Consume Spectrum Responsibly

  • Posted by Simon Byrne
  • On August 15, 2016

CX117_Aug_16

In the live production industry, we like to think that our use of wireless spectrum on events, take precedence over other users.

This thinking is grounded in the desire to deliver a perfect production to our clients and audience. Reliable wireless microphones and in ear monitors are particularly critical for a successful event.

However, the reality under the law, is that our use has zero priority over other users. Fortunately, other than TV broadcast stations, there are few other users in the valuable 520 – 694 Mhz band. This is good because this band of RF real estate is a choice part of the spectrum for two reasons. It’s wavelength propagates well so less transmission power is required (IE longer battery life), and it turns out that it is comparatively easy to make equipment in this frequency range (reliability and price).

Most of the wireless audio and wifi type gear we use is regulated by the Radiocommunications (Low Interference Potential Devices) Class Licence 2015. Rather than licencing users, the class of equipment is licenced. Manufacturers and importers are required to ensure their product meet the technical requirements of the licence including record keeping and labelling in order for it to be sold and used in Australia.

The class licence states that a receiver tuned to the transmitter will not be afforded protection from interference caused by other radiocommunications devices. That is, ’no interference’ and ‘no protection’ basis. As well as no protection, users must take steps to ensure that their devices don’t cause interference to other radiocommunications devices.

Seeing that your rights are close to non existent, you need to be smart and cooperative about how you manage the frequencies used on your events.

Scan and monitor – The major manufacturers provide excellent and free RF scanning and coordination programs as well as online tutorials for their wireless products. They also contain a geographical database of the TV stations in your area. You should take full advantage of these tools to coordinate your wireless devices.

Have spare frequencies planned – Not just for you, but for other users too. For example TV crews are renowned for turning up minutes before an event which can cause grief. With spare frequencies standing by, you can quickly and confidently change on either your gear or ask them to.

Hey venues! It is 2016. Perhaps it is time that you guys factored visitors into your frequency coordinations. Rather than giving a list of your frequencies already in use, how about providing a list of frequencies that visitors can confidently use without compromising the venue’s existing allocations.

Frequency squat before the show – If battery life permits, turn and leave on your transmitters well before the event starts. I typically use the batteries from the last show and put fresh ones in just before the start. By doing this, you are staking your claim over those frequencies. Remember, another user is not permitted to cause interference to you so if you are already up and running when they arrive, they should not claim a frequency that you are already using.

Transmission power – Only use as much as you need and put it where it is needed and no more. Using too much power for your situation creates a higher overall RF noise floor as a consequence of stronger intermodulations (interactions between adjacent frequencies). For example I reliably get 50 metres range with just 10 milliwatts of power, as long as I use the right receiver antennas.

RF Venue Diversity Fin – A true diversity antenna which achieves diversity by polarisation rather than spacing 2 antennas apart.

Most of the time that means LDPAs (Log Periodic Dipole Array Antenna), often called paddles. My favourite antenna for general purpose use is the Diversity Fin from RF Venue. It is a compact paddle design with true diversity configuration in a single unit.

Paddles are analogous to cardioid microphones meaning their pickup pattern is directional, dramatically reducing the overall RF noise floor.

Haven’t got the money for paddles? Get yourself some of the Kent Electronics log periodic printed circuit board antennas (link below).

Use good quality coaxial cable between your antennas and receivers. You will have signal loss in all coaxial cables but some are much better than others. RG8X is an excellent compromise between signal loss, flexibility and cost.

By the way, in a well planned system, you can get away with using 75 ohm cable instead of 50 ohm. Quality 75 ohm cable is cheaper to make than 50 ohm so it might be an attractive alternative. Basically, a standing wave sets up between a 75/50 ohm junction resulting in about a 5% loss per junction. This loss may be quite acceptable provided the antenna a reasonably close other transmitters.

Insert attenuators in the receive antenna chains if you have plenty of signal, but a high noise floor. By the way, if your paddles are side of stage, you have plenty of signal. Yes that’s right, if you have plenty of RF strength, you can confidently insert an attenuator to lower the overall noise floor and increase the reliability of your systems.

Physically separate your IEM transmitters and your microphone receivers as well as their antennas. The input stages of receivers are sensitive. It does not make sense to put significantly IEM transmitters in close proximity to them when your actual wireless microphones are many metres away, even if they are on different frequencies.

PCB Log Periodic Antenna – Just solder on a connector and fit a mount and you have an excellent paddle. Can be painted without affecting performance.

Work in with others – This is the big one. All users of wireless devices have a legitimate use and just as you want reliable transmission, so do others. It is rare for not enough spectrum to be available so with adequate planning, I have never come a situation where all user’s needs have not been met.

Lastly, if you find yourself in an extreme environment where the 520Mhz to 694Mhz is not enough, you can apply to ACMA for a short term apparatus licence to use frequencies in the now illegal 694–820 MHz region (at a cost). ACMA recognise that there are some rare circumstances where extra spectrum would be helpful. In these cases, an application can be made to use other spectrum and ACMA will assess each application on a case by case basis depending on what spectrum is available in the area.

 

 

 

 

 

 

 

 

Useful Links –

Mini Circuits – Suppliers of lot’s useful attenuators, splitters, RF amplifiers and attenuators. http://www.minicircuits.com
Kent Electronics – Suppliers of cost effective log periodic printed circuit board antennas http://www.wa5vjb.com/products1.html

RF Venue – Suppliers of Antennas, RF Distribution equipment and Spectrum Analyser products for use with wireless microphones.

ACMA – Information regarding short term apparatus licences


This article first appeared in CX Monthly Tech News.

I am a contributing writer to CX Magazine. CX Network is the voice of technicians in entertainment and audio visual across Australasia.

To read this article online (and ALL of their articles dating back to 1990 for free!), head over to the CX Network.

http://www.cxnetwork.com.au/cx-magazine/cx117-august-2016/

Lot’s of great stuff!

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