but it’s too much trouble. Easier to just use gqrx at one frequency and eyeball it.

I’ll take a look at that LNA, thanks!

1 Like

Yeah no filter = no good. The price we pay for living in a modern world.

But it is a try and error. I am using the blue Pro Plus Stick with a Jetvision Antenna.
Adding the dark blue filter from Flightaware does not improve my site. Seem to be that i am far enough away from electronic obstacles

1 Like

Absolutely! There are a ton of variables involved and only actual/individual testing can tell the real story.

3 Likes

Ok, it seems that I might get a chance to get back to home this weekend.

I changed the gain to 42.1 as advised and the reporting has actually dropped. Unfortunately, I have not been able to see the graphs that I installed as I am not on the same network, but this weekend I will.

If the error’s are greater than the 5% that was advised, do I go up with the gain or down with the gain?

Is the default gain set to 30 or?

Will I be tuning the gain to 42.1, 42.2, 42.3 etc.? or lager steps like 43, 44, or downwards even?

Thanks!!

there is no “hard and fast” rule as to where the gain should be set. the greater than 5% threshold is simply a way to measure if you are getting bad decodes and overloading your SDR. if you have an airport very close by you can watch for low flying aircraft that are close to your antenna. if you see “interrupted” tracks than you can consider lowering the gain. the choice is if you want maximum aircraft (and are willing to not get good data occasionally on very close aircraft) or if you want to insure that you get good “fidelity” on the close aircraft.
the default gain is -10 (this is “agc” but it doesn’t work for ADS-B signals so it’s the equivalent of a gain of 55 or so.)
if you wish to reduce the amount of signals greater than 5% lower the gain (a smaller number). here are the “allowable” gains. always enter a positive number (except for -10, which very few people on the forum will recommend. you can enter any number the the software will choose the closest “allowable” gain to the value you entered…but easier just to enter one of the values listed below.
Allowable gains for PiAware

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

Note special value of -10.0 is piaware AGC setting…approx. gain is 53 to 55

OK, I’ve got some results of my testing.

TL;DR: My antenna’s better but let me tell you why :slight_smile:

Let me set the stage first…

  • I live in a very strong RF field generated by multiple FM and TV transmitters.
  • I live about 27nm due west of Denver International Airport (KDEN) but because it’s dead flat for hundreds of miles to the east of me, and because I’m about 1000ft higher, I can see the main terminal from my roof.
  • Directly to the west of me are the Rocky Mountains. If you’re not familiar with the Rockies, they just “stick up” with no preamble. They’re a 5 minute walk from me.
  • Both my antenna and the FA antenna are mounted on my north-facing deck, which if not for one tree, would also be able to see KDEN.
  • Both antennas are wired as follows:
    • 10m RG-174 N-male (antenna end) SMA-male (entrance panel end) They’re the length they’d be if mounted on my roof.
    • SMA-female bulkhead connector through the entrance panel.
    • 1m RG-316 SMA male-male.
    • RadarBox “Green” 1090 FlightStick with built-in filter.
    • No other filters (but that changed later)
  • Two instances of dump1090-fa running, configured the same, except for the serial number
    /usr/bin/dump1090-fa --device RBX10900 --gain 40 --max-range 360 --fix --modeac <networkstuff>

This is the 8-hour plot for the FightAware 1090MHz ADS-B Antenna: (all distances in nautical miles)


Over the past few days, I’d been playing with filter/no-filter and at about 0900 today, I took the filter OUT to get a baseline reading. With no filter, reception was DISMAL and if you look at the Signal Level chart, you’ll see why. The receiver is just overloaded. Even though the FlightSticks are supposed to have filters, no amount of gain adjustment could improve the stats. At about 1140, I added a RadarBox Blue ADS-B filter back in and you can see the difference double filtering made. I tried asking RadarBox support where their filter is in the receiver chain but haven’t heard back. I’d suspect it’s NOT at the front :). No amount of gain tweaking improved the stats better than what’s shown.
Oh, the spikes in tracks are from tis-b rebroadcasts from KDEN.


This is the 8-hour plot for my "semi-spider" (more later):


The scales are the same for the FA antenna charts. There is NO additional filter in this chain. Just the one in the FlightStick. Adding the additional filter didn’t change the results enough for me to notice. As with the FA antenna, I started with the gain at 40 and at about 1300 increased it to 43. The signal levels went up but I don’t think there’s a difference in usable messages. I’d have to leave it there for a while to tell.


Conclusions: My antenna performed “better” IN MY ENVIRONMENT than the FA antenna and demonstrates that while a commercial antenna will beat a custom one 95% of the time, there are just some situations where you have to go custom. In this case, I think the combination of the flatter far field and the sharp bandpass of my antenna offset the high field strength of those FM and TV transmitters that look down on me from Lookout Mountain.


For reference, here's the 8 hour plot from my "production" rooftop setup with an earlier version of my the spider: These plots are NOT in the same scale as those above.


And finally, here's the semi-spider:

image
I took the suggestion @geckoVN made earlier in this thread and used double length wire for the radials and just wrapped them around the corner screws. Less drilling, fewer screws to fiddle with, performs the same. Just FYI, a simple 4 legged spider does NOT do as well. Still needs the 8 legs.

3 Likes

That’s really surprising. Many commercial antennas only have three legs. Shows there is not substitute for actually trying it.

Could I suggest a more appropriate Coax?
At 1090MHz, RG174 reduces your signal 1~1.5dB per meter RG316 is about the same).
LL195 or LMR200 would be vastly better.

Yeah I think it really depends on the wavelength and environment. The 3 and 4 radial antennas seem to do better at longer wavelengths where the tips of the radials can be about λ/4 apart and still be long enough to provide a good groundplane. I think keeping that distance at 27cm with 3 or 4 radials in an ideal environment works fine but my environment is far from ideal :slight_smile:

Actually I confused myself. I used RG142 M17/60 double braid shield with ptfe jacket. Has a loss of about 0.45db/m. I used RG174 for a different project with a much shorter run (because I had it on hand).

Oh yeah… The “production” install uses N female-female combination lightning arrestor/bulkhead adapters so I don’t transition to SMA until inside.

8 legs for Spiders is nature’s selection :wink:

20200901_200846

Resizer_15990051556100

3 Likes

What I found out in my case, with mixed close-by planes (airport at 2 mile) and far-away planes, was that the Dynamic Range of the signals were too much for the normal 8 bit receiver sticks.
I could not find a gain that worked perfectly for both.
So I have switched to Airspy, for their better DR.
Airspy Mini or Airspy R2: 35dBm IIP3 RF front end, 12bit ADC @ 20 MSPS (10.4 ENOB, 70dB SNR, 95dB SFDR).
R2 has a 10MHz “panoramic view spectrum” and it can do easy 20MBPS and even 24MBPS (not very well IMO).
Mini can do 20MBPS (not very well IMO) and it’s “panoramic view” is limited to 6MHz. Strictly for ADS-B it’s sufficient.

PS: I have a FlightFeeder connected to the same antenna feed like my Airspy system, so I can compare their output in real time. I have used both Mini and R2.

2 Likes

i had the same situation regarding close traffic and distant traffic so i also switched to Airspy Mini…a BIG difference in my ability to see both close traffic without interrupted tracks and distant traffic.

2 Likes

The Airspys are pretty pricey though. $99US for the mini?