raklet

This is my first foray into the wireless world so suggestions, RTFMs, etc are more than welcome. I have been reading the forums and web and know that I need to do my LOS and RFLOS calculations.

Here is what I have gathered about my topography:

Site A: 4376 feet elevation
Site B: 4361 feet elevation
Obstacle: 4400 feet elevation highest point

Distance from Site A to Site B: 2.64 miles
Distance from Site A to Obstacle: 1.05 miles
Distance from Site B to Obstacle: 1.59 miles

I ran this fresnel calculator and was told that my 1st Fresnel Zone is 36 ft. and the 60% level is 30 ft.

I am assuming this means I need 30 ft of free space from the radio waves to the obstacle at the point which the waves pass over the obstacle. So, if Site A is 4376 ft, I would need a 24 ft mast to get LOS and then an additional 30 ft to obtain RFLOS. Are my assumptions about the information from the calculator correct? Do I need a 54 ft mast?

*Note about the obstacle. It is the tail end of a hill with a not too steep slope. The hill is barren with light growth of scrub grass, no trees, and no major rock outcroppings. Does the type of obstacle make a difference in the calculation of RFLOS? Perhaps my mast does not need to be as high as I have calculated?

See attached picture for an overhead view of the site.

Thanks for any help,

Raklet

wrzwaldo

Don't be a dink... Read the rules!


http://www.netstumbler.org/showpost....69&postcount=2

raklet

Attachment removed.

VBulletin allows restricting image size. Why not set it to 50 kb and be done with it?

Dutch

Because we expect people to read the rules, and to THINK for themselves. If anybody feels that is to much to ask of them, they know what to do...

As for the attachment you had posted, You could have rescaled the picturedimensions down, without loosing any detail with regards to what you wanted to demonstrate with the picture.
Remember, there are still people accessing the web via dial-up...

Dutch

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raklet

Ok. My apologies. I didn't repost the picture because I don't have access to it right now. Now that I have been duly chastised (yes, it was deserved ) perhaps we can return to the question at hand?

ShadowFX

Now that you have been bitch slapped, it’s not likely they’ll be back. As I do not know anything about Wireless long hall installations, I’ll give you the benefit of my RF path analysis as I remember it from college and actual field work in automating the lighthouses on the east coast of Canada.

Your interpretation of the results is almost correct. They are not incorrect, but incomplete. That is to say, you will also need a 71 foot mast at site B to clear the obstacle, or else increase the height of the mast at A.

Does the topography of the obstacle affect the system calculations? Yes, it most certainly does have an effect on the link budget. If the obstacle is gently rounded and barren (no vegetation) it can be treated like a stationary obstacle. If however it is tree covered, its apparent height may change with the seasons or even with the weather (humidity). The other consideration is its shape. Is it a knife edge obstruction? If, in relation to the wave length of your signal, the obstruction is considered a knife edge, some advantage can be gained by using this phenomenon to lower the height of the antenna considerably.

The considerable plus or minus performance difference between calculations and reality come from multi path signal reception phenomenon. Keep in mind that every time RF signals bounce of something, their phase flips 180 degrees. Each Fresnel zone is 180 degrees out of phase with the zone on either side. So if the second Fresnel zone were to reflect off an obstacle, it would be received, out of phase with the direct path signal, there by cancelling some of the signal, causing a “minus” condition with respect to the link budget calculation. Here’s where it gets tricky. If you bounce the second Fresnel zone and skim it off the top of a knife edge obstruction, you end up with a “plus” condition with respect to the link budget calculation. This happens more than we would like to imagine, but it can never be counted on as a solution.

As a side note, while I was installing VHF corner reflectors (2m a side) on Burin Island light, we assembled the reflectors on the base of the 10m masts (both island and main land). The masts were needed for LOS+ clearance of “The Rock”, smack in the middle of the path. To make a short story long, the system worked better with both antenna on the ground than it did with both antenna 10m in the air.

Final note, I used a different calculator
http://www.firstmilewireless.com/calc_fresnel.html
and it came up with 22.2-22.3 feet. You may want to do your own math.

This site is cool
http://gbppr.dyndns.org:8080/fresnel.main.cgi

Shawn