Amateur Radio

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antenna question (lemmy.zip)

submitted 1 month ago by [email protected] to c/[email protected]

13 comments fedilink hide all child comments

cross-posted from: https://lemmy.zip/post/33672853

So I live in a small city of around 50,000 people and we have a router that's 200 feet up on a tower with a 5.8 DBI antenna.

There's a guy 17.5 miles away who wants to get into the mesh and his node is on a 30-foot flagpole and also has a 5.8 DBI antenna.

There is no major elevation change between the two nodes and according to a distance calculator, the line of sight between both antennas should be about 24 miles, which would cover the distance with no problem.

With that said, the nodes are not connecting together. And I'm wondering if that's because of the 5.8 DBI antenna gain on both sides, or if there's something else I might be missing.

Edit: On a side note, I live 3.7 miles from the router, and it has trouble hearing me, but I do not have trouble hearing it with my T1000E. And I'm also wondering if that's because of the antenna gain on the router side.

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[–] [email protected] 1 points 1 month ago (1 children)

Have you considered the Fresnel zone? You know, the ellipse between sender and receiver where most of the signal power is located? If you are 200+30 feet up, at 17.5 miles, my back of the napkin map says you are barely having a line of sight, so much of the Fresnel zone is probably obscured by the earth.

Some random online calculator says that at 433 MHz, you have a Fresnel zone radius of 230 ft, (145 MHz has 395 ft, 866 MHz has 161 ft). So half of the signal path is blocked at 433 MHz, and apparently the rule of thumb is it should be at least 60% free. I am too lazy to do actual math right now, but that might be a reason.

[–] [email protected] 1 points 1 month ago

Well, the frequency we are working at is 900 megahertz, so that might explain a lot of it. Also, I think the coax might be part of the problem because he is using three feet of RG316, which appears like it's very lossy, and so I'm not even sure if power is making it to the antenna or if the cable basically is the antenna