FlightAware Discussions

Unexpected signal reception direction

Hi. Newbie #420 here! :slight_smile:

I’ve decided to donate my retired Pi 3B+ and a RTL-SDR v3 to this project.

I’ve only been sending data for the last 12 hours or so, but I’m already noticing a perplexing pattern…

My antenna is attached to an (approximately) east-facing, second floor, window…yet, most of my contacts, and my farthest range contacts, are coming from WSW, according to my stats. My location setting is accurate, so that’s not an issue.

Is my home really RF-invisible (or even a ‘lens’)? I very much doubt it! :open_mouth:

Can anyone give some input?

This is what is up there at the moment.

Can you not reposition the antenna?

Yes, I can definitely move the antenna. But that’s not the point of my enquiry.

The antenna is set up with eastward line of sight. How is it possible to have stronger reception in the opposite direction? By your image, you seem to be suggesting that it’s a factor of how many planes are in particular locations, and I get that.

I don’t expect too many contacts to the north for that reason. But there are two major airports to the east, within 50 nm, and yet my stat’s pie chart clearly shows a preference to WSW.

At the time of writing, there’s a plane track at FL40 on my local tracking page. The track starts roughly 20 nm ENE of my location, goes almost directly overhead, then ends nearly 100 nm WSW of my location.

This simply doesn’t make sense to me! I watch the sun rise from this window every morning! And unless the earth reversed rotation without my knowledge, that’s EAST! So, why is my reception umbrella so strong to the WEST?

Thank you.

Terrain and tall buildings can have a significant impact on the ability to receive in particular directions. It could be that the direction you are expecting to receive most from is actually obstructed.

You could go to heywhatsthat.com and generate a plot for your location that will give you an indication of how your local terrain will affect your reception.

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Toggle the “P” switch on normal mode. That display is with “persistence”.

What an interesting tool! I did generate a plot as seen from as close to my antenna’s location as possible.


Panoramic view shows a rise in terrain to the east, and a valley westward. And the ‘Horizon’ map plot shows restrictions north and south (expected), and a clear bias to the west (unexpected). Add to that the fact that I’m on the west edge of town. I guess my question is answered…

I have a west facing window, but it’s full of air conditioner right now. I doubt setting the antenna near a giant noise source would be helpful. Ottawa and Gatineau might be ‘visible’ from that window, but I guess I won’t be picking up Montreal and Mirabel as easily as I thought! XD

Thanks for another lesson in RF propagation!

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RF (radio frequencies) are by nature, a very strange animal. They will bounce around in unexpected ways. I equate it to a pinball machine. The ADSB signal (the plunger) being the signal from the airplane, comes from one direction. But when the steel ball is carried to the receiver, (the bottom hole in the pinball machine) the RF signal bounces around in a very unpredictable manner.

Trees, buildings. inside or outside walls, mirrors, etc., all will cause RF signals to bounce around in strange ways. The airplane broadcasts its position in the packet burst, if your antenna is positioned in an eastern facing window, the “bounce” of the signal will show up as the actual direction that the ADSB signal broadcasts.

Positioning of an antenna has a great effect on transmitting and receiving of a broadcasted signal. Sometimes as little 1 to 6 inches can make a huge difference of the antenna placement on receive or transmit. As all Ham radio operators know, you have to experiment with your equipment to find out what works best for you.

We all wish it was just “plug and play” but it’s not.
Good Luck!

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I appreciate the explanation, but I actually know how RF signal propagation works in general, despite how it may seem. I simply forgot to use my brain for a little while. It can happen when you get results that are exactly opposite to what’s expected.

I don’t have my license…yet…so no callsign for me. :slight_smile:


If your link takes me to a map with your position marked accurately (west of Motreal), then you want to click on the “up in the air” button (make sure that your altitude is set correctly). Down at the bottom, you will see two boxes, that default to 10k feet and 30k feet (change these to see different ranges, if you like).

Now zoom out - you will see a small gold and a larger blue ring that show your theoretical range at each altitude level. From my view, you should be able to pick up planes at 10k feet to just east of Montreal, and at 30k feet to past Quebec City.

This map allows for terrain, but not for trees, buildings, & stuff like that.

Try putting your own actual values in and share what you get!

Edit: Hit “new panorama” at the top, make sure that “Elevation” is set to the height of your antenna above ground level. Give it a name and hit “Submit Request”. Wait for the commercial, and then when the map comes back, hit “Up in the Air”. Zoom out to see the rings.

There is a way to make these rings display on your local “SkyView” page, but I can’t remember how to do it just now.

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Here is mine, for example. The concentric rings are my range circles, set at 100/150/200 miles, and the jaggedy circle is from HeyWhatsThat.com

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Man…that site gets cooler every time I hear about it! :laughing:

The panorama I linked had elevation set to 20 feet, which is approximately where the top of my 2nd floor window is. I set the rings to 5k and 10k, and it somewhat accurately represented what I was seeing in my results. Most planes coming to or from Mirabel fade out at roughly 5k feet, Montreal at roughly 10k feet.

There’s definitely some real world difference, as expected. I would imagine that these rings are a “theoretical maximum” at best.

Oddly enough, even with the difference factored in, the ring suggests I should be able to pick up planes at 5k feet at Ottawa Int’l. As far as I’m aware, that isn’t the case. But, I’ve been concentrating on eastward signals, and could very easily have missed seeing a plane go in at Ottawa.

I absolutely MUST know how to include these data into SkyAware! :open_mouth:
Surprisingly easy, it turns out! :sweat_smile:

Thanks for the tips! That helps immensely!

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If you really want to jump down the rat hole, you can check out VRS (Virtual Radar Server). It can consume data from your Flightaware server and present it on its own map - but that map also has the ability to create a persistent “splat map” that shows your actual maximum detected range - the most distant plane that your server “heard” in each direction. It’s a great tool to compare to your theoretical range.

Here’s mine, with about a month of data:

The concentric circles are at the same distance as on my SkyAware map, so 50/100/150/200 miles


And down the hole I go! :sweat_smile:

Next stop, VRS-ville!

Enabling the range-plot is definitely not intuitive and the instructions seem incomplete, so if you get hung up post something here and you will get help.

I “think” I have it figured out. But, there’s no planes about at this time, so I’ll have to wait and see

I used the following guide, starting at Step 11:

That looks right. If you have it set up correctly you will have some additional menu items on your map display.

On your map, hit the Menu button, and you should have an option called Receiver Range. If it’s there, click it, and you can chose which altitude range(s) you want to display. If the receiver range is not there, or is grayed out, something isn’t set up correctly.

Also, the receiver range display is a browser setting, and not a server setting. So you will need to enable it on any web browser that you want to view from.

Note to self: Click the ‘Save’ button before closing the WebAdmin page… :face_with_raised_eyebrow:

Now that I’m doubly familiar with the process, the menus are now showing up. I’ll only be using the one laptop to access this, and only locally. So, I don’t expect it to be a problem.

Thanks again for the help!

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Glad it worked out! I’d like to see your results after a few days.

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Just picked up a flight roughly 50 mi SE of me. Ranging display is working!

I’ll take a screenie in a week or so, and update this thread :slight_smile:

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