On Sun, 18 Oct 2020 12:12:01 -0700, Johann Beretta
wrote:

All he has to do is search google for "fresnel zone calculator"

At 1,056 feet (0.2 miles) the Fresnel for 5.1 GHz is 7.1 feet.
(for 5.8 Ghz it would be 6.7 feet). The higher the freq, the smaller the
zone.

The question was for an 800ft link.
800ft / 5280ft/mile = 0.152 miles
Please adjust your computation accordingly.

You can intrude the fresnel by 40% (max), but I try to avoid even that.

That depends on whether the intruding material is absorptive or
reflective. You can get a way with much less clearance if the signal
is absorbed. Yes, the signal level goes down, but it also stays down
and does not vary. However, if it's reflective, then it will refract
(bend) part of the signal, creating the opportunity for fades, nulls,
cancellation, etc. It can also create reinforcement and stronger
signal levels, but those tend to change radically if anything moves.

40% intrusion is a usable number for real links, but only works if you
have a sufficiently large fade margin, also known as SOM (system
operating margin). 20 dB would be a good minimum. I carry 20dB and
30dB attenuators in my toolbox. If the system still works reasonably
well with 20dB loss inserted at one antenna, it will probably be
reliable. If it dies completely, you need a bigger antenna or more
transmit power.

While I'm ranting on the topic, fade margin (or SOM) is related to
reliability (or downtime):

SOM dB Reliability % Downtime per year
8 90 876 hrs
18 99 88 hrs
28 99.9 8.8 hrs
38 99.99 53 minutes
48 99.999 5.3 minutes
58 99.9999 32 seconds

99% reliability might sound great, but that means your link will be
useless for 1% of the year, or 3.6 days per year. Don't go below 20 dB
fade margin, which is 70 hours of downtime per year.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558