How close can you go..?

[Steve1812]

Trying to get to grips with the Sennheiser 300 G2 body packs and receivers.

They work fine at our theatre where we have 12 of the things.

Weird problem yesterday where one of them kept cutting out on the stage but worked fine when in the sound booth.

Battery, mike lead/head set etc all in good condition.

Checked the frequency and it had shifted closer to one of the other sets - there is a ghost at this theatre Im told...

 

Heres the question - In the groups massive experience, how close can one transmitters frequency be to a another one?

 

ie if a transmitter is on 840.00 MHz - how near will another need to be if there is interference - would 840.10 MHz be to close etc.

What is the minimum difference in frequency you guys like to see?

These units can be adjusted in 25 KHz increments.

 

Dont shoot me down in flames Im new to this type of radio mike.

 

Thank you,

 

Steve1812

[lonfire]

Hi,

look here:

Sennheiser Intermodulation and Frequency Management Software

 

regards

chris

[Ynot]

It's not just about proximity of frequencies to the next one - more about harmonics - that Senn site above will give you the info required.

However, your licence should give you the available frequencies, which should be adhered to.

You DO have a licence, yes?

[boswell]

He won't have post 2012 :** laughs out loud **:

[Bobbsy]

To try and sum up all the cryptic (and not overly helpful) comments you received:

 

First, as lonfire and Ynot have said, there isn't a simple formula that frequencies can be x Hz apart. There will be a minimum distance determined by the FM deviation and the frequency filtering in the gear but, even if you exceed this minimum, you may still get interference from harmonic/intermodulation interference. The only way to work out which frequencies can work together is through complex calculations--which, luckily, can be done for you by that Sennheiser software that was linked to.

 

Second, the issue of licensing. There is only a small slice of licence-exempt bandwidth--enough for a maximum of 4 channels--between 863 and 865 MHz. Anything outside this range requires a licence. The next tranche of frequencies (and easiest to license) are known as "UK Shared" and are just below the licence free channels making it easy to mix and match between the licensed and licence-exempt frequencies. The UK shared licence is for 14 specific frequencies (not all of which work together) but you'll easily get your 12 in. Contact JFMG for your licence which will cost £75 or £80 per year depending how you pay. If your mics are rented, they should already be licensed and the company should have provided paperwork.

 

Third, if mentioning 840MHz was more than just an example, you need a more expensive form of licence, also from JFMG.

 

Finally, the mention of 2012 was because all the regulations change then (due to the move to digital TV) and the frequencies mentioned above will no longer be licensable or legal.

 

Hope this helps. For your original problem, check into your license and, if UK Shared, plug those frequencies into the Sennheiser software to get a 12 channel frequency scheme.

 

Bob

[Steve1812]

Thank you - yes, the info you give does clarify the other replies - I did say I'm new to this. It's an "amateur" theatre and I do not know if licenses exist, though they now know of 2012 and its consequences. Cheers for the "idiot" guide, it does help.

 

Steve1812

[paulears]

You didn't say if the rack is out front, so when you are on stage this is the greatest distance, or if the receivers are stage side - as in closest.

 

Another thing to keep in mind is that another property of radio systems is that the receivers can be de-sensitised by transmitters working very close to their frequency. So let's say the actor is at the furthest working location in the performance area, so their signal strength is at it's weakest. An actor wearing the transmitter pack on the next door channel stands next to your receiver rack - let's assume you're using local aerials, not remoted ones. The weak signal that 'just' give solid reception might go noisy, or vanish under the squelch when the very strong signal gets into the receiver. De-sense is quite important as it can make working out why things go wrong suddenly, quite difficult. A turn leaving a hand-held on top of the receiver rack can easily wreck something that worked fine many times before - then the next show works fine if the hand-held isn't put back in the same problem location.

 

If the frequency programmed in had changed, then let's face it - somebody is twiddling. You cannot select random frequencies have have them trouble free. The intermodulation problems mentioned above can also be made worse by changing locations - increasing or decreasing levels of certain frequencies and allowing the mixing to take place. Sennheiser put channels into banks to help with this, but most seem to simply be operated in frequency mode, people just going up or down to what they think is clear space. This is exactly why the big firms send a technician to look after frequency planning at big prestigious events - it really is a science.