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The world of RF (I)

rf

We present some necessary tips to take into account within a TV channel and how all microphones, pointers (monitoring systems), intercoms among others, should work properly.

Juan Moreno*

It is no secret to anyone that technology is advancing more and more by leaps and bounds, the implementation of new analog or digital transmission systems and the ease in which today we can acquire a wireless system, has generated an awakening in sound engineers focused on RF. In addition to this, the electromagnetic radio spectrum has become a challenge, because the professional or amateur world wants to have fewer cables during their transmissions.

New digital television technologies have worsened the picture, leaving within the radio spectrum, very minimal areas to operate our wireless systems or In Ears. That is why we cannot think that just by turning on our system we will operate without any interference or interaction with other wireless systems, digital or analog TV that are in the spectral environment.

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The objective of this article is to provide the necessary tips that we must take into account within a TV channel and how all our microphones, pointers (monitoring systems), intercoms among others, must work properly, for this we will use some similar principles that apply to the management of audio waves, however, it will be necessary to see some concepts of radio frequencies to understand the operation of these.

1. Recognition and evaluation of the workplace
Regardless of the brand of the tool or software that is being used to perform a frequency scan, we must start by making a measurement without anything being turned on (some manufacturers of wireless systems allow to turn off the RF transmission while the system is still on) Sennheiser is one of them, Lights-Screens-Microphones-In Ears-Intercoms ... etc.

Having a panorama of the electromagnetic spectrum in place will allow me to locate the frequencies (carriers) that are in the environment and mark them with a name that allows me to identify them during the event. The objective of this pre-show scan will allow me to assign frequencies for my systems without interference. An aspect that we must take into account when making a coordination of frequencies is to identify the analog and digital television channels, the latter having more importance since their bandwidth occupies the total of 6MHz of bandwidth and change in quantity and location according to the country where we are.

2. Inventory
Due to the high congestion within the spectrum and even more with the appearance of digital television channels, it is necessary to locate and name within our Scan (step number 1) the systems that we are going to use in the service, differentiating these, from analog television channels, Intercoms, radio stations, lights, LED screens ... etc., using colors, names, zones and type of equipment. This is why each manufacturer has developed a specific mode of recognition that will allow you to know the range of frequencies or bandwidth where they operate. In certain softwares such as WSM it will be necessary to have the following information in order for our analyzer to identify the frequencies that best suit us:
a. Manufacturer (Sennheiser)
b. Equipment reference EJ. EW 300 G4-A ... ETC
c. Operating Range or EJ Bandwidth. At 470-558 MHz

This last information is embodied in different ways depending on the brand we are using, in some it will be a letter or in others a letter followed by a number which will be found on the back of the receiver or transmitter (in the case of an In Ear); e.g. Sennheiser A, B, G... ETC. On the other hand, providing this information to a software will allow us to ensure a unique transmission and assignment of frequencies in each equipment in addition to avoiding side effects such as those intermodulations that may occur due to interaction between transmission antennas, a topic that is very important when buying a team or installing the antennas within an event which will have a good number of wireless systems.

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3. Antennas
To know what type of antenna we have to use, it is necessary to explain the different types of antennas, and what is the correct location and installation according to the sense of their polarization, directionality and gain.

Antennas are of fundamental importance for operating wireless systems, as they constitute the interface between the high-frequency radiation in a cable and the space around us. The antennas allow the high frequencies of the circuits to be transmitted in a controlled manner and then retrieved in a cable that carries them to the receiver. They are three-dimensional structures with two poles that conduct electricity. For this reason they are also called dipoles. RF cables have an impedance of about 50 ohm, while the air around us has an impedance of 377 ohm (a physicist can calculate this in detail). Antennas are used to equalize impedance in the same way that, for example, a horn is used to equalize the impedance between the air and the diaphragm of a speaker. A speaker is much more efficient with a speaker attached to it.

On the other hand, antennas can be classified by the type of polar pattern (directional or omnidirectional) and can also be classified as active or passive.

At the time of a Show, the parameters initially mentioned will be fundamental and indispensable for the correct location of the antennas. The polar pattern will help us to focus the RF in such a way, that in the case of the In Ear, we are not dirtying the spectrum by issuing RF signal to everywhere, similar case with wireless microphones which must have antennas that only capture the desired information rejecting everything we do not want to capture.

This is why manufacturers recommend having a prudent distance between transmission and reception antennas of at least 3 meters, to prevent them from interacting with each other or having a distance of 1/2 wave or a complete wave between 1/2 wave antennas which come with wireless receivers. Below, we will show some recommendations on the proper use of antennas and their proper position.

As an important note, you should know that the more antennas you use, the worse the RF will behave so it is better to have distributors for (Microphones) or combiners for (In Ears), avoiding as much as possible interactions between transmission antennas which can produce an effect called intermodulation or (unwanted frequency amplitudes), different case in the receiving antennas because the proximity or proximity could affect the polar pattern of capture generating something called comb filter.

For this reason we recommend investing in systems that are digital which are not so likely to present intermodulation products however this is not 100% safe (check with the manufacturer) in the case of Sennheiser systems EW-D, D6000, or the top series of Sennheiser D9000 will operate without problems and without the need to lower the transmission power, in conclusion, they are very linear systems.

An important aspect to consider is the gain of the antenna and the size of it. To understand this aspect it must be clarified that the size of the antenna and the wavelength are directly proportional, the longer the wavelength, the larger the size of the antenna, and the smaller the wavelength the smaller the size of the antenna.

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There is a wide variety of antennas with different shapes. Its dimensions depend on the wavelength of the RF transmission. The simplest form of antenna is a piece of coaxial cable that has been split in two with the protective cover on the right and the copper wire on the left. Basically an antenna/dipole is nothing more than an open capacitor of a resonant circuit in series, which is the basic circuit of every transmitter/receiver. If the length of the exposed wire and the protective cover correspond to approximately 2 x 1/4 of the RF wavelength, then it will be able to work. However, there are other antennas that will give us a greater gain such as the Yagi type, Log Periodic or circular polarization, these antennas will provide a gain of 7 to 14 dBi greater compared to those of 1/2 wave or 1/4 wave, offering greater coverage and therefore distance in giant scenarios.

Antennas can send and receive, even at the same time. Of course, a handmade antenna like the one we just described does not meet the requirements of regular use, but in case of emergency it can be of great help. Even a small wire, such as that of a clip placed in the contact in the middle of a BNC connector, offers considerably better features than having no antenna at all. That's why engineers have created antennas that are electrically reliable and mechanically robust.

Keep in mind that the antennas according to the previous information can operate as transmitters or as receiving antennas, however, only the passive ones will be able to work transmitting signal or receiving, instead the active ones will only work as receiving antennas.

On the other hand, the height when installing the antennas is an important point. The effective distance of a wireless connection depends on several factors. The most important is the line of sight between the transmitter and receiver. With a clear line of sight and favorable conditions it is possible to establish a connection of up to 1 kilometer in transmission powers of 250mwts, but if the transmission between the transmitter and the receiver is hindered, for example, by a crowd, it can become a problem to try to transmit even 10 meters away.

After decades of experience, there are some rules and recommendations on how to place and use transmitters. For example, a practical recommendation is to keep moisture out of the microphone, transmission antenna, and transmitter frame. The worst case is the direct contact of the transmission antenna with the skin, with which the loss of effective radiated power can reach 99%.

Finally, avoid having long cables and 75 Ω (typical in MADI or SDI video) that generate a significant loss in the signal you are sending or receiving, in case of using cables with a loss of more than 5 dB for most manufacturers is relevant, so you must insert a Booster to lift the signal and recover the lost dB.

Finally, it is worth noting that there are no symmetry requirements in the antenna system. Feel free to mix and match any type of antennas to achieve proper coverage. The lengths of the antenna cables on sides A and B of a Diversity receiver also do not have to match. It's about keeping the loss to a minimum.

4. Antenna distribution
To begin with, remember that installing a multiroom system requires several stages and elements such as: BNC Cables, Boosters, Antenna Distributors, Antenna Combiners, Antennas and a configuration of the equipment in terms of transmission powers and construction materials, the latter being in most cases, not modifiable.

As a first step, we must have a recognition of the place and if possible a plot that allows us to see the distances between each of the spaces. Having an estimate of these distances will allow us to know the amount of antenna cable of (50 Ohm) that we require and therefore the loss that this same cable will have according to the manufacturer, thickness of the cable, if it is a single filament cable or it is a braided cable and the frequency at which we are going to operate our wireless systems. Remember that the higher the frequency of operation, the greater the loss when the information travels through the cable.

There are two general types of antenna combiners available: passive or active. Passive antenna splitting is performed with simple in-line devices that provide RF impedance adaptation for minimal loss. Even so, a single passive division results in a loss of about 3 dB, which can translate into a loss of distance.

To allow the combination of antenna signals and overcome the loss of passive combiners, active antenna combiners are used.

Using an antenna combiner which is commonly used for In Ears will allow us to avoid a loss of 3dB for each spliter (it must be remembered that for an installation of 4 environments 6 passive Spliters will be necessary which translates into a loss of more or less 18dB). Next, we will show you how we can avoid this and the importance of using active antenna combiners.

Now the question will be, what should be my concern when using antenna combiners to perform antenna distribution for various environments? Your only concern will be to compensate for the loss obtained with long cables which do not exceed the loss of 5dB. For this, the use of active antennas or the use of Boosters will be of great importance when it comes to compensating for losses in BNC cables. make sure the basic antenna requirements are covered.

In short, when deciding where to place antennas, it's not about "how far away can I zoom out my antennas?", but "what kind of cable should I use?" With the right combination of cable and amplifiers, you can get there. Simply add or subtract the variables shown here, keeping in mind that you should try to keep the total loss at less than 5 dB.

Antenna system gain formula: (Antenna gain) + (Total amplifier/amplifier gain) - (cable loss) - (Split loss)

Moreover, some receivers come equipped with built-in cascading BNCs for the antennas, eliminating the need for external antenna distributions. Typically, all cascade receptors must be in the same frequency range, and there are a finite number of receptors that can be cascaded. Refer to the manuals of each Sennheiser equipment for these limitations.

For newer digital equipment such as the D6000 cascade use 8 systems and in the D9000 digital system, use only 4 levels through your BNC connectors.

Keep in mind the following tips and tests that you must perform once the installation is finished including all the above:
Once the installation and the proper antenna coverage are finished, the user must go to the first enclosure and turn on the microphone with the antennas of that enclosure disconnected, the idea is to be able to visualize through a measurement software called WSM that there is the least amount of signal captured by the other antennas which are installed in the neighboring rooms. For this it is important to configure an adequate transmission power for the perimeter where it is going to be used. In other words, at best only two-thirds of the power produced by the transmitter is radiated by the antenna. Thus, from a transmitter of 30 milliwatts only a maximum of 20 milliwatts is radiated. This value is known as radiated effective power (ERP).

Then we will repeat this same step in all the rooms (the antennas of the room where the microphone is must be disconnected only to do the tests, after the tests we must check that everything is working and connected both in the antennas and in the receivers).

Having a receiver receiving x number of times the same information can be a problem for those signals that are in a different phase resulting in a phenomenon called Drop Out caused by multivia interference, for example, However, True Diversity systems guarantee that this type of phenomena do not affect the performance of the system.

Keep in mind that the amplifier should always be at the end of the cable antenna, not at the end of the receiver where what we would be amplifying is the floor noise. Some directional antennas may already have a built-in amplifier. Again, be careful not to amplify the signal too much!

Another solution is to use multiple receivers and antennas tuned to the frequency of a single transmitter. The audio outputs of the receivers can be combined in a mixer to allow continuous signal pickup from multiple locations. However, some form of audio level control should be employed as the audio level of such a system will increase by 6dB each time the number of active receivers doubles. That is, if the transmitter is picked up by two receivers at the same time, the overall audio level will be higher at 6dB than when only one receiver detects it. Automatic mixers can control this effect.

• RF over fiber is a possible solution when it is expensive, but necessary when:
• For installations where the distance between antennas and receivers would cause unacceptable loss
• For long coaxial cables over 200 m (650 ft)
• When excessive gain is required to compensate for cable loss
• Careful planning and execution (no plug and play)

*Juan Moreno is Sennheiser's Business Communications Sales Manager for Colombia, Ecuador and Peru. You can contact him through juan.moreno@sennheiser.com

Richard Santa, RAVT
Author: Richard Santa, RAVT
Editor
Periodista de la Universidad de Antioquia (2010), con experiencia en temas sobre tecnología y economía. Editor de las revistas TVyVideo+Radio y AVI Latinoamérica. Coordinador académico de TecnoTelevisión&Radio.

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