The only one I can think of that makes sense to me, after cogitating on it for a few minutes, is that signals hitting the board even at very oblique angles hit enough of the wider metallic surfaces which are the active elements of the antenna to still be received even if at lower strength, but that those that happen to hit the board precisely on the edge may not be. Since airplanes in flight are always in motion this effect wouldn’t last for more than a second at most so isn’t noticeable. If I’m correct then the thicker the board is the more messages would be lost, but probably still not enough to matter unless the PCB was extremely thick, like several inches. 
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If you think about local field strength and how an electromagnetic wave interacts with a dipole, it doesn’t matter if the dipole is a flat plate and which angle that plate is at.
Well … within reason.
If you’d get to more than quarter wavelength you would probably start getting issues as when a zero transition arrives you get counteracting induced currents.
But this width is chose for how the wave resonates once coupled into the antenna.
As it’s signifantly shorter than a quarter wavelength, you won’t get much effect from any self cancellation at the zero transition and the power coupled into the antenna is the same regardless of the angle.
As to why it’s not more effective when orthogonal to the wave: It doesn’t really interact with more of the wave front because it’s wider, at least not significantly.
This is because there is such a thing as an effective cross section (i think that’s what it’s called anyhow).
The electromagnetic waves have a certain “width” to them, so the antenna will interact with a significant horizontal width of the wavefront which again is much larger than the width of the antenna.
So any additional width of wavefront being coupled into the antenna is negligible.
This is all very unscientific and based on my very poor understanding of electromagnetic waves.
But that’s how it works in my head.
Maybe @prog wants to try a better explanation.
This video seems relevant: https://www.youtube.com/watch?v=H3VVY9vBGoI
Seems … effective aperture is the better keyword than effective cross-section.
Yeah not sure if that’s what really applies … as i said my understanding is poor but my guess about the topic would be:
Dimensions a lot shorter than the wavelength: The electromagnetic wave doesn’t care
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Come to think of it, about 30-plus years ago I lived in a fairly small fourth-floor studio apartment in Seattle and was an avid shortwave listener. No outdoor antennas was the rule so I took a length of aluminum foil doubled-over to make it thicker and sturdier, thumb-tacked it to the wall above my main room windows that faced east, and fed it to my receiver via a length of wire with an alligator clip on one end clipped to the foil, and it didn’t appear to be particularly directional at all as I was able to hear South America. Australia and Tahiti, also San Francisco Center on SSB, so what you say makes sense. The length of foil was a lot shorter than a wavelength at HF frequencies and faced east-west.
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Anyone interestedin this US$ 4.40 PCB antenna?
Price: US$ 2.37
Shipping: US $2.03
To United States via Cainiao Super Economy Global
Estimated Delivery on Dec 03
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The best thing I liked about above PCB antenna is honesty of the seller. He declared a realistic gain figure “Gain 2.0 dBi”, while most sellers will declair blown up figures like 5 dBi, 9 dBi, even 12 dBi.
₱220.00 in the Philippines, plus ₱84.15 shipping, which is around $5.98 total. Everything to do with electronics, radios, computers, cameras etc. is always more expensive here. Still, that’s not a lot of money. You’d want to fabricate a watertight case for it if you wanted to mount it outdoors. I’d also be wary of the SMA connector snapping off from the weight of the coax. Run a pigtail of lightweight cable from the antenna to the main run of coax and/or put a blob of epoxy cement on top of where the connector’s soldered to the board to help strengthen it.
It is mainly intended for indoor use. Fix it to window glass by adhessive tape, that is how I am using it, although I purchased this one in a white platic radome (which can be used outdoors) couple of years ago.
With it I also purchased RG174 pigtail, 2 meters long, with SMA-male on both ends of pigtail. One end of pigtail screwed to this pcb antenna, other end screwed to the dongle (ProStick).
If a person had a low-loss high-quality splitter and used lower-loss coax he/she could set up two or three of those antennas in windows on different sides of the house for increased coverage.
I tried that one day by taping my cantenna to the top of an east-facing window that wasn’t blocked by a neighboring house, but the splitter I have is just a cheap one. As I suspected might happen my message count was around half of what it was due to the loss introduced by the splitter, so that experimental setup was undone after testing it for a few hours. Still, it was a fun thing to try on a lazy Sunday.
@jaymot
Why not use this splitter cable (tee), and increase gain setting by 3 dB?
EDIT:
If you are using RG6 cable + F-female to SMA-male adapter/pigtail to connect Cantenna to the ProStick:
Because I don’t have one of those cables and I already have the SP-02 5-2400MHz splitter. I’m more interested in getting my own stripline ADS-B (in a weatherproof enclosure) antenna higher as a more permanent solution to my range issues than investing money in a splitter coax that would only be temporarily used. The auto gain in dumpt090-fa keeps my gain at 49.6 already. Can’t crank it up any higher without enabling AGC.
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