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Can 100 mW transmitters be safely used in theatre applications or is 100 mW far too much power?


--Clearing up the Issue of Higher Transmitter RF Power and IM

For many years, Lectrosonics has built wireless transmitters that are higher-powered than those from other vendors. In addition, when we say a typical power of 100mW, we don’t mean that we had one engineering sample reach that power with a westerly wind. We center our production on 100mW with small manufacturing variations both up and down. We have built 100mW units for what we feel are four good reasons:

  1. Increased range
  2. Fewer dropouts
  3. Reduction of interference
  4. Better signal-to-noise ratio

There have been discussions about problems with higher powered units like ours, but the only real negative of higher power when properly implemented is a slightly increased drain on transmitter batteries. Since most of the battery power is used supporting digital processing in the Lectrosonics’ transmitters, the increased RF power is only a minor consideration. Another way of saying it is, if you have made the choice of digital processing in the transmitter, you might as well have increased RF power too, since it doesn’t change the battery life that much. The new generation of rechargeable batteries can reduce battery costs for all wireless systems and battery usage isn’t quite as important a consideration as it was several years ago.

--Design Considerations

Careful design has removed the two other problems that are sometimes discussed when higher power is considered. The possible problem of increased intermodulation (IM) in the output stage of the transmitters at higher power has been solved by using an output isolator in the antenna circuit. This prevents two transmitters that are physically close together from creating IM products. This isolator is unique to Lectrosonics’ transmitters and is in all transmitters except the LM series. Possible IM in the receivers has likewise been solved, because Lectrosonics’ receivers have always had higher power RF stages for substantially better IM rejection than competing receivers, so the increased transmitter power is no problem whatsoever. 

The technology used in the Lectrosonics products is all fine and good but the real question revolves around what happens in actual use. What about a real world situation with lots of transmitters on a stage? We’ve been hearing stories that 100mW transmitters are absolutely unusable in a stage environment and that 100mW systems will wipe out the wireless operations of theatres over city blocks, if not the entire Eastern Seaboard.

--Real World Numbers

Let’s first apply a little common sense and then run some numbers, first looking at general reception issues and then those specific to transmitters:

If intermodulation was really a consistent problem with 100mW transmitters then it would be only slightly less of a problem with 50mW transmitters, all things being equal. The only way to change from a supposedly “real” problem to a rare occurrence would be to make a radical change in transmitter power such as down to 5mW or less. A 3dB difference in power doesn’t mean much when typical signal levels on a stage are making 50dB swings as people move around. Just as an increase in power from 50mW to 100mW doesn’t solve all dropout problems and only extends effective range by 30%, reducing power from 100mW to 50mW won’t solve all IM and transmitter interference problems if there were any to begin with. In the same way, even a 3 to 1 change from 100mW to 30mW is insignificant when signal levels at the receiver can vary by 100,000 to 1, i.e., the previously mentioned 50dB.

To show why there shouldn’t be IM problems at either power level, let’s do the numbers. We will analyze the signals that would be present at a receiver from a 100mW transmitter in the worst possible situation and then in a real world situation. Even better, we will use published numbers from other manufacturers rather than our own measurements (even though they are about the same). We will do a third order analysis since it is accepted as being the worst case. Second and fourth order products are not a problem because they are totally removed by the receiver’s RF filters and fifth order and higher are at much lower levels than third order.

Posted 1 year agoby LectroAdmin