BH Links not working as predicted.

We have installed 3 PTP400 - 5.7 Ghz BH links (OFDM 30/60) with the Andrew 2ft Dual-Polarized Parabolic Dish – PX2F-52. None of them are working as well as expected.

I have been in contact with 2 installers that Motorola recommended and have received two different opinions on the subject. One said that we had an alignment issue.
The other said we had an equipment problem and that it might be the cable from the bh to the dish or the feedhorn. He also said that we should have used 4ft drums instead of dishes.

We aligned the links twice and saw no improvement.

East to Darmstadt
Local ant height – 190ft
GPS - 37.95498N, 087.56768W
Remote ant height- 120ft
GPS - 38.09863N, 087.60208W

East to Grimm
Local ant height – 190 ft
GPS – 37.95498N, 087.56768W
Remote ant height – 120ft
GPS – 37.96770N, 087.43272W

East to Mt Vernon
Local ant height – 190 ft
GPS – 37.95498N, 087.56768W
Remote ant height – 120ft
GPS - 37.99378N, 087.66420W

Does anyone have any ideas?


As I’m sure someone else is about to point out, you’ve provided zero info on the type of problems your experiencing, or any kind of data from the link itself.

Does the link drop, or just not pass as much data as you think it should?

Within the radio itself, are you seeing an inability to function at the optimum settings, or are there errors on the ethernet side of the link that might point more towards a cabling issue?

A little more background would do well to get the ball rolling . . .

The links are stable but are showing -90db on the Darmstadt and Grimm links and -62 on the Mt Vernon link. The predicted Rx Power for all 3 links is -60db.

The Grimm link is getting 2 Mbs and the recieve modulation mode is QPSK 2/3.
The Darmstadt link is getting .73 Mbs and the recieve modulation mode is BPSK 1/2.
The Mt Vernon link is getting 7.42 Mbpsand the recieve modulation mode is 64QAM 3/4.


If I make the assumption that all three backhaul links are pointing roughly in the same direction (east off of your tower), two things come to mind;

1) Self-interference: Have you isolated the frequencies so that the backhauls themselves are not stepping on each other, frequency-wise? Forcing the backhauls not to frequency hop would impact the overall throughput, but it’s the only way I know of to keep the links stable. Otherwise each antenna would constantly be jockeying with the others to find the cleanest part of the spectrum.

2) Alignment: If you’ve already got the backhauls on seperate frequencies, and spectrum analysis at both ends of the link shows no other 5.7 messing with you, the backhauls may just need to be dialed in better. It’s quite possible to link up a pair of these without them being well-aimed, as they are designed to overcome interference. They’ll just keep turning themselves down to a slower throughput to try and stabilize the connection.

The best analogy I’ve heard was from the guy who trained me: He said to imagine the radio wave pattern like a flower bloom, with the exterior petals like lobes of radio waves that you could inadvertantly align the link to. Those outer lobes would basically give you a ‘false positive,’ as they would sustain the link, but keep you from pursuing the sweet spot at the center of the bloom, so to speak, where the signal would be optimal. It’s really tedious to do it right: Alternately sweeping horizontally and then vertically to zero in on the distant antenna.