Wireless Making Sense of Shure's Wireless Frequency Finder

Stevens R. Miller

Well-Known Member
Well, once again I am baffled by something I thought would be easy to understand. My company has eight Shure SLX4 receivers, and eight SLX1 transmitters. Six of the SLXs are in band H5 and two of them are in band J3. When I got the system (on loan so I can learn how it works), all eight were already configured and synced to group/channel settings the company regularly uses without problems.

Now, Shure's User's Guide suggests that all receivers should be set to the same group within a band. As you might expect, that's not how whoever set this up did it. Only two of the H5s are in the same group and both of the J3s are in different groups.

First question I have is, why is it a good idea to put them into the same group? (Guess: the specific frequencies in a group are chosen to create intermod products that don't land on frequencies in the same group.)

Now, one needs a clear channel, of course. When I use the automatic channel finder on my second J3 when it is set to Group 3, it happily chooses Channel 5, which works fine. But, I recently discovered Shure's Wireless Frequency Finder Web site, which tells me that J3's Group 3, Channel 5, is not available. Apparently, that's because its 577.750 MHz frequency is smack in the middle of WETA-TV's television channel 31 band segment of 572-578 MHz, and that they are only ten miles away. Except that WETA is on Channel 27 (virtual channel 26), which is 548-554 MHz.

Second question, then, is why is Shure claiming I can't use Band J3, Group 3, Channel 5 when, as far as I can tell, it's in the clear?

Next, Shure's Web site lists six bands that it says are recommended for use in my area, one of which is J3. For each recommended band, it lists the number of mics I can use on the available groups and channels. For H5, for example, it says I can use Channels 8 through 12 of Group 6. I actually am using a Channel in Group 6, but it is Channel 2, which Shure says I can't use (because it is used by WMDO-LD on Channel 22). None of the rest of my H5 SLX4s are using Group 6 at all, yet all work fine. For J3, Shure says I can have zero mics on that band.

Third question: why is Shure recommending band J3 when, at the same time, it says there are no available channels for me to use in that band?

Fourth question: why is Shure telling me that WETA is on television channel 31?

All answers gratefully received, with dubious and uninsured legal advice available as compensation.
 
First question I have is, why is it a good idea to put them into the same group? (Guess: the specific frequencies in a group are chosen to create intermod products that don't land on frequencies in the same group.)

You are correct, the groups are free of intermod products. It is up to the end-user to program around TV stations.

Second question, then, is why is Shure claiming I can't use Band J3, Group 3, Channel 5 when, as far as I can tell, it's in the clear?

I use PWS IAS to get all of my data -- it pulls from the FCC TVDB and can also pull user scans from folks who have uploaded them to their server. When I look at a general overview of the Northern VA area I see WETA transmitting a 15kW signal from DC and I see another station broadcasting a 6kW signal from Crozet, VA on Channel 31. I would feel confident overriding Shure's preferences and using anything within Channel 31 unless you're in downtown DC.

The channels IAS recommends to not tune over are 15, 17 + 18 appear to be emergency services, 26, 33, 34, 35, 36, 38, 40, 42, 44, 46, 48, 50. There's a scan from last spring in the database taken from Wolftrap's Filene center - based on that scan I would feel confident using Channel 26, 33, 38, 40, 42, 44, and 46.

Third question: why is Shure recommending band J3 when, at the same time, it says there are no available channels for me to use in that band?

Shure doesn't actually know what's in the air - it is pulling from the TVDB. Shure is also quite conservative with channel estimates with the SLX in my experiences with them. With quite stringent settings in IAS (every TV station IAS suggested to black out, 5th Order Intermod Products disallowed) I get 8 candidate frees in the J3 band and 13 in the H5. These were separate runs, no idea what happens if I tried to make a single coordination with the maximum number of both ranges.

Fourth question: why is Shure telling me that WETA is on television channel 31?

Because the FCC has a 15kW signal for WETA registered in their database. They also have a 75kW transmit listed on channel 27, but it's a bit further away.
 
I use PWS IAS to get all of my data -- it pulls from the FCC TVDB and can also pull user scans from folks who have uploaded them to their server.

Well, that sounds immensely useful. Can I download or access it from the Web?

Because the FCC has a 15kW signal for WETA registered in their database. They also have a 75kW transmit listed on channel 27, but it's a bit further away.

That explains it, though I'm puzzled as to why WETA itself doesn't mention the low-power transmitter on their site (or, at least, not where I could find it quickly).

So... am I to understand that making use of frequencies already in use by existing television transmitters is allowed by FCC? With 30mW packs, quarter-wave wire antennas, and the walls of an auditorium, I would imagine very little interference could be detected much beyond our venue, but it does seem unfamiliar. As a kid, learning the regs for my ham license exam, it seemed that Thou Shalt Not Interfere was something FCC wanted engraved on stone tablets.

Along similar lines, I'm curious about what professional venues do to avoid interfering with each other. A lot of them are packed in pretty much cheek-by-jowl on Broadway. Do they coordinate with each other?

Thanks for the info!
 
Well, that sounds immensely useful. Can I download or access it from the Web?
https://www.professionalwireless.com/product/intermodulation-analysis-system/ -- not cheap, but it works well.


That explains it, though I'm puzzled as to why WETA itself doesn't mention the low-power transmitter on their site (or, at least, not where I could find it quickly).

I don't know too much about how TV works, but it could be uplink from their main studio to the broadcast tower.

So... am I to understand that making use of frequencies already in use by existing television transmitters is allowed by FCC? With 30mW packs, quarter-wave wire antennas, and the walls of an auditorium, I would imagine very little interference could be detected much beyond our venue, but it does seem unfamiliar. As a kid, learning the regs for my ham license exam, it seemed that Thou Shalt Not Interfere was something FCC wanted engraved on stone tablets.

Yea, you can tune into DTV stations if they aren't absolutely stomping you. You just can't tune over emergency services or Channel 37.

Along similar lines, I'm curious about what professional venues do to avoid interfering with each other. A lot of them are packed in pretty much cheek-by-jowl on Broadway. Do they coordinate with each other?

It's pretty simple with the Broadway venues. The NYC audio community is very tiny, we all know each other. There are 3 rental shops that service Broadway and each one has a lead coordinator. While they are rivals, their RF departments do talk to each other and so they all know what's going on in every theater. The day a show powers up the RF system for the first time you give a call to your neighbors, let them know you're powering on, and then do so. In my experience you do this during the day to shoot out issues with your system alone and once that's stable you keep it on through your neighbors matinee or pre-show checkout to make sure you aren't causing anything crazy but hopefully the shop coordinators caught any direct hits when they did their work in the shop.

Because we are all unlicensed, interference is accepted and we do whatever we can to mitigate risk. I generally have an extra Helical antenna in my rigs so I can slam it on my analyzer and hunt down interference when someone is around me that hasn't announced themselves (television shoots, reporters, and the surprise street concert are the big offenders) and I run around with my scanner and my laptop to find them, and then quickly re-coordinate their wireless for them to known good frequencies.
 
That explains it, though I'm puzzled as to why WETA itself doesn't mention the low-power transmitter on their site (or, at least, not where I could find it quickly).

WETA broadcasts on channel 27 in Washington, DC. The confusion is from a construction permit for channel 31 in Dickerson, MD. A CP means it isn't built yet, but they have the right to build it during a 3 year period. The expiration date of 12/17/17 means it'll either be on the air or a dead project by then.

Also note that DTV stations use virtual channels. For example, the local CBS affiliate here broadcasts on channel 39, but my TV set displays it as channel 7. The advertised channel doesn't necessarily associate with the channel used by the actual RF carrier. In the case of WETA, their virtual channel is 26 for both transmitters.
 

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