A couple of questions.

I have a couple of questions:
First, here is my heat map from VRS at 30K

As you can see, I have fairly symmetrical coverage. However, number of planes is not so symmetrical. I seem to get most of my tracking on planes with a head on or tail on orientation. In particularly, I am disappointed at the NUMBER of planes I get to the west (there are not that many to the east). It seems the data signal is stronger with doppler, either positive or negative. is that the case?

Second question: My antenna is on roof top. I am in pretty good shape in regard to structures, but there are tall trees around the house. With the coming of fall, and the leaves dropping, I appear to be picking up more and farther contacts. Is this a result of fewer leaves obstructing signals?

Thanks for any and all insight!

  1. You might check the actual air routes and see where the most traffic is coming and going. That could give some insight. Doppler would be relatively insignificant to the receiver, well within the frequency tolerance for 1090MHz decoding.

  2. Yes, most likely. Leaves can attenuate the signal.

I am sitting right under the NE to FLORIDA route, but there are others, parallel to the west. There is one over the Richmond - Charlotte corridor, more or less, that I just get intermittent hits on.

The speed of light is extremely fast ( 300,000,000 m/s ). It is also extremely fast relative to the speed of a plane ( 270 m/s ). The effect you are seeing is not due to the speed of the plane. Sensitive equipment can measure this difference but is it extremely small 270/300,000,00 = 0.00009% difference when moving toward or away.

The effect you are seeing might be due to the polarization. Not all antenna on perfectly vertically polarized or circular. You can see this with the old rabbit ears antenna for TV. People would adjust the angle they made and they would get a better reception. The trick is to have the same polarization of the transmitting antenna. This can’t really be done for ADSB since the angle and direction of the transmitting antenna is changing (plane are flying around). In most cases having the antenna perfectly vertical I have seen about at most a 10% difference and in most cases 5% in mismatched polarization. This difference is usually impossible to see on my sites unless you have some really bad antenna angle.

We did find someone reporting a range problem and it was found that a branch hit the antenna and knocked it off from vertical. An Antenna at 45% from vertical you will see about 1/2 the range in some directions. An antenna close to horizontal will get just 5-10% of the range.

The other effect that can cause lopsided map is that you are not able to see the horizon in some directions. People think that planes are flying above us but in actuality they are flying very close to the horizon. If you plot the angle you have to look up to see a plane based on it’s distance you will see that it is practically straight forward. The easy way to think of this with triangles and try to visualize where you have to look to see a plane.
A plane that is 1 mile away and flying at 4 miles up you will need to look up to see.
A plane at 4 miles away and flying at 4 miles up you will need to look up about 45 degrees.
A plane at 10 miles away and flying at 4 miles up you will need to look up just a few degrees from the horizon (straight forward).
A plane at farther away and flying at 4 miles will just be a degree or less above the horizon. If you could see a plane that far with your eyes!

So practically all the planes on your live map are flying at the horizon of the antenna. If you see some obstruction at the horizon it will be blocking the signal. This can be a small apartment or office building in the direction to the east or west from your description. Or may be a large tree.

Yes. Definitely yes.
Getting the antenna above the tree line is the biggest boost to range. After you raised it above the tree line you don’t gain much. See above about the horizon if you want to know why.




Thank you for taking the time for such a comprehensive reply. I appreciate it.