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Mar 05, 2016, 12:07 PM
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Understanding the Fresnel Zone


So I have been flying FPV for going on three years now, and thought I understood what the Fresnel zone was, but as it turns out, I was completely wrong. I thought the FZ was something you "flew into", as in, flying behind a building on 5.8 would be "flying into the Fresnel zone." Alas, this is completely incorrect.

I was doing some web surfing on the FZ and stumbled across this illustration:



..and BANG! I realized I have been wrong all along about what the Fresnel zone is.

The definition from that site is this:
Quote:
The Fresnel zone is a football-shaped space between two transceivers – the longer the distance and the lower the frequency, the further across the Fresnel zone is.
Of course, for our application, the FZ would be a football-shaped space between a transmitter and a receiver.

I found this site useful as well, for visualizing what is going on.

Since I tend to be a very visual person, I made a quick illustration of what I think it might look like if you could actually see the FZ from the point of view of your video receiving antenna. Please forgive the MS Paint cartoon version of my flying site, and I can't claim that it's completely scientifically accurate.



So, in my illustration there are two FPV aircraft flying at the same distance away from the receiver. One has a 1.3ghz VTX and the other has a 5.8ghz VTX. The FZ would probably look a bit more "swirly" if I was using CP antennas but that isn't really important.

Based on the way I'm visualizing this, I'm making the following assumptions:

- The 1.3ghz aircraft can fly farther behind the building, because its FZ is larger, and it will take longer for 60% of the FZ to be obscured, causing the loss of the video signal. Same goes for the trees, although the nature of the 1.3ghz radio waves will allow them to penetrate the leaves more than the 5.8ghz radio waves will.

- If the 5.8ghz aircraft flies behind the building, the signal will be lost more quickly, as the FZ is smaller.

- If we went to someplace like eastern Colorado where there aren't any trees on the perimeter of our flying site, the 5.8ghz aircraft could actually fly lower to the ground on the horizon, since its FZ is smaller.
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Mar 05, 2016, 12:08 PM
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The Fresnel Zone explained (20 min 57 sec)
Mar 05, 2016, 12:08 PM
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Fresnel Zones and FPV - An Example (1 min 4 sec)
Mar 05, 2016, 05:42 PM
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Yeah, after watching Bruce's video I had questions alright.
I have no idea why the "lobe" from a rubber ducky should even be "squished out" in the direction it's pictured, simply because "gain". Why wouldn't it still be circular, but flattened like a pie? How does that make it directional? It would seem to make it about altitude, not direction - unless the rubber ducky is horizontally positioned - in which case quad antenna should be horizontal.

But most advices given say to put both antennas vertical.


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