Radio/ Wireless Mic ADU’s

May 25, 2019

What Should You Do?

Sometimes you can do more harm than good with the use of Antenna Distribution Units (ADU’s) with their remote antenna and boosters etc… How can this possibly be true you may well ask?

It’s not that long ago that even the more expensive radio/wireless microphone systems caused very high stress levels, as they would operate fine in an empty hall/theatre while rehearsing, then would drop out all over the place when you filled the venue with an audience. One of the reasons this can happen is because we humans (along with the odd guide dog etc) are basically made up of carbon and water and consequently we are a good source for soaking up radio frequencies (RF) thereby weakening the signal between transmitter and receiver, and therefore causing the loss of signal. There are also many other reasons for the weakening of the RF signal, for example fire doors, radiators, walls with reinforced steel etc.

Radio/Wireless microphone system manufacturers usually quote an operating range of up to 100mtrs, however this is direct line of site and in ideal conditions, such as outside on a fine day, away from moisture in the air, and where signal reflections aren’t such a problem. When indoors, the usable range is realistically closer to 25mtrs, and so enters the ADU to try to improve matters.

Good Quality Cabling

What people don’t always realise is the importance of the RF cable you use with your remote antenna feeding your ADU. RF cable isn’t like for example microphone cable, where you can run 50mtrs or 100mtrs without any noticeable loss of signal whatsoever. You must use 50ohm coax cable to start with (not 75ohm TV coax) and the thicker the cable, along with the shorter the cable run, and the fewer bends there are in the cable, the better. Once you start using UHF frequencies and upwards, there is a very high loss of signal escaping through the cable. RF doesn’t like to be forced to go down a cable in the first place as it wants to radiate in all directions. For example, you are more likely to pick up a stronger RF signal if you are 25mtrs from the transmitters using 3mtr – 5mtr RG58u antenna cables feeding remote antenna at the back of the venue, than you are using 50mtr RG58u cables feeding remote antenna placed much closer to the transmitters. The signal loss through the cable really can be that high.

Over Amplifying

Something else people don’t always realise, is you can over amplify an RF signal which is just as destructive as having a very weak signal, so you must find the correct balance between too strong and too weak a signal. Don’t worry, you won’t blow anything up with over amplification, it just won’t function at its best and you may well get interference and up go those stress levels again.

You should also be aware if placing your racks of receivers near the stage managers desk that you will run into trouble if people swap transmitter packs from person to person and find a handy place to put the pack they’ve just removed is right on top of the receiver rack, don’t wonder why you are now getting interference across the other RF channels, which will happen, because you have a live transmitter(s) within the 3mtr no go zone (oh yes there is a minimum as well as a maximum range for operating these systems) and a very strong signal is doing its best to be picked up by every receiver in your rack. A transmitters job is to spread the word as far and wide as it can, and a receivers job is to try and grab any signal it sees as having a remote possibility of being its own transmitters signal.

Precise Tuning

All the above assumes you did actually tune the transmitter and receiver frequencies correctly, while you were in the usual rush to get things done? Always use a frequency chart when setting up radio/wireless systems, as there is a very good reason they list a jumble of numbers, rather than round off to the nearest round figure. NO you can’t use all 16 frequencies on those manufacturers systems out there who kindly give you all these on channel 70, when even the best systems available can only manage four – six systems working together. They are spot frequencies to help you select those best to use in a particular venue.