Total n00b question. I’m using the FlightAware antenna/filter setup. I have a flat roof on my building, but my easiest mounting options result in the antenna being oriented horizontally (either along the roof itself, or extending off of the roof on one side. Does this matter in terms of the signal that I’m able to receive? The actual visible horizon doesn’t change much, but I guess I’m mostly wondering whether it matters for signals that are coming from relatively low to the horizon line.
The adsb transmissions are Vertically Polarized. The reception of an antenna is therefore best when it is vertical. As the antenna tilts from vertical, its reception of vertically polarized wave reduces. When the angle of tilt becomes 90 degrees, i.e. the antenna is horizontal, reception of adsb is NEARLY ZERO. However, in this position, it can receive “on the air” TV transmissions which are Horizontally Polarized.
All radio systems have an alignment. The 4 types are Vertical , Horizontal, Right Hand Circular , and Left Hand Circular. Some radio systems like cellphones will have antenna in both the vertical and horizontal positions.
For ADSB (vertical polarization), having the antenna ±10 degree from vertical will usually not change your range. Most airplane antenna are not perfectly vertical and they can bank and turn which will changes the angle even more.
We had support calls where sites were seeing less positions and it was traced back to an antenna that fell over or something that dropped on the antenna. I haven’t done measurements on how fast the fall off vs angle of the antenna but it is definitely noticeable once it tips over enough.
Circular polarization is mostly used when you have no idea which way the signal is going to come in from. They are used with acrobatic quadcopter and satellites which are both changing orientation relative to the receiver.
It matters so much that when my coco gets blown off straight by wind I get a significant drop in signal, it doesn’t seem to take much either. to mess with it.
Back in the 70’s I worked on a radar that had vertical and circular receiver antennas.
When it was raining or snowing, the radar performed better with circular. The theory is, that the echo changes from vertical to horizontal every time it hits an object between the target and the radar.
This works with ground scatter as well. If you point your beam antenna at skyscrapers you can get a lot of vertical, and horizontal, and so a circular would add them together.
Image 1
A “vertically polarized” electromagnetic wave of wavelength λ has its electric field vector E (red) oscillating in the vertical direction. The magnetic field B (or H) is always at right angles to it (blue), and both are perpendicular to the direction of propagation (z).
Image 2
A circularly polarized wave as a sum of two linearly polarized components 90° out of phase.
In this animation, one linear component is Vertically Polarized (red), and the other is Horizontally Polarized (blue)
The signal loss between opposite polarised linear antennas (vertical to horizontal or vice-versa) is in the order of -23db (quite a lot!)
The signal loss between linearly polarised (either vertical or horizontal) to circularly polarised antennas is in the order of -3db (not a huge amount).
A while ago I made a quick tests with a homemade Skew-Planar (bottom right most antenna in abcd567’s pictures above) cut for 1090MHz and didn’t see a big drop off in received signals.
My theory is that some adsb signals are being ‘skewed/slanted’ in terms of polarisation.
That is, they hit some object on their way from the aircraft to my antenna and the polarisation is no longer true vertical or horizontal, but somewhere in-between allowing the circularly polarised antenna to better receive these signals at less loss than a true vertical.
I didn’t make extensive tests at the time, but will try to do so again when I get some time to spare.
My tests were in a built up area, so may be less relative on a hill top site with no nearby obstructions (causing reflections).
The skew planar is a true omni-directional CP antenna - meaning it will receive equally as well in all directions, unlike most of the other antennas in abcd567’s pics above.
The helical beam is obviously directional (top left pic), the Lindenblad is omni-directional and circularly polarised (top right pic) and the Quadrafilar Helix is circularly polarised but more end fire (more like a beam) - (bottom left pic).
A circularly polarised antenna may work equally as well as a vertical in some locations - that is in densely populated areas with lots of tall objects/buildings in the vicinity that potentially may be distorting the signals polarization at the receiving antenna.
I don’t think it wouldn’t be a total waste of time to look a bit more at CP antennas?
The Skew-Planar is relatively simple to make and presents a pretty good match to 50 ohm cable.
In GPS, CP rejects first-order reflections, as those reflected signals hit the antenna with the wrong polarization.
In some sat work, such as 137MHz weather, and 2m/440 ham birds, CP antennas do not have a sharp null directly vertical, which normal vertical antennas (such as ground planes and co-linears) have, which means that the signal drops off when the bird is directly overhead. The resulting signal drop can be a big deal in satellite work; not so much for ADS-B where you have an aircraft pushing 20 - 100 Watts overhead.
On the down side, the phasing line required in a lot of CP designs is cut for a specific frequency, and needs to be cut accurately for best performance.
The animation of Circular Polarized wave in my last post shows one thing very clearly: The Circular Polarized wave can be broken into two linear waves which are polarized at right angle to each other (blue wave is polarized along X-axis, red wave is polarized along Y-axis).
However there is a second condition for Circular Polarization i.e. the two linearly polarized components should have a phase difference of 90 degrees. This condition is not obvious in the animation, as it is difficult to notice phase shift due to speed of animation. I have now slowed down animation to 1/10th speed, and have attached the slow-speed copy below. If you watch the animation carefully, you will notice that when blue wave is at its +ve or -ve peak, the red wave is at its zero, and vice-versa. This means a phase shift of 90 degrees.
**Now what happens if the phase difference between the two component linear waves is not exactly 90 degrees? In this case, the resultant wave becomes Elliptically Polarized. **
Please see the diagram of an eliptically polarized wave and its two linear components below. We still have two linear waves (red and green) polarized at right angle to each other, but if you watch carefully, at top of diagram the red wave is at its zero, while green wave is between its zero and peak. This means that the phase shift between red and green waves is somewhere between zero and 90 degrees. The resultant wave (blue/purple) is Elliptically Polarized.
