how to align highly directional antennas for LOS

[sarahr202]

Hi everybody.

I was reading about highly directional antenna . Some of them have beam  width of less than 6 degrees.

My question is how do we to align transmitter antenna with receiver's antenna to establish line of sight communication ?

thanks and have a great weekend.

[Scott Fella · Accepted solution]

Most of the time you align be checking the rssi value at both ends. Some bridges have alignment tools but just for close range, but most vendors will have you align using rssi.

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[sarahr202]

Thanks Scott

How close is close enough to use rssi. for alignment?

[Saravanan Lakshmanan · Accepted solution]

you could use laser pointers.

[Scott Fella · Accepted solution]

You must look at your best signal and usually it take one person at each end to align, especially if you can't see the other end.  If the distance is close, you can usually eyeball it and make small adjustments to the antenna.

[sarahr202]

thanks Scott

If we can not see the other person, does it not mean we don't have clear line of sight?

  My understanding is we want to have a clear line of sight for optimal communication on ptp link.

[Scott Fella]

No... Clear line of site means no trees, buildings in the way. Sometimes the instance makes a difference in being able to see the other end or not. I don't know how you would use a laser when you can't see the other end. Some outdoor bridges use gps to align and the is a iPhone app to help align bridges. But that goes off of the gps on the phone. Might get you close then you need to use rssi readings. When looking at the horizon, that is 3 miles away, so if your further than three miles, then you will not see the other end. You still can link up bridges if the bridges are made to reach that far. You need to calculate the fresnel zone as that tells you how high up the antennas should be to clear the obstructions between the two sites.

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[Leo Laohoo]

Clear line of site means no trees,

LOL.

I laugh at this line.  We had a case where we were doing point-to-point.  The site said, "there are no trees between A and B".  9 months later the link went down.  Days of troubleshooting and nothing.  We went there and the next thing we saw was a friggin branch waving due to strong winds.

The knucklehead who initially told us that there were no trees responded, "But it was only yaay high 8 months ago."

No kidding, Sherlock!

[chan · Accepted solution]

Beamwidth is determined by only 3dB off of the center, so it is not like losing signal outside the 6 degree range.

Reading RSSI is the way, but the reading is very slow, so you have to give few seconds to allow the AP to recalculate the RSSI each time you adjust the antenna.

[sarahr202]

Thanks Chan

Beamwidth is determined by only 3dB off of the center, so it is not like losing signal outside the 6 degree range.

But it does mean losing power at antenna site . Any power not received by antenna at receiver site is the lost power.,  depending upon the beam width of signal and receiver's antenna misalignment, the lost power could be little or very huge.

Have a nice weekend.

[Leo Laohoo · Accepted solution]

I agree with Saravanan.  The best way to align a point-to-point dish antenna is by laser.  Alignment using RSSI values will also work but it's a slow process.

[Scott Fella]

Not only have I seen trees grow... I've seen buildings built in the path of the bridges:)

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