5700BH20 causing RF-based interference to licensed service

Ok, so I’ve run into an interesting quandry. I’ve got a 5700BH20 on a 1mi link which has traditionally had no problems operating.

Recently I have been made aware that this radio is causing an extremely high noise level and significant receive problems for a licensed service on 144MHz. As a radio amateur, I am further able to confirm that this is indeed the case, when I have power to the BH20, it increases the RF noise floor from approximately S1 to S9 levels, sometimes even S9+10.

Regardless of which frequency channel I operate the BH20 upon, the noise levels on the 144MHz system are still significant enough to cause harmful interference, which thus puts this BH20 in violation of Part 15 rules.

This unit has been in place for approximately 3 years, the affected station has been in place for even longer. This problem has only raised its ugly head in the last 4 months.

Is there a history of older units going “out of spec” and generating more noise with age? Or some other problems of which I’m not aware?

Other suggestions? Replacing with 5.2 or 2.4 is not an option as there are already several access points in those frequency allocations, and 900MHz will not provide adequate bandwith, and would cause harmful interference to known licensed services in the area on that band.


We had case similar to this. We use towers from the national broadcasting company. Canopy has made interference to the UHF equipment there. We changed the radios and fixed the grounding. No problems since.

Is the 144 MHz station a HAM base? Is this situation occurring on a tower? Do the Canopy BH’s have reflector dishes on them?

What is the physical separation between the 144 MHz antenna and the Canopy unit? Would it be possible to move the Canopy unit? Have you tried changing the modulation on the Canopy unit to 10 Mbps as opposed to 20 Mbps? I think in theory both modulation schemes utilize ~20 MHz of RF spectrum, but I would be curious to see what would happen if you changed to 10 Mbps.

If there are no dishes on the BH’s I would highly recommend putting them on. You would be reducing your beamwidth and decreasing the chances of interference.

Also - what other unlicensed infrastructure is at this location. Canopy, non-Canopy? Any CMM’s?

Update to previous… replaced the offending unit with another 5700BH20, and now we’re back down to S1 noise levels on the 144MHz transceiver. Seems to me that the offending unit had some type of failure, not sure exactly what.

On the tower is another 5700BH10, as well as a 2.4 AP. CMM is located one hop upstream, at the other end of the 5700BH20. Separation from the main 144MHz antenna is 2 wavelengths at 144MHz.