“New” wireless interference from old tv station?

[dflower]

Anyone want to take a stab at this one?

Rack of six Sennheiser ew100 g4 A band with body packs. Never had a problem until December. Now, the whole rack is wonky. It’s not constant interference, but more like an FM trap at the end of people speaking, and not on just one mic, it jumps from mic to mic and even when the frequency changes. I didn’t believe it until I saw it. It doesn’t follow any pattern, it’s not better or worse depending on distance, and like I said, it doesn’t follow any pattern or is predictable.

Did some research and there’s an old analog tv station still broadcasting evangelist programs on channel 29 uhf, so that’s in my spectrum, but when we programmed the receivers away from that, we’re still getting it. Someone suggested that it’s a new cell tower that popped up, someone else suggested an exorcism. I’m just afraid the exorcist would come after me, not the mics.

Any thoughts?

It is a delicate balance between finding the best frequencies, having the right antennas, adjusting the squelch. Without a spectrum analyzer, I would suggest you do it the hard way - once you get the antennas and squelch settings the best you can, every time you have an issue with a mic, write down what frequency it was set to (your bad frequency list) and use the ew100 receiver to find a new frequency. When you search for the new frequency, turn off that transmitter and make sure all the other transmitters are turned on. With intermittent interference, this may take some time, but I am thinking you will eventually find usable frequencies. I have a location where it looks to me like the very old and heavily rusted transformers on the telephone pole outside the auditorium are leaking a very wide band of rf noise, so I end up finding new frequencies for the 30 wireless mics about twice a year as the transformer ages and changes. And yes I have called the power company.....

 

[FMEng]

The FCC is asleep to unintended radiators. The market is flooded with cheap electric devices that make noise all over the radio spectrum. Almost any kind of appliance can do it, even consumer LED bulbs. It'll keep getting worse until it severely affects cell phones, and then Congress will act and an effort the size of taking the census will happen.

 

[eadler]

@ACTSTech I didn't see where you said... what group/channel or, ideally, frequency are these tuned to?
Note that based on just the TV channels you listed earlier I expect 25, 28, and 30 to be potentially unusable due to intermod. As others have said, we'd want to look at the full spectrum situation to be sure but if you can provide what frequencies you're on now we might be able to hazard a guess as to what specific may be happening.

Edit: For reference, A is 516 - 558 MHz (42 MHz)

 

[TimmyP1955]

"75 Ohm/50 Ohm Mismatches

First of all, a 50/75 ohm mismatch has a resultant VSWR of 1.5:1. That translates into 96% forward transmission with only 4% reflected power. Less than 1dB.

Even using 50 ohm coax exclusively presents mismatches if its length isn't exactly tuned to the frequency - and only one frequency - it's carrying. Since it's always carrying multiple frequencies, it's always a mismatch, though again by an insignificant amount.

Secondly, the antenna you're deploying is varying by as much as 65 ohms (about 37.5 to 100ohms depend on frequency(ies) and near field obstructions); you'd be lucky to see a system VSWR better than 2.0:1 - about 89% forward power.

Lastly, the front end of mic receivers, and I don't care what make/model, including the venerable ole' Sennheiser 1046, is not exactly 50 ohms; it's somewhere between 50 and 75.

Reality is, your fully "50 ohm" system is perpetually mismatched."

_____

Henry Cohen

Production Radio Rentals

 

[BCAP]

Ben mentioned this unit already - I use an RF Explorer 3G, similar to the one sold on RF Venue's site. You can set it in a location with a computer using either RF Venue's Vantage software and have it run scans you can then input scans into Workbench. You can also get for free Windows software that works with RF Explorer which is good for tracking and logging activity over time. There is a chintzy little plastic stand they sell as accessory - it breaks easily but it's handy to have if you're going to set the unit somewhere for a long while. IMHO a spectrum analyzer - or something like the RF Explorer could be the best best way to identify a possible outside conflict in your area.

Do you have the ability to easily take the rack of G4s away from the location and run the test in a different area? That might also be a way to confirm your issue is location specific and not some other factor.

Good luck