FlightAware Discussions

Antenna Movement

Hi All,

Short story : I am wondering how much movement in the antenna is an issue eg. if it moves 0.5m in the wind, is that a problem? What level of movement is acceptable?

Long story.

N00b here. Started playing with an SDR to listen to airband, had no luck. While getting around to making an antenna for airband, I came across ADS-B and found that interesting, so built a linux VM and have an ADS-B feeder running to various sites.

I’ve just built a spider antenna to try, waiting for some connectors to arrive for me to be able to connect it. I plan to try it in a few locations:

  1. Inside near a window
  2. outside, on top of my retaining wall (about 1.5m higher than the window location)
  3. On my roof, attached to my existing tv antenna.

I live in a valley. I have a few other feeders in a ~30km radius, and they are mostly getting 10,000 - 50,000 positions a day, where I am only getting 1,000. I have a few options:

  1. Give up - after all, it’s airband I was interested in…but if I can make this work, I’ll stick around with ADS-B running, and probably by a rasberry/orange pi

2.Setup the spider inside and call it good - any data is better than none on the network, right?

  1. Mount antenna up higher. In which case, for me to get a decent height to try and shoot out of the valley better…I’d need to go 50m to clear the valley, but that isn’t happening. But I might be able to get 2-3 metre above my current tv antenna, if there is value in it…but it would come with some antenna movement, so wondering how big of an issue that is.

I am happy to investigate the option of mounting higher, and I’ll probably test it if nothing else - I might depending on movement, zip tie a pvc pipe for a day and see the results. If it works out, my dad thinks he has a spare extendable steel post antenna that I might see if I can mount (not ideal for my style of house, but should work something out)

Cheers
Chris

Movement shouldn’t cause any reception problems so long as the antenna stays roughly vertical. Keep in mind that the transmitter is moving in an unpredictable direction at maybe 900km/h…

The main concern would be the physical constraints - is the antenna and mounting point going to stand up to the stress?

Thanks. Yes, I think I can make the physical restraints work…if my wife will let me put it up (new house, doesn’t want poor aesthetics).

I am thinking I’ll do a budget PVC pipe solution for testing, I think I have a spare 6m, 100mm pipe. I’ll stand that up and attach to my bobcat to hold it up for a day, to see if I get much better results.

If I do get much better results, then I’ll try a shorter ~1m extension on my existing tv antenna and compare again, then work out if it’s worth the effort for the the larger solution. I don’t have high hope considering my position in the valley.

1 Like

Have you looked at “HeyWhatsThat” to get an idea of what your maximum theoretical range is? It seems as though in practice, small changes in antenna height don’t really return large changes in range unless they are lifting you above immediate obstacles.

  1. go to heywhatsthat.com
  2. Click on “new Panorama” tab at top center
  3. Search for your address or enter lat/lon of your location, and enter your antenna height above ground
  4. Enter a name for your search
  5. Click the “Submit request” button and wait for your results
  6. In the top/right corner of the map, click “Up in the Air”
  7. Below the map you will see two text boxes - one in yellow with 10000 and one in blue with 30000. These are your altitude rings. You will need to zoom the map out to see them. As you change the values you will see the rings expand and contract based on how the local geography affects your view.

This tool doesn’t (can’t) know about that big 'ol oak tree in your neighbor’s yard, or stuff like that, but it will give you a good idea how close you are to the range that you should be able to get.

4 Likes

thanks, great tool and I’d have been lost without your guide.

I think I am reasonably stuffed from that information
This is from my house to the local airport.

In reality, I am not getting anywhere near that good coverage.

My parents might have a chance…but they don’t have fixed internet access :frowning:

Wow, I’m glad it was helpful. None of it is original information, I learned it all from others here.

I’ll still try the spider I’ve made when the connections arrive and see what happens.

1 Like

well, here’s an example. left side is adsbexchange, and right side is my actual data. alt of ~3700ft at the time.

and another to the west.

and another at 19,000ft

Same here. I ended up with a antenna attached to a PVC pipe, installed high in a tree :slight_smile:
The 100mm pipe is a bit excessive, I have used 1-1/4" Schedule 40 for mine, but only 3 m long. Pulled up from a tree branch with paracord.

yeah, it’s just what I thought I had for testing…but I didn’t, the best I had was 3m of 20mm. So I attached that to a tent pole and got it above my roof line…I didn’t get any better results than just having it in my window, inside.

Looks like you are in some deep hole in the ground… Unless you can get above that ridge, you won’t get much better results.

ok, so I think I have two or more issues. I got lucky today and a plane was doing a survey. Unfortunately I realised late on, and was working, and didn’t have things ready…but…what I did learn.

  1. The cut down whip that came with USB SDR is not much chop
  2. The not cut down whip that came with the USB (I got 2…same thing but different connectors) is better than the cut down one.
  3. The spider I made is about as good as the standard whip (though I did only get to use it for 2 passes, and they there further away)
  4. Possibly interference or faulty SDR, as the plane came directly (literally) over me @13,000ft and I didn’t have a solid track. This was using the cut down whip. So could be the whip, or could be the unit.

Did you place the magnetic base of whip on a metal plate? This step give much more improvement compared to improvement by cutting the whip alone.

Best is to do both i.e. place the magnetic base on metal plate + cut the whip.

First - those antennas don’t pick up signals overhead that well. Very low sensitivity due to mismatch of the wave polarization planes. If you think, the antenna length is “perfect” when you look from it’s side, like from horizon. The higher angle the airplane makes to the horizon, the shorter the projection of that antenna looks for the incoming RF. To extreme, when the plane is straigt above, the antenna looks like a dot from the plane’s perspective. So it will pick up zero signal. The horizontal metallic reflector on the bottom helps reflecting some of those incident waves up towards the antenna whip, increasing sensitivity to those higher angles.
That’s why the longer whip seemed to work better - the projected length of the whip was shortened for higher elevation angles like you were experimenting with, being close to the theoretical lambda/4. However, that longer length it will affect the long range reception.

Secondly - if the trace start to break down when the plane is close, check the dropped samples log, IMO the signal is too strong and overloads the USB stick.
Not enough Dynamic Range on those generic 8 bit receivers, that’s why I switched to Airspy.

Third - when a GA plane is close and low altitude, any serious banking (like those turns) will obscure it’s ADS-B antenna (and also change the polarization plane), because antenna is installed usually on the belly of the airframe. Commercial airlines have two antennas - one on bottom and one on top of the airframe.

2 Likes

That’s a really small cone usually.

That’s indeed something to check.

But if someone has just no reception, usually there is some interference around.
HDMI monitors / cables are a favorite source of interference, high up on the ranking list.
Cable modems close by can also interfere.
Otherwise mobile towers …

1 Like

It’s a progressive weakening, based on cosine law, they have a theoretical horizontal lobe characteristic (or a doughnut if you see it in 3D). Half-power value is at something like 30 degree elevation.

When radials are installed, they improve a lot the reception at higher elevations, by reflecting the RF to the vertical antenna. Even the ground distance affects that, because it reflects the RF too.

https://www.qsl.net/kk4obi/Standard%20vertical.html

1 Like

But that mostly cancels out due to the inverse square law.
As the distance between aircraft and receiver is reduced, the signal gets stronger due to the lower distance and weaker due to the lobe characteristics of the antennae.

If you don’t have a really bad noise issue … the cone should really be no larger than maybe 5 nmi across for aircraft passing above you at 40000 ft.

No matter it seems in this case the reception is just terrible across the board.

2 Likes

Yeah, correct I was just thinking about lobes.
It’s indeed more complicated when you look at the distances and also because transmitters have different powers.
Some intercontinental commercial aircraft push a strong signal from far away, while a C172 has a week signal even from a few miles away.

1 Like