Why use aluminum trays for hand transmitters or bodypacks?

Juan David Moreno *

More and more microphones are used every day. Big scenarios have become the biggest challenge an RF engineer can have, and artists want most of their instruments to work without the need for a cable. The staging and the show that will be offered to the public should be as impeccable and professional as possible.

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It is for this reason, that all those who work behind the scenes in the coordination of frequencies know and understand that there is an interaction between bodypack transmitters and hand transmitters which are turned on and off in short periods, and sometimes very close to each other. This interaction between transmitters will cause in non-linear systems, an effect called intermodulation products, a phenomenon that will affect our frequency coordination, to the point of causing a disaster in vivo and affecting the artist who is playing at that moment.

In another situation this intermodulation effect may be momentary, due to the interaction of two singers who put their microphones together to make a chorus, or in the case of two guitarists who approach their backs to make the most anticipated guitar solo of the night . It is at this moment when this phenomenon occurs, which may affect all the previous work of the RF engineer.

Also, one of the points of greater attention and that we must always keep in mind is to avoid this interaction before handing over the microphones to the musicians. For this reason, rental companies often have a person in charge of the microphones, who is responsible for placing them on the top of the rack, storing them one on top of the other.

However, this is not enough and many times the transmitters are turned on within said lid due to an order from the monitor engineer, who wants to perform a Line Check before the event begins. The result of this test will be even worse, due to the number of microphones interacting with each other.

In accordance with the above, we always suggest using aluminum trays, which will allow this intermodulation phenomenon to not affect our frequency coordination. The technique of aluminum trays comes from '' the big leagues '', the man many call "the father of frequency coordination", James Stoffo.

"Of all the tools I wear in a show, the aluminum trays make the biggest difference," James Stoffo.

James has worked in professional audio with previous versions of wireless microphones such as Shure UHFR models, and discovered that multiple transmitters turned on behind the scenes, waiting for audio signals, result in intermodulations that impair and destroy hours of careful coordination work. frequencies and the ability to accurately monitor RF levels.

What is really happening when you put a transmitter in an aluminum tray is the following: not only are you preventing the transmitter from interacting with others but you are creating a large number of wave fronts or reflections inside that tray that cancel out one with others, resulting in very little of the RF signal coming out of the tray. "

One of the last recommendations we make to those who do RF is that the trays will give us the possibility to better organize our wireless equipment to the point of placing in these boxes other capsules that the artist will use or different lapel or headset microphones , which are part of the event and has a specific order within the show.

Is there any technology that can make these interactions between transmitters not so harmful? Technologies such as Axient Digital will provide a better performance of the system, making it more linear than those systems of older and similar technology. Turn on transmitters and put them all together at the same time will no longer be a problem. We will be able to occupy spaces of 6 MHz with 47 microphones, reducing the distance between the carriers, without them coming into conflict and much less without generating intermodulation products due to the proximity between the transmitters.

* Juan David Moreno, Market Development Latin America Shure. You can write to moreno_juan@shure.com.co

Richard Santa, RAVT
Author: Richard Santa, RAVT
Journalist from the University of Antioquia (2010), with experience in technology and economics. Editor of the magazines TVyVideo + Radio and AVI Latin America. Academic Coordinator of TecnoTelevisión & Radio.


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