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Rack-mounted Sennheiser EM 3732 wireless mic receivers (left) and SR 2050 IEM transmitters out with the Shania Twain tour in 2015.

RF War Stories: Fast Fixes When It All Starts Falling Apart At Crunch Time

Several real-world experiences from the field in coming up with quick answers to unplanned issues impacting wireless system performance.

The very first time I spoke about RF matters was at a Canadian trade show around 2006. I was a last-minute fill-in for someone who couldn’t make it … I actually threw a PowerPoint together while my late friend and mentor, Neil Muncy was on stage giving a talk on grounding (now that I think of it, I was Neil’s roadie … he had limited mobility at that time, and I was there to help him set up).

In any case, I gave a talk about the importance of frequency coordination, something that was still a pretty new concept to most of the folks in attendance that day. After the talk, there was the usual Q&A and someone asked me, “So, what do you do when it all goes to hell?”

Well, I had to think about that one for what seemed like a good minute and finally replied, “I have no idea … that’s never happened to me!” That said, I have had a few … moments, and here are some of them.


In 2015 when I was out as RF tech on Shania Twain’s “Rock This Country” tour, our New York City date was my first time working at Madison Square Garden. While I had done a few RF gigs in NYC, I this was my first time in this storied venue, and I’d certainly heard many anecdotes, both about working at MSG and the challenges of working with RF in the crowded NY market. So, I was wary of what I might encounter.

Because I was on edge about it, I’d done a scan and a preliminary coordination in my hotel room the night before and then been pleasantly surprised at the results. The next day, in the venue, I repeated the whole process and was still happy with what I came up with.

Set-up went fine, sound check went fine, as did getting our new direct support act coordinated and up and running. (I always coordinate all acts on any bill with my headliner.) After dinner, I got a call on the radio from our crew chief/system tech, who was at front of house keeping an eye on the support act’s first show when he saw the band vocal channels lighting up on the main console. When he told me that the band’s BG vox mics were taking major hits, I thought “Oh boy, here we go…the other shoe has dropped!”

I hurried back to my rack, which lived under stage left, and sure enough, the band vocal channels, all six of them, were flashing every light they had, muting and un-muting and generally going crazy. As calmly as I could, I got out the spectrum analyzer that had come with the kit, a TTi PSA2700 (which I hadn’t used much, having brought my own system, but the TTi was ideal for looking at individual frequencies), and tuned it to the first band-vocal frequency.

On close inspection, I could see a low-level transmit, right on that exact frequency! That looked like a bad sign, but rather than panic, I thought I’d look at the next one. Same thing – a low-level transmit on exactly the frequency that the receiver was tuned to. I picked another channel at random…same story! What were the chances of there being an interfering signal on the exact frequency of each of the six band vocal channels? Pretty small.

Then it occurred to me that the chances were 100 percent that there were six transmitters tuned to those exact frequencies, and that was the band’s packs (they all sang on headset microphones) which were down a long hallway somewhere, in their dressing room. Which is precisely what the “issue” was.

I had delivered the band’s vocal and in-ear packs to their dressing room before I had gone to dinner and, when they had switched them on, the packs had bravely tried to reach their receivers, through a maze of concrete and steel and a few thousand bums in seats … and the receivers were gamely trying to receive those signals. In other words, not a frequency problem, and the correct action to take in this situation was to do nothing, which is what I did.

There are a couple of important takeaways from this story. First, if I had panicked, run down to the dressing room, retrieved all the packs and moved them to new frequencies, besides rattling the band, I would have appeared to have fixed the problem because when the packs were tested on the new frequencies, near the stage, they would have worked fine. That is, assuming that the new frequencies were as clean as the ones I had assigned in the first place, which is not a given in today’s tight spectrum environment.

Second, when encountering this type of “dropout,” it’s really important to know where the hardware is when it’s happening. If the artist’s mic is in their dressing room, or the artist is in some tunnel under the stage for a repo, or in a quick-change booth etc., you may not have a real problem.

This is an especially important factor to keep in mind with today’s “intelligent” receivers and monitoring systems. I’ve seen these devices absolutely lose their minds, flashing every kind of warning message under the sun, like “INTERFERENCE DETECTED!!!” when the real issue is that the stage manager has taken the star’s mic to their dressing room.

Conversely, I was doing a show at the Theatre at Madison Square Garden (now The Hulu Theatre) a few years later when I had a similar, but different, issue. For those who don’t know, this venue is a 5,600-seat theatre, at street level under the Garden, which is on the sixth floor.

Now on this gig, a corporate event, I was getting some RF hash on one of the Telex BTR-800 intercom channels, but only after one particular floor manager would speak. Initially, there were a few things that looked like they might be the issue such as the fact that she was stationed out in the house, furthest from the stage (and the receive antenna) and was wearing her pack facing away from the stage but I eventually ruled all of that out.

This time, I got out my own TTi (I’d since bought one), a PSA3600, which is more sensitive than the 2700, and took a close look at her frequency. Sure enough, there was a very low level transmit either on, or very close to hers (I can’t recall which). By really low, I mean around -90 dBm, so almost down in the noise floor.

Nonetheless, when I used the demodulate function on the TTi, I could hear that this was a steady transmit, albeit without any program present. What was it? I didn’t bother to confirm this, but I’m 99 percent certain that it was a Lectrosonics IFBT4, with a 250 milowatt output, which would have been used for a hockey game at the Garden the night before and was making its way through six floors of concrete and steel.

How was this making noise? It wasn’t, exactly … modern RF receivers mute their output until they get a signal from their transmitter telling them it’s OK to un-mute, essentially gating out any RF signal below a certain level. In this case, the transmit that was bleeding through from upstairs was holding the gate open for a few seconds after the floor manager had keyed (and released) her pack, allowing random RF noise through. So, this was a frequency problem, and the solution was to move her pack to a new frequency.

Say What?

One of the classic memes of RF interference is undesired or unintended voices blaring through a wireless system, something that was used to great comedic effect in the 1980s Steve Martin movie Roxanne. In real life, I’ve encountered this exactly twice, and both times I used the same method to figure out the source of the interference.

The first time was at an arena gig in Toronto, the same corporate as the one at the MSG Theatre mentioned earlier in this article. We were in rehearsals. I can’t remember how I became aware of the issue, but I may have just noticed some activity on a receiver. In any case, I plugged my headphones in to see (hear) what was going on and heard audio on a channel with no transmitter turned on.

Weirdly, the first thing I heard sounded like someone warming up on a stringed instrument, like a banjo or a mandolin. However, I kept listening and eventually I heard a woman’s voice say, “Well I’ve got one brother, now where’s the other one?” and I immediately called out “Who just said “I’ve got one brother…?”

The lead floor manager was standing near me, and she told me that the second FM had just said that, over what turned out to be the HPPL or High Power Party Line. HPPL is an intercom system based around two walkie-talkie frequencies (which are high power, typically 2 to 5 watts ERP) spaced 5 MHz apart. One frequency is the outbound “talk” and the other is the inbound “listen.”

The advantage of one of these systems over a traditional wireless intercom is that they have great range, typically what you would expect from a walkie-talkie. The downside is that, like walkie-talkies, only one person can talk at a time, and they need special hardware to interface with traditional intercom systems. These are often deployed with wireless camera systems as the two have similar coverage areas, such as an entire stadium on a sports event.

In this case, it turned out that the HPPL was being provided by a good friend of mine who was set up about 20 feet behind me. Once we identified the source of the interference, we resolved the issue by moving his transmit antenna around the corner and part way up the nearest vom. Getting his Tx out of the direct reception field of my receive antennas did the trick.

Now, frequency wise, this was a different situation than on the other show because the transmit frequency was in a walkie-talkie range (450 to 470 MHz in this case, but be warned, there are other WT ranges) and the affected receiver was in the low 600 MHz range. I have an expression – “power trumps frequency” – meaning that if you have enough power, there’s every possibility that you will affect the front end of a sensitive receiver, even if they’re quite far apart frequency wise. Note that filtering your receive antennas will help mitigate this situation but if there’s enough power on the offending transmit, even that’s not a sure thing.

The second time I encountered this problem was on the Shania Twain tour mentioned above, also in an arena. This time, an announcement came through an open receiver input, in this case the one for the boss’s acoustic guitar. This was during sound check and as we were not checking that input at that point, the transmitter was turned off.

The other “fluke” thing about hearing this was that the only reason that anyone heard it at all is because the monitor console was in an “all inputs open” scene for line check. Once again, I made inquiries as to who had just announced, “Dr. So-and-so, the restaurant will be opening in five minutes” and found out that this announcement had been made over the arena’s emergency announcement system.

Because the antenna for that system was located in the grid, immediately over center stage, there was no possibility of relocating it. After some discussion, it was agreed that this was essentially a non-issue as the acoustic guitar input would only be open when its transmitter was on, and the emergency announce system wasn’t going to be used during the show, unless there was an emergency.

Onsite Ingenuity

And finally, on a more recent event, I was made aware of an issue with the various Riedel Bolero intercom systems that were being deployed for a recent event in rural Iowa. My official role there was as lead frequency coordinator, but as I’ve mentioned in other articles, lately, that position seems to also mean “RF advisor/problem solver.”

Late on the pre-event set-up day, I was informed that Bolero users were encountering an unusually high level of interference and asked if I could look into that. Well, after determining that: 1) The 1920 to 1930 MHz (DECT band) spectrum was clear of interference; 2) That there was not enough DECT band intercom packs deployed for them to be interfering with each other, and 3) That there were no other DECT band devices present (easy to do in a cornfield in Iowa). I decided that the most likely issue was adjacent-band interference from the temporary cellular tower that was located just a few hundred feet away from the field.

The 150-foot cell tower at the event site in Iowa that created a lot of havoc.

What to do? By this time, it was early evening and many of the affected parties had left for the day, so I decided that the best approach was to head for the catering tent … and borrow a couple of aluminum serving trays. Then with the able assistance of Solotech’s Nick Denoncourt, who had flagged the issue to me, we conducted tests to see if shielding the back of the Bolero antennas with an aluminum tray would mitigate the interference.

The Riedel Bolero antenna with the baking pan attached to the back.

As you can see from the scan screenshots below, the situation went from barely usable (19:55:53 time stamp) to very usable with the tray (21:14:37) and back to very congested (21:21:50) when the tray was removed as a before/after test. These tests were performed on the one antenna that was closest to the cell tower, both in terms of distance and in height.

These Bolero scans show how the RF situation changed at the rural Iowa event with the use of the aluminum trays.

I went back to catering, mooched another 10 trays so there was enough to do every antenna deployed around the field, called and/or texted all of the crew involved and told them what the plan was and left for the day.

On event day, it was determined that the trays helped with the antennas that had their backs to the cell tower, less so with the ones that were either facing the tower or at 90 degrees to it. However, all those antennas were further away from the tower and somewhat lower in height, so the issue was resolved well enough for the game to proceed.

To wrap this up, the one thing that I hope readers take away from this article is that when RF issues come up, they are not voodoo, not bad luck and not Murphy’s Law, but problems to be identified, worked, and resolved using the proper tools and a logical approach.

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