Radio Frequencies

[iamchristuffin]

Hi all,

 

I got round to wondering how close two Radio Microphones (in this case Sennheiser ew100 G2) can be in frequency before there is interference? I tried it in a warehouse with two ew500 G2's, and got interference with a difference of about 0.3MHz, but I was wondering if this was always the case, or if there was differences (dfferent systems/rooms, etc.)

 

Many Thanks,

 

Chris

[Andrew C]

It's been done here several times. Have a look at Sennheiser's web site, where it will show you clearly. In short, you can get 4 in the UHF licence exempt band, Group 8.

[aidso]

you should be able to get it down to about 0.26MHz but it all depends what other stuff the transmitter is sending. I.e if pilot is on. But I try not to go any closer than .5MHz

[6th Order]

and search the blue room for old topics on the same subject, where eventually someone says it's more complicated than 'how close' and mentions intermodulation.

[johndenim]

This begs the question,

 

How many G2 transmitters can be used at the same time?

 

John denim.

 

(Sennheiser website seems to be down) :0

[Shez]

How many G2 transmitters can be used at the same time?

 

Depends on how much money you've got available for licencing...

[Bobbsy]

you should be able to get it down to about 0.26MHz but it all depends what other stuff the transmitter is sending. I.e if pilot is on. But I try not to go any closer than .5MHz

 

I don't know where you got this number but it's just plain wrong.

 

The peak FM deviation on the Sennheiser G2 system is +/-48kHz so, with the filtering working perfectly (it doesn't) and no regard for Intermodulation (you have to regard it) the closest you could get would be 96kHz--round off to 100kHz.

 

However, as has been said you CANNOT simple assign an arbitrary channel spacing that will be safe because, as you turn on extra units, IM problems become real and serious.

 

This was all discussed within the past couple of weeks. Look at THIS TOPIC but be sure to read it all because the first couple of replies started with the same sort of misinformation that this one has.

 

Bob

 

Edited to add: Re-reading the original question, I think it's time for a brief and simplified lesson in FM radio theory.

 

When you set the frequency on an FM system like radio microphones, what you're actually selecting is the "Centre Frequency". However, as soon as you apply an audio signal (or video in the case of, for example, satellites) this is modulated onto the original carrier causing it to "deviate" a specific amount above and below the centre frequency. In the case of audio, the higher the level of the sound the wider the deviation. In the case of the Sennheiser G2 system, the deviation at peak audio level is 48kHz above and below the original carrier.

 

Therefore, saying you tried it in a warehouse and found you could be 30kHz apart is an invalid test. Unless you know that both mics were transmitting at peak level at the same time, then you weren't covering all eventualities. Alas, the time this will come back and bite you will be on the big finale scene when suddenly all your mics will start to drop out at just the wrong moment.

 

As noted above, channel spacing is only part of the issue though. Besides interference from adjacent transmitters, every FM system generates extra unwanted frequencies at specific spacings away from the main one. If you turn on enough transmitters you will eventually get one of these spurious frequencies that interferes with one of your main channels. That's why, instead of just saying "put them XkHz apart" Sennheiser give you, for free, their SIFM software which lets you calculate what frequencies are safe. Indeed, the way intermod works, you tend to be better off to have slightly uneven spacing. The link to SIFM is in that previous topic I mentioned...I won't copy it so you're forced to go do some reading!