I am investigating some interference with public service radio equipment.
I am seeing emissions from a Cambium ptmp unit in the 4.9GHz band.
The operator syas he is set to 5.6GHz as primary and 5.4 GhHz as a backup.
I hold a spectrum analyser in front of the antenna and I still see radio on 4.9850GHz
Is this common? Has anyone seen this before?
Here in Bermuda some of 4.9 is licenced by the government for various things including radio systems for public transport etc.
Thanks
Laurence Fox
What happens when the radio in questions is temporarily powered off? Depending on what spectrum analyzer you're using and how it's designed, holding it very close to the antenna might be overloading it and causing issues with it. You may need to add an attenuator if the signals are too strong to give you a cleaner/better view of what the radio is actually transmitting.
That being said PMP450 radios typically have a very tight wave form and it would be highly unusual for someone operating on 5.6GHz to show up down at 4.9GHz. I have three guesses about how this could be happening... 1st, you're over saturating your SA as mentioned above or 2nd, the operator doesn't know (or is lying) that he's operating in 4.9GHz, and 3rd, the radio itself is bad and having spurrious RF issues and needs to be replaced.
I would also like to add that the radios are tested to meet the regulations for whatever country they will be operating in. There are strict out of band emission limits, but the power may not be absolute zero in other bands.
Typically, the regulations around the use of unlicensed frequency states that the radio must not cause harmful interference to other operations, so frequency changes might be necessary to eliminate interference depending on the proximity of the radio systems, even while the radio is meeting the regulatory requirements.
@bermudafox wrote:
The operator syas he is set to 5.6GHz as primary and 5.4 GhHz as a backup.
I hold a spectrum analyser in front of the antenna and I still see radio on 4.9850GHz
I am going to add a 4th scenario to Eric's list, IMD. The fifth order IMD of 5.4 and 5.6Ghz is 5.0Ghz, extremely close to the 4.985Ghz you are seeing on the your spec-an. If he -is- transmitting around 5.6 and someone else is around 5.4, and all these antennas are very close togther, IMD's can appear due to these frequencies mixing in some non-linear fashion (loose nuts, corroded junctions, poor grounds, etc). Your spec-an, unless overloaded as Eric suggested, should still lock on to his fundamental frequency at 5.6, not the IMD. With that being said, I think Eric's #2 on the list is the most plausible answer.
One more thing, can you go into a little detail with your issue?