RF Tips According to Steve Caldwell
As the amount of available clean RF spectrum for use by operators of wireless microphones decreases, staying on top of best practice operations is more critical than ever. It takes a great deal of planning, patience, and skill to pull together a show—particularly when managing larger-scale events with very high channel counts. One man who certainly has no shortage of experience is Steve Caldwell, RF Engineer at Norwest Group. We caught up with Steve ahead of his next project managing RF for The Royal Edinburgh Military Tattoo in Sydney. What unfolded was a series of priceless tips for anyone involved in setting up and running wireless gear in today's challenging RF environment.
"We break down large-scale RF projects into five key areas," opens Steve. "Essentially, we're looking at spectrum management, wise hardware choices, maintaining said hardware, careful spectrum monitoring, and the ability to troubleshoot effectively. This, at least, is how I break things down."
From here, I ask him if he minds breaking each of these categories down, with a few key tips on each, and Steve was more than happy to oblige.
"Spectrum management starts with diligent information collection and identifying the spectrum resources available", Steve continues. "The first piece of information that must be acquired, before any spectrum planning can commence, is to obtain details on the RF environment at the event location. This preferably should be performed by an RF monitor or spectrum analyser that can plot information over time, or at the very least log a max-hold trace. If the scan can be done during the period of the day that the event is, then you will gain a lot more accurate image of what you are dealing with. A lot can change across the RF spectrum during different times of the day. If you are able, and resources allow, a scan collected over 24 hours is ideal.
"Apart from obtaining information about the RF landscape, an important detail is to collect information about other users requiring spectrum resources during the event. Not only other licence-holding radio microphone and IEM users, such as broadcasters and ENG crews, but also for communication providers such as two-way radio, and any model of duplex communication system that may use the spectrum of interest. All of these systems (especially the latter), will influence the outcome of an RF solution, and should be included—to some extent—in your solution calculations. Often all of these systems can be resolved simultaneously in your solution."
"…an IMD hit does not require ANY spacing from another IMD. You can pack IMDs on top of one another as densely as you are able to make them. By doing so, you create dense IMD "junk bands" between the blocks of devices. This leaves more unaffected space for more fundamentals within the spectrum."
Without a clear view of the spectrum environment, it is equivalent to running your show blindfolded. Once you can see clearly, an effective solution is easier to obtain, Steve goes on to explain how he "resolves effective solutions", starting with a great spectral efficiency tip.
"When resolving your spectrum solution, try to condense each type of radio equipment within the equipment's switching bandwidth (its tuneable range). By default, most frequency calculators will utilise all of a device's switching bandwidth for the number of devices required. This essentially spreads the fundamentals, and therefore any IMD (intermodulation distortion products) over a proportional amount of space (for instance, 3 times the devices' fundamentals bandwidth for 3rd order, 5 times for 5th order, etc). By constraining the spread of the fundamentals in the spectrum as much as possible, you are also constraining the spread of IMD. The reasoning behind this, even though a calculator will avoid an IMD when placing a fundamental, is that all calculated fundamentals must avoid IMD hits by a certain spacing – up to 250kHz or even more. However, an IMD hit does not require ANY spacing from another IMD. You can pack IMDs on top of one another as densely as you are able to make them. By doing so, you create dense IMD "junk bands" between the blocks of devices. This leaves more unaffected space for more fundamentals within the spectrum."
Ever the active planner, Steve leaves nothing to chance and always recommends including extra resources in the solution to allow for last-minute users.
"Always try to include a modest, additional quantity of space for popular equipment types — especially equipment used by broadcasters and ENG crews, such as Lectrosonics, Wisycom. If you do get ENG crews turning up, you at least have a selection of compatible frequencies to offer. If no last-minute guests appear, you have some extra compatible frequencies to use. These frequencies often do not require anything more than basic calculation parameters, such as 3rd order IMD, and even then, only fundamental spacing is all that is required."
The old saying "measure twice cut once" is a good metaphor for what comes next as Steve wraps up his tips for spectrum management.
"Once the spectrum plot has been resolved, make sure that you check the resolved frequencies are actually usable on the equipment they are specified for. A lot of equipment model variations are manufactured for various parts of the world, and quite a few have minor variances in their coverage. Even variations in the generation of specific models and bands of equipment can change. It's a lot easier to correct this now, than after you have deployed frequencies to equipment. Don't forget to check with other parties you have resolved frequencies for."
HARDWARE CHOICE AND DESIGN
Clearly, the physical equipment chosen needs to suit the physical and RF environment. As we moved on to hardware choice and design, Steve had plenty of tips on how to effectively design an RF rig fit for modern production environments. Even if you have little control over the transmitters and receivers, you can still make large gains by carefully selecting and managing your antennas.
"Often, the choice of equipment will be beyond your control. However, if you do have the option, base your choice on providing the best performance while keeping in mind what you've learned during the spectrum analysis. The types of antennas used should also reflect the coverage area of the performance space. Quite often standard active log-periodic paddles will suffice. However, if covering a large stadium, choose antennas that have an improved front-to-back ratio, larger capture area (physically larger antenna, such as a stacked array or fractal design), and also try to use antenna preamplifiers that can be adjusted in smaller gain increments. Half-wave whips are rarely the best choice for any situation beyond small room coverage. Even smaller, tightly spaced venues can benefit from directional antennas (Log-periodic paddles, circular polarised helicals, etc), as it's often not about what the antenna receives in the front, but what it rejects behind it. Using a directional antenna to ignore the RF noise radiated from a digital monitor console, or a two-way radio base antenna is one example."
Moving further into the world of antennas, which is often overlooked, the tricky business of how to balance cable lengths and antenna path distance reared its head, as well as exploring the practice of physical antenna placement.
"Antenna placement is one of the more important aspects of setting up a radio mic or IEM system for a large event. Getting an antenna close to the performance area, at the expense of a longer feedline, is an advantage in some situations. However, there is a lot to be said for keeping the antennas at a farther distance, especially when dealing with a very large performance coverage area. The reasoning behind increasing the distance from the performance area to the antennas is in keeping the ratio between the closest and most distant performance area low in comparison to the antenna distance. For example, if an antenna is placed right at the side of a large stage, the variation in signal strength from the closest performer to the farthest performer is quite high. This can make setting gain structures very difficult and can also cause receivers (even receivers not tuned to those frequencies) to reduce their RF gain in response to them being swamped by closer transmitters. This makes receiving a farther transmitter very difficult for its receiver. An antenna type chosen to adequately cover a distant performance area might overload the receivers when transmitters are placed closer. If the antennas are moved away from the performance area by a reasonable distance, you have less chance of swamping the front end of the receivers and are better able to set gain structures, as the RF signal strength has less variation during the performance.
"Along with determining antenna distance from the performance area, the location and height of antennas can play a huge part. Both azimuth and height play a part in the positioning of antennas. It goes without saying that height is your friend. The higher you are able to place your antennas (within reason of course), the better your RF link will perform across the venue. There is no point in placing your antennas at front-of-house, if during the event they are blocked by hundreds of bodies of water. The other aspect is the azimuth of the antennas in relation to the performance space – do you place them side-of-stage, upstage centre, front-of-house? In fact, this is often determined by the type of transmitter (or receiver) the performers are using. It's quite obvious that IEM antennas should be placed as far upstage as possible to allow a more direct line-of-sight to the receivers placed on the back of a performer. With regard to microphone bodypacks, it depends if the packs are worn on the back of the trousers, in which case upstage is also the best position, or in the breast pocket. Modern handhelds, however, have quite a unique radiation pattern from their side-firing helical antennas. When held, or on a stand on stage, the pattern they emit is a typical toroid 'doughnut' however, it is somewhat tilted back toward the handheld body, and is not 90 degrees perpendicular to the mic, so side-of-stage is ideal. There are significant nulls in the radiation pattern on axis of the handheld, so placing antennas directly in front, or behind a performer with a handheld is the worst-case scenario. When covering large stadiums, this aspect of antenna positioning will very rarely come into play and is probably more about the location of performance spaces within the stadium."
Distribution System Design
When channel counts run high, antenna distribution is your friend, but the process needs to be handled with care. Steve shares his experience with us.
"When building a radio microphone system with significant numbers of receivers, there are certain things that will help in maintaining optimum performance. If your receiver count exceeds the number of outputs available on a single distribution unit, cascading is used to feed more distribution units and their receivers. However, I would not cascade any more than three distro units in series. Beyond this you risk increasing your Noise Factor (NF), which is the ratio of RF signal-to-noise (SN) on the input to the SN on the output of a device, to levels that reduce performance of the receivers. As the distro units have amplification to counteract the RF signal dividers, they introduce noise even with unity gain. In this case, I would use a master antenna distro unit to feed other distro units. This way you have a maximum of two active distro units in the antenna signal path. If you consider that up to 5 distro units may be fed by the master, that's 25 receivers or 50 channels if using dual receivers, 100 channels if using quads. One of the other aspects of RF systems is the function whereby loss in a system is proportional to frequency. Therefore, if you do use the cascade function in a distribution system, and you are also using different bands of receivers, place the higher frequency band receivers closer to the incoming antennas. Lower frequency receivers on the last distro."
RF operations are often referred to as "a dark art", and it's not hard to see why. Most sound engineers get into sound production, originally at least, to work with audio, not radio frequencies. There's a fair amount of physics involved, which can feel overwhelming at first. All aspects of audio are part-art, part-science, but with RF, this is particularly true. With all the tech and physics talk, it can be easy to neglect the basics of hardware maintenance. And let's face it, no amount of RF coordination wizardry matters if a hardware connector fails, for example. Steve goes on to stress the importance of looking after the physical kit.
"The most common connector used with radio microphones and IEM is the BNC type, closely followed by the N-type connector. These connectors use the same electrical mating surfaces, and only differ in the mechanical retention used. However, the mechanical retention used by the BNC is, by comparison, quite loose. This causes issues with the electrical shield connection when continually mated and unmated. Combined with the heavier RG-213 type cables, this shield connection becomes loose and then relies on the equally loose bayonet retainer to maintain a ground connection. One of the most effective maintenance procedures you can perform is tightening up these shielding 'fingers' on the BNC male connectors. This can actually make a huge difference in the performance of a radio system, both microphones and IEM. By simply spreading these shielding fingers outward, you are providing a firm shield connection for the connectors. This allows better current ground reference for both RF frequencies, and also the DC bias voltages provided by receivers and distros and used by antenna preamplifiers."
Equally important is the process of checking your antenna distribution system. Once a distribution system is built, Steve recommends checking the connection integrity of the antenna patch interconnections.
"You can check this pretty simply by tuning all the receivers (per band, if different bands are used) to the same frequency, turning on a transmitter tuned to that frequency, and verifying that all the receivers show similar RSSI on their signal meters. Often, removing or covering the transmitter antenna to reduce the RF level will allow the receivers' expanded scale meters to better represent the signal strength each unit is receiving. It is then a simple matter of comparing receiver antenna signal strength between channels, and between individual receivers. Once you have checked that all the receivers have the same signal strength, unplug one of the antennas at the first distro, and make sure that all the receivers lose that antenna channel. If they lose signal on both channels or there is no change, then the receiver has been patched with both antenna inputs connected into the same channel in the distro. This is actually quite a common occurrence, and defeats the diversity used by the receivers."
No assessment of wireless hardware would be complete without checking the bodypacks. As one of the most abused parts of a radio microphone and IEM system, the transmitters (or IEM receivers) seem to lack a proportional amount of maintenance. Most of the connections used on a radio microphone bodypack are quite fragile and can easily become damaged. Spotting a damaged audio microphone capsule connection is easy, however detecting a damaged antenna connection may not be as simple. Steve elaborates with a pertinent tip for one aspect that's often overlooked:
"Often the centre pin of the antenna connection will appear to make a connection when held in the hand, but then fail once fitted to a performer. Unscrewing the antenna and checking for a broken pin will save a lot of time. The pin can often break off in the socket of the transmitter. Checking that the antenna is also the correct wavelength will also retain its performance. Often, different frequency bands of transmitter may use the same length of antenna."
In many aspects of life, taking a leap of faith into the unknown might be considered an admirable trait. In the world of RF, seeing is believing and believing is knowing. Visually inspecting the RF environment can make the difference between success and failure. Choosing the right radio scanner, however, is vital.
"The choice of RF monitor (scanner) can make the difference between seeing the real RF noise floor or the receivers' self-noise. Also, narrowband spikes adjacent to your fundamental frequencies in the spectrum may be missed by wide resolution bandwidth receivers, and poor amplitude resolution can be caused by units with bad dynamic range. A lot of the cheaper units available these days also have no front-end filtering, allowing high power signals to adversely affect the performance of the receiver, even when scanning out of those bands. One other thing to look for is the ability of the scanner to record its scan data, for importing into a spectrum management calculator. Ensure that the scanner is capable of recording the data in fixed step sizes of your choosing, rather than as fixed points per span. The ability to demodulate and listen to the spectrum is also invaluable.
"In order to effectively monitor the performance of your transmitters by comparing measurements made over time, your antenna must remain stationary. Based on this, you must choose an antenna position that will remain constant. Choose a position that won't require moving during the event."
Monitoring from the Distribution System
"A very effective means of monitoring your RF spectrum is to take an RF feed for your scanner from your radio mic distribution system. This way you are seeing exactly what the radio microphone receivers are seeing in the spectrum. It will allow you to see what external influences, such as TV transmissions, two-way radio, etc are hitting the receivers, and taking into account your filtering and gain structure, at what level. Remember that the resolution bandwidth product of a radio mic receiver may be quite a bit wider than that of your scanner, unless it is adjustable. As noise is proportional to bandwidth, your receivers may see more noise floor than you are seeing from your scanner. It's really all about relative measurements – seeing the results of things that you do to change or improve the system, rather than the absolute levels."
"Unexpected transmission is something that almost all RF guys will experience at some point. . . . Ask them if they are open to you including them in your spectrum management plan. Most people will be grateful to be included, and it offers them at least rudimentary protection, at least from your transmitters."
Full visibility of the RF environment enables you to chase down and deal with rogue transmitters that might otherwise spoil the show. Steve is no stranger to this problem and had plenty of advice on how to detect, identify, and chase down rogue transmitters.
"This process doesn't have to be daunting. One of the most useful techniques is to use the audio functions on your spectrum monitor to actually listen to the audio from the transmitter. This is getting difficult with digital modulation systems, however. Listening to the transmission will give you more information about its owner and whereabouts than anything else. Are they news crew? Are they inside the venue (can you hear the PA in the audio)? Also, look for sidetones present in the transmission. IEM and IFB will often contain a 19kHz pilot tone. Does the frequency of the transmission give you any clues? If the frequency is on a 100kHz channel step it is likely to be an older Lectrosonics. All this information will give you clues as to where to look for the source.
"Unexpected transmission is something that almost all RF guys will experience at some point. The main thing to remember is, most of these people that just turn-up and turn-on are undoubtedly less experienced with the concept of spectrum planning than you are. In reality, this behaviour is as bad for them as it is for you. The one thing you do not want to do is to confront them telling them they have to turn off their transmitter. Often, they have as much right to use the spectrum as you do, and you will just end up making an enemy rather than a friend. Ask them if they are open to you including them in your spectrum management plan. Most people will be grateful to be included, and it offers them rudimentary protection, at least from your transmitters. Give them a frequency and your phone number, and most of them will become another pair of eyes for you when trying to locate other rogue transmissions."
Last but not least is the (sometimes) stressful process of troubleshooting. When speaking to Steve, he broke this topic down into two sensible categories: 1) Prevention 2) Mitigating any issues that occur.
"Once a radio microphone is given to a performer and they make their way onto the stage, your opportunity to fix any issues is very limited. Unless you're furnished with a Shure Axient or Axient Digital system, and you've set up showlink, you cannot change frequencies should a spectrum issue occur. Other than spectrum issues, one of the most common issues is the failure of lapel and headset microphones attached to bodypack transmitters. The best way to combat this is to get a hold of the show run or microphone plot and listen to the audio from each microphone receiver before the artist goes on stage. This way you can catch any issues with poor connections on headsets before it is too late. You may use your RF scanner to do this. Most receivers come with headphonessockets for this purpose. The Wavetool program provides an excellent way of monitoring radio mic receivers, as it will display metadata information from the receivers, while allowing you to PFL the audio from any receiver. Wavetool also contains algorithms that automatically detect pops and cracks from faulty microphone headsets and lapels.
"Dealing with any issues that occur while a microphone is in use is a different matter. Once again, unless you are showlink enabled, you cannot change the frequency of a radio mic system once the artist has that mic on stage. If you find you are having an issue with a mic that a performer is using, there are several things that can be done to mitigate the issue. If the microphone starts dropping out completely, this is most likely due to lack of signal strength. The first thing to do is to locate the receiver, either physically, or through its software, and drop the squelch threshold setting. This lowers the minimum RSSI required to keep the receiver un-muted. This is a stop-gap procedure while you find the real cause. It may be that one or both of the antennas have been compromised. Check the RSSI metres on the receiver and see if one or both antennas are showing low level. If only one antenna is low, then check the orientation and position of the antenna, and the connection of the feedline to the distro. If both antennas are showing low level, then it is most likely a lack of RF power from the transmitter. If the receiver continues to drop out after lowering the squelch threshold, then this is where your hot spare microphone comes in to play. If you have plenty of signal strength, but you are still experiencing dropouts, then the most likely cause is interference of some kind, either from another transmission or from increased noise floor. One of the most common causes of interference is friendly fire – one of your own transmitters is on the same frequency. Often someone programming, or replacing a faulty transmitter will switch on a new unit that is on the same or close frequency, or even a frequency that is causing an IMD hit. Show plotting that requires the use of multiple transmitters used on the same frequency can be fraught with danger and should be avoided. Two transmitters on the same frequency are easy to spot, as the activity of the antenna diversity switching on the receiver increases significantly. This is often associated with the inability of the receiver to identify the transmitter type or read any other metadata from the transmitter."
In a final word, Steve shared his 3 rules of thumb for using radio microphones:
- Turn down the power – Less is more in the world of RF, especially with IMD creation. As an IMD product's power will increase 3dB with every 1 dB increase in its contributors, it's very easy to let this get out of control. Turning up the power on a transmitter should only be used for situations where that power is being consumed or absorbed by its user, or where unusual distances are required, and are then provided to those transmitters.
- Height is your friend. – there is no better way to increase your coverage area than to increase the height of your receiving (or for that matter, your transmitter) antenna.
- Use a #@$%ing cable!! – In all seriousness, if the situation can be done with a piece of copper, then do it. Radio microphones definitely have their place, but they also open up possible issues that may very well be avoided.
When preparing and setting up your radio microphone system, always undersell its performance, and then over deliver. Do not go into a venue stating that "I can get coverage everywhere", "we won't have any issues covering that!" because as soon as you say that, its all over. Build the system that does what the client wishes, and then surprise them when not only does it do this extremely reliably, but it does a lot more.