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Weird Frequency Coordination Question

Discussion in 'Sound, Music, and Intercom' started by manuallyfocused, Mar 10, 2018.

  1. manuallyfocused

    manuallyfocused Active Member

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    Occupation:
    Set Designer/TD/Carpenter/Painter/Photographer
    Location:
    Los Angeles
    I have a rack of mixed Sennheiser receivers that I just put together this year and am still trying to fine-tune. Generally speaking we have a pretty quiet RF environment in our location, along with a thick-walled institutional building that helps block outside signals, but I'm suddenly having trouble finding free frequencies. I realize that RF environments change quickly, but I'm seeing behavior that I don't understand. Normally I hook everything up and scan with one receiver, then use the "list" from that receiver to populate the frequencies on the rest. In the last few days, I've been getting a lot fewer free frequencies than before when I scan (7 at most instead of more than 20) and scanning with different receivers on the same band yields widely different amounts of free frequencies (scanning with an EW300 gives 7 free at most, scanning with an EW100 gives me multiple groups of 12 free). I haven't yet tried pulling individual receivers out of the rack to test them independently, but that'll be the next step after a small event tomorrow for which I need at least 8 functional channels.

    My rack is split into two sections so I can mix and match for different size events-
    Section 1 is a 4-pack of EW300 G3 A band receivers, all powered and distro'd by an RFVenue Distro4.

    Section 2 is 3 EW300 G3 A band receivers, 6 EW100 G2 A band receivers, and 3 EW100 G3 receivers, one each in A, G, and B-bands. There are 3 RFVenue Distro4s in this rack, powering and providing antenna distro for these 12 receivers, and I can cascade out of the 3rd Distro4 to the other section's antenna input to tie them all together. Everything is used with a pair of Audio Technica ATW-49 antennas connected with 50' 50 ohm coax cables.

    I realize that it's a bunch of disparate gear kludged together, mostly because of budget constraints, but it's all functioned pretty well for the better part of a year. Is there anything I'm overlooking that might be causing this weird behavior?

    Thanks for any help!
     
  2. Brenden Friedel

    Brenden Friedel Active Member

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    Occupation:
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    Make sure when you scan the transmitters aren’t on. Only thing that should be in is the receivers. I had this exact problem the other day using WWB. I used the scan feature and realized all the receivers were in so I didn’t get any open frequencies
     
    Ben Stiegler likes this.
  3. TimMc

    TimMc Well-Known Member

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    Sennheiser B band is effectively gone with the auction of the 600mHz TV spectrum for mobile phone and mobile data (video) use. Chances are good that you are seeing T Mobile 5G being rolled out. In spite of the oft quoted date of 2020 for the move the reality is that T Mobile can fire up their new toys as soon as the TV stations turn off their old transmitters.
     
    RonHebbard likes this.
  4. manuallyfocused

    manuallyfocused Active Member

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    Occupation:
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    Location:
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    Since I only have the one B-band receiver still in the rack (soon to be replaced by a G-band, and I'm really only scanning with the A band receivers, as they cover 14 of my 16 channels) the T-mobile rollout shouldn't be affecting me, unless they are somehow jumping into the 516-558 MHz range. It's possible that other users in the area have switched to the A range from old B-band equipment in advance of the rollout, but as far as I know we don't have any neighbors with heavy RF usage.

    The issue I'm really seeing is that a 100 series receiver (less powerful, fewer programmed frequencies) is seeing a very clear RF environment, while a 300 series receiver is seeing a much more congested environment, and I'm curious as to why. Perhaps it's time to look at investing in an RF explorer or similar piece of equipment so I'm not reliant on the internal scanning of my receivers, or maybe go the laptop/WSM software route?
     
    Jay Ashworth likes this.
  5. AudioGreg

    AudioGreg Member

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    I have found other gear like ethernet switches, switching power supplies, and SDI video devices near my G3 receivers or antenna cables will seriously increase the RF noise floor and produce scans similar to what you have been seeing. Relocating things in my rack, and moving to paddle antennas away from said rack fixed all of this issue for me. I saw this issue in A, G and B bands...

    FWIW...we are using G3 Bank 6 Ch 1-8 in the B band, these fall into the "duplex gap" and continue to function well even in a new TMo area...knocking on wood

    Also as a result of the ongoing repack, the A band is filling up with TV in a lot of markets...LA going to be all TV in A band very soon :-(
     
    Last edited: Mar 11, 2018
  6. FMEng

    FMEng Well-Known Member Fight Leukemia

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    An RF quiet environment in Los Angeles? I don't think so. Maybe some of the first repack TV transmitters are starting to go on the air for testing.
     
  7. TimMc

    TimMc Well-Known Member

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    Unlicensed use of transmitters capable of tuning outside the duplex gap or other permitted frequencies LOSE THEIR TYPE ACCEPTANCE and become illegal to use the moment T-Mobile turns on their equipment.
     
  8. TimMc

    TimMc Well-Known Member

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    Why you get different results from different models is a question best addressed by the manufacturer. Did you ask Sennheiser?
     
  9. manuallyfocused

    manuallyfocused Active Member

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    Occupation:
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    Location:
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    We're a ways outside of LA proper in the western San Fernando Valley, so I'm keeping my fingers crossed that we don't get too congested out here with the repack.

    I have not yet contacted Sennheiser, but I may do that soon. I got a bit more insight after yesterday's event, and observed some more variations in behavior:

    Connecting my paddles to only section 1 of the rack (1 Distro4, four EW300 receivers) yielded a scan with 17 free frequencies. Same receiver, connected at the end of the chain of 4 Distro4 units yielded 7 free frequencies. It looks like the noise floor gets raised significantly as more Distro4s are attached, or there's something else going on with the way my rack is set up. I don't believe this was the case when I first set the rack up, maybe a call to RFVenue is in order to see if the Distro4s are acting up?
     
  10. FMEng

    FMEng Well-Known Member Fight Leukemia

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    That's good troubleshooting. It could be as simple as signal overload. Starting with a 6 dB antenna, and up to 2 dB of gain per cascade, the signal going into the last distro amp could simply be too hot. Add in a few TV stations and a cell site or two, and the result is intermod products. Inserting a pad, or switching to a whip antenna would point to whether this is the culprit.

    That's one reason why I'm not a fan of active cascade outputs. The other thing I don't like is that the noise floor increases after every cascade. I'd rather passively split from the antenna to the active distro inputs.

    Of course don't overlook the possibility of a bad BNC cable. Make sure you don't have a center pin that's too short on a male connector.
     
    RonHebbard likes this.
  11. eadler

    eadler Member

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    Occupation:
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    Have you checked a TV Whitespace / channel availability map? There's a few vendors that have them (free, online). Note that these don't necessarily show wireless microphone license holders properly. There are, in most areas, two channels reserved for 'wireless mic' use.
     
  12. Chris Jolocon

    Chris Jolocon New Member

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    Definitely check what everyone has mentioned thus far. Also, I've found on the G3 Sennheiser receivers that the squelch setting on the particular receiver effects the results (available frequencies) when you perform a "scan new list."
     
  13. TNasty

    TNasty Active Member

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    Occupation:
    Technical Adviser. Aux Police Officer.
    Location:
    New Jersey
    Common culprits include:
    -Cheap switching power bricks (or really power supply without any Faraday shielding)
    -Big appliances (my ham radio picks interference up like crazy around the tray drop in my dining commons, sometimes it even manages to match my CTCSS tone and then my radio spews out mish mash for all to hear, loud and... well, not clear.)
    -Rollout of new RF equipment (just like the mobile comms already mentioned. You should also take a look on radioreference.com to see if there is anything in that range... if I remember correctly some P25 systems can operate around 600MHz)
    -Intermodulation
    -Intergalactic magnetic waves?
     
    Ben Stiegler likes this.
  14. JD

    JD Well-Known Member

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    Watch out for mains operated LED replacement lamps! Huge chunk of out A band goes away when the house lights go on. The harmonics thrown by some of the power supplies in the cheap LEDs are amazing. Try this: Switch everything imaginable off. Do a scan. Turn everything back on. Do a scan. If you get drastically different results then start eliminating things one at a time. Trust me, a simple malfunctioning wall-wart could kill much of the available spectrum! Use to be they had transformers in them. Now most all are switch-mode supplies. Heck, even the new Sennheiser supplies are switch-mode. Remember, although cheap is suspect, even a good supply may fail in a way that spits out RF.
     
    Ben Stiegler, RonHebbard and TNasty like this.
  15. FMEng

    FMEng Well-Known Member Fight Leukemia

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    That's a good point, and one even the manufacturers seem oblivious to. Boston Acoustics famously made an HD Radio with a "line lump" power supply. The problem was if the power supply was anywhere near the radio antenna, it interfered with itself, causing lousy reception.
     
    Ben Stiegler and RonHebbard like this.
  16. Jay Ashworth

    Jay Ashworth Well-Known Member

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