“New” wireless interference from old tv station?

ACTSTech

Well-Known Member
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?
 
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?
Calling @FMEng and @TimMc
Toodleoo!
Ron Hebbard
 
What city and zip is this? Knowing this will help us find a quiet channel bank. The TV repack is still going on so, chances are good that a new or stronger TV carrier has landed on ch 29. It's hard to find accurate info because many databases don't show the repack.

You can network all six receivers together, then go to "easy setup" and perform a "scan new list." Then "assign new frequency" on all the receivers. The transmitters will need to be synced.
 
Nearest city is Youngstown, OH, 44555, but the interference, I believe, is coming from an old LP analog broadcast station. I’ve been following the repack and cursing the FCC since I keep losing channels that I used to get, but here’s a rundown of the area. I’m only including channels near the Sennheiser frequencies.

WFMJ (Youngstown OH) Transmit Channel: 20 506 - 512 MHz (Digital)
WWBP-LP (Freedom PA) Transmit Channel: 27 548 - 554 MHz (Low Power Digital)
W29CO (Sharon PA) Transmit Channel: 29 560 - 566 MHz (Analog)
WNEO (Alliance OH) Transmit Channel: 29 560 - 566 MHz (Digital)
WKBN (Youngstown OH) Transmit Channel: 31 572 - 578 MHz (Digital)
WYFX (Youngstown OH) Transmit Channel: 32 578 - 584 MHz (Digital)
WFMJ (Youngstown OH) Transmit Channel: 33 584 - 590 MHz (Digital)

So to me, anything below 548 SHOULD be somewhat clear, but we’re still getting the phantom jumping from pack to pack. I’m looking at my bank chart saying MOST of banks 1 and 2 are in the free. The group is interested in renting an additional 6 mics but the way this is jumping, they’re going to have to be in a different band, or universe, or galaxy... It’s not that I’m not used to a gremlin, it’s that I’m not used to a really smart and random gremlin.

Side grumble, how can both an analog and digital tv station exist at the same time on the same frequency and no one seems to mind?
 
Analog can exist if there is room in the local spectrum. The main thing you might not know is that 1) the FCC database is not current by a factor of months; 2) not all TV stations on on their final frequency, at licensed antenna height, and possibly not at full authorized power.

It's possible that one or more of your area TV stations is on an interim frequency or the transmitter location is closer to you (temporary tower/transmitter, back up transmitter location, etc.

Finally, it's less likely... but we have a venue that's near the airport. The airport has Cessna Citation factory, LearJet, Sperry & Honeywell and Garmin avionics labs including radar. When a Beatles tribute act moved to IEMs for a more authentic visual presentation they started using them at that venue. When the act's A1 asked how things were, "George" said "it's good, but can you turn off the flanger inside my head?" The best we can figure was some kind of high power avionics testing was creating an intermodulation product because subsequent acts have not had the same issue. This is pre-600mHz repack, too, so the FCC database was likely accurate... and it took some extensive trial-and-error frequency changes until they found a combination that worked well. Four frequencies is all they needed...

So you might call up your TV stations and ask for Engineering. Inquire if they're on the final freq assignment, HAAT (height above average terrain), and output power.
 
I was hoping for the zip of the theater, not the nearest city. If this is the Youngstown Playhouse, you have six UHF transmitters within 2.2 miles of that location. It could be that the very recent addition of repack channel 29 has caused an intermod or overload problem. I would be suspicious of active antennas and active splitters. An overloaded RF gain stage can splatter garbage across a whole band. Yes, you can have too much RF.

Turn off the mic transmitters and scan the known empty channels. If you don't find a quiet, 6 MHz channel on the RF meter, then start looking at the antenna system. If a receiver with just a stock, whip antenna gives you a clean channel when it didn't before, you found the problem. Do the whip antenna test with the power turned off on any active antenna components because they can re-radiate.

There is only one repack channel in the area, the above mentioned WNEO channel 29. WNEO could be running on both 45 and 29 until 3/13/20. WFMJ has not been granted channel 20 yet, so it isn't likely on the air.

Do some more investigation and let us know what you find.
 
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It's not a good assumption to just look at Freqs "near" your A band channels because intermodulation products can come from all sorts of X+/-Y summations.

look at www.rfvenue.com
They offer a very affordable neat 24/7 spectrum monitor (under $300 I recall) which will give you the actual truth about your airspace both in real time and historically. I also always start with using a tool like Shure Wireless Workbench (Senny has. one too) which calculates the intermod products at higher orders of all known signals ... then recommends the optimal channels to use.

as noted above. Too much RF gain in your antenna or receiver stages can make this worse, too. Whenever you turn up receiver gain, you are raising the background noise level as well.


Are you using the built-in antennas on each receiver, or a diversity fin antenna set up with proper orientation and separation?
Where is your antenna(s) and recovers relative to the stage? How far? Can you get at least the anetenna closer to the stage?

drawings, pictures, frequencies, details plz!

Ben
 
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My first question is, how were frequencies assigned to each system? Did you use a tool such as Shure Wireless Workbench or IAS to generate a set of compatible frequencies? If not, I would highly recommend doing so. You can easily find an up-to-date list of channels to avoid here: https://www.shure.com/en-US/support/tools/frequency-finder (use these, not the ones returned in WWB).

My next question is, what kind of antenna distribution and antenna placement is used for this installation? Can you post some photos of the installation? Note that some units (such as the RF Venue distribution units) have been known to present issues like this in the presence of high power RF.
 
Excellent tech questions from @mbenonis

Toss in any amplified receive antennas or the placement of directional RX antennae (pointing at the transmitters?).

My gut reaction is that a full scan and time/heat map would be very helpful in tracking down the sources of this IM.
 
Folo question to Ben and FMEng:

I have a pair of paddles on my remaining 6 channels of Analog Shure; is there a minimum distance those have to be apart? They're on a pair of 844 splitters, each has a ducky on the other jack; in my booth, 60ish feet from downstage.
 
Given the short wavelength at those frequencies, even a few feet separation is ok. I usually go for the width of the tech table or booth for convenience. But ... those duckies gotta go. (Ya gotta put down the ducky, if you wanna play the saxophone ...).

why? Mixing 1/4 and 1/2 wave antennas of different gain on the A/B sides of diversity receivers is not optimal, and can lead to more RF and audio noise and artifacts. Instead, daisy chain the 844s so that both antennas feed unit 844-A, which feeds 4 receivers as well as the second 844, which feeds your remaining 2 receivers.

ideally you are using low loss, super-shielded 50ohm cable from paddles to the 844 inputs


plz poke me if Ididn't convey the cascade topology of antennas splitters and receivers clearly enough.

Ben
 
Given the short wavelength at those frequencies, even a few feet separation is ok. I usually go for the width of the tech table or booth for convenience. But ... those duckies gotta go. (Ya gotta put down the ducky, if you wanna play the saxophone ...).

why? Mixing 1/4 and 1/2 wave antennas of different gain on the A/B sides of diversity receivers is not optimal, and can lead to more RF and audio noise and artifacts. Instead, daisy chain the 844s so that both antennas feed unit 844-A, which feeds 4 receivers as well as the second 844, which feeds your remaining 2 receivers.

ideally you are using low loss, super-shielded 50ohm cable from paddles to the 844 inputs


plz poke me if Ididn't convey the cascade topology of antennas splitters and receivers clearly enougH
No, been a ham since 78. Just didn't realize 844 had pass-throughs.

I didn't realize that the rubber ducks and the paddle antennas had different wavelengths though. On reflection, I suppose it should have occurred to me that a log periodic would be a half wave antenna from its construction.
 
So now I've pulled up the original 844 manual. You can cascade to 2 tiers, using any of the 4 outputs to the next 844.

so 3 receivers and cascade off the top level, remaining 3 receivers at the 2nd level.

hats off to you as a ham ... you understand a lot. I thought I was heading in that direction but got seduced by am /fm broadcasting instead.
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So Ben, you were out in the K-FOG? (pun to those not in the Bay Area, and a now-gone radio station).
 
So now I've pulled up the original 844 manual. You can cascade to 2 tiers, using any of the 4 outputs to the next 844.

so 3 receivers and cascade off the top level, remaining 3 receivers at the 2nd level.

hats off to you as a ham ... you understand a lot. I thought I was heading in that direction but got seduced by am /fm broadcasting instead.View attachment 19240
Cool. In that rack I have one feeding 4 and one feeding 2, so I can loop from the latter to the former, in each direction.
 
So Ben, you were out in the K-FOG? (pun to those not in the Bay Area, and a now-gone radio station).
My wife would say I wake up in that every day! My broadcast engineering and programming was all in the Midwest ...WNUR, KRLX which I got to transition from am carrier current to FM, launching KFAI-Fm Fresh Air community radio in Mpls, rebuilding WCCO-FM, and launching WOJB, the voice of the Ojibwa nation in Hayward, Wi.
 
So, @ACTSTech you went radio silent there for a bit. Everything work out? You have some serious brainpower here in this thread. FM Engineers, Shure Wireless Support, AES Members, just to name a few credentials. Let us know, we are happy to help!
 
Yes, the shure post about this says (nutshell) far more important that you use low noise, low capacitance cable then worry about 75 vs 50 ohm. That said, if you have a choice, order 50 ohm.

Yes, agreed. RG-6 is a great value and quite low loss, though I only really recommend it for users who understand the tradeoffs. In most cases, I recommend RG-8 for runs under 100' (RG-8X for under 50'), and LMR-400 or Belden 9913 for urns over 100'. For runs over 300', LDF4-50A is the pick. I generally don't like to compensate for line loss with amplifiers unless I have to, because it's too easy for them to end up in an overload state and generate intermod.
 

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