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Parrothead

Metfone has internet access running out Highway 57 (Southwest toward the farm). But, the access is limited to just off the highway. So, I have been working on a solution, to get the internet extended to the farm. I finally received, compliments of a friend, two TP Link CPE-510's. One will function as an Access Point for the other unit, while it will serve as a Client. They are good for about 15 kilometers, according to the specifications. I just have to complete the tower builds (one has to be looked at again, due to my findings, below) and have the antennae mounted up top. Both are powered via POE (Power Over Ethernet). 

 

Screenshot 2016-04-20 03.04.jpg

 

Screenshot 2016-04-20 03.jpg

 

There is a 3.35 meters (11 feet) difference in the elevation, with the highway being on the lower end. I have to take this into account when mounting these units. So, I now need an extra story high, at which to mount that unit. 
 
Sort one thing out, and two other issues crop up. 

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Oz Jon

Paul,

 

I've drawn up your radio path (on an exaggerated vertical scale) [attached] ... assuming you have the 5GHz model

 

The critical high point on the path is the 150ft feature at .9 miles from the farm.

 

You need at least 16ft=60% Fresnel clearance above that point (the 21ft=80% clearance I've drawn is a good choice).

That ensures that you won't get signal dropouts due to ground reflections.

 

Antenna heights of 22ft at the highway and 27ft at the farm will do that job nicely.

 

You could come 5ft lower and still have a pretty good link.

 

Note that there is a 3 to 1 ratio effect on mast heights at each end ( a 3ft lowering at the highway is countered by only a 1ft rise at the farm )

 

Enjoy (and play safely with the mast/antenna erection exercise!)

 

[leave the money on the fridge!- LOL!]

 

edit - changed to a better jpg

 

post-1173-0-19435300-1461150178_thumb.jpg

Edited by Paul

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Oz Jon

Paul,

 

I note from the product datasheet that it is a 5GHZ device and it's antenna beamwidth is at least 15 degrees.

So you won't need a very stiff mast.

Sounds like a 35ft bamboo pole at the farm will do the job.

Cheers!

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Oz Jon

Just as a matter of curiosity, Where did you get those detailed heights from?

 

The Google Earth values (by running the mouse along the route) are pretty crude!

 

Is there a trick to it?

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Parrothead

Paul,

 

I note from the product datasheet that it is a 5GHZ device and it's antenna beamwidth is at least 15 degrees.

So you won't need a very stiff mast.

Sounds like a 35ft bamboo pole at the farm will do the job.

Cheers!

 

Yep. It broadcasts 45° horizontal bandwidth and 30° vertical bandwidth. Dual antennae, dual polarization.

 

From the farm, it is about a 113.0° heading. So, Once they are mounted, eyeballing them will get 'em fairly close.                                                                                                                               

Edited by Paul

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Parrothead

Just as a matter of curiosity, Where did you get those detailed heights from?

 

The Google Earth values (by running the mouse along the route) are pretty crude!

 

Is there a trick to it?

 

Google Earth and an altimeter APP on my phone, is all I have used so far.

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Parrothead

Antenna heights of 22ft at the highway and 27ft at the farm will do that job nicely.

 

I don't understand the Freznal calculation? Shouldn't they (both CPE's) be closer to the same altitude, relative to the ground?

 

I mean, I realize they broadcast a fairly wide bandwidth. But, the highest broadcast signal I can zero in on from one to another, the better off the signal, as a whole, will be.

Edited by Paul

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Oz Jon

I don't understand the Freznal calculation? Shouldn't they (both CPE's) be closer to the same altitude, relative to the ground?

 

I mean, I realize they broadcast a fairly wide bandwidth beamwidth. But, the highest broadcast signal I can zero in on from one to another, the better off the signal, as a whole, will be.

 

No Mate, It's all about unwanted ground reflections.

 

Even with the antennas pointing exactly at each other, they will still radiate quite a lot of signal onto the intermediate ground.

The ground acts as a reflector and some of that signal also arrives at the receiving antenna, slightly delayed by it's slightly longer path.

 

The red half-drawn ellipse (the 100% Fresnel zone one) is the location of all possible ground reflecting areas which would result in a ground reflected signal arriving at the receiving antenna delayed by 180 degrees (out-of-phase) with the signal coming directly from the transmitter. But the reflection itself causes another 180 degree phase shift, so the 2 signals, tend to add to each other.

 

Usually 100% clearance is too expensive in tower heights and a even more clearance (2nd Fresnel clearance) can result in signal cancellation. A very small clearance also results in cancellation.

 

60% - 80% Fresnel clearance is a good economical solution. 

That range avoids signal cancellation and may even provide a bit of signal enhancement.

 

In your case, the only potential unwanted ground reflection point is .9mls from 1 end.

Everywhere else is well clear of the 100% ellipse.

 

I drew a maximum recommended 80% clearance, just touching the critical spot.

 

You can raise one and lower the other antenna height, but you need to keep the chosen clearance (80% as drawn) just touching the .9ml spot.

 

You can reduce the clearance down to 60% Fresnel if you wish - that's still a commonly acceptable solution - reducing antenna heights at both ends.

 

Believe me mate - I played this game for 40+ years.

 

Anything you're not clear about - just ask

 

edit: same result - improved explanation ... here is someone else's explanation

 

http://radiomobile.pe1mew.nl/?Calculations:Propagation_calculation:Fresnel_zones

Edited by Oz Jon
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Oz Jon

Paul,

 

Your path is very short, so you don't need to make any allowance for atmospheric variations (they have the effect of appearing to vary the earths curvature from time to time)

[similar to the visual distortion effect of summer heat on a bitumen road]

 

Almost flat land (probably wet) provides lots of good radio reflector areas.

 

With 80% Fresnel clearance, there is a risk that some reflecting spots may be at 2nd Fresnel clearance and tend to produce signal cancellation.

 

So, I recommend that you reduce that risk and install lower masts to suit 60% Fresnel clearance - easier and cheaper to do too!

 

ps.

When you've got it all working with good signal strength, if you get occasional signal drop-outs, caused by reflections, let me know.

I've got a couple more tricks up my sleeve that may fix that problem.

Edited by Oz Jon
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