Radio Mic Interferance

[SoLiEn]

Theatre company I'm doing the sound for has 4 x Trantec s4.5 radio mics, frequencies minimum of .500 apart, the two in particular are set on 863.025 and 863.725, so they are .700 apart, and I am still getting interferance between the two of them, has been giving me a headache, anyone got any suggestions as to why its doing this?

 

Dan

[Shez]

Firstly (and slightly pedantically) 863.025 shouldn't be used as you're infringing on the licensed ch.69 band. The lowest freq you can use without a licence is 863.1

 

The gap between frequencies isn't as relevant as the choice of frequencies themselves - picking them based on the gap will always end messily as you're bound to get intermodulation interference between them and the maths to work that out is a little more complex. If you need four licence free frequencies, try 863.1, 863.5, 864.3 and 864.9 - these will generally be quite happy working together.

[SoLiEn]

And how can I go about changing the frequencies on the Belt pack, it only flits between those four!

 

those are the frequencies that Trantec supplied them on. they have not been altered since.

[Shez]

(According to the spec sheet for those, they only use a deviation of 22kHz, so 863.025 should be alright from a licensing standpoint.)

 

Are you trying to use them alongside any other systems e.g. channel 69 kit? Only one of those four frequencies will work with the shared ch69 ones.

 

As those four are licence exempt, you could be getting interference from other nearby users - pub karaoke is often a culprit.

 

Can you describe what this interference is like? Is it always there or does it come and go? Does it affect both those channels or just one of them? Is it like white noise or can you hear intelligible sounds?

 

Are you using an antennae distro or just the receivers' individual aerials? How far away are the receivers from the stage? Do their RF power meters display reasonable levels?

 

Lots of questions I know; it can be tricky to remotely diagnose RF problems...

[Simon Lewis]

Further to Shez's excellent questions, can you physically separate the four receivers?

[Lamplighter]

Dan

It is unusual though not impossible to get interference between just 2 units. As you mention 4 are used could you post all four frequencies. This may just reveal the problem.

Brian

[SoLiEn]

Frequencies are:

 

A)863.025

B)863.725

C)864.100

D)864.500

 

(last one I think is correct)

 

in answer to questions:

 

no 69 kit

 

no other radio microphones in area as far as I know, no pubs doing karaoke

 

interference, is where Freq A encroaches and is received by receiver B, therefore cutting out the transmissions from Beltpack A

 

Using an ADU, Have checked all connections, no cross wiring anywhere

 

RF Meters display good quality Levels

 

and Yes I can physically separate all four receivers. although they are currently in rack mount.

 

 

Cheers Guys

 

Dan

[Shez]

Trantec's website suggests that the freqs might be 863.025, 863.725, 864.300 & 864.650.

 

Do you have access to a frequency meter which will tell you if each pack is transmitting dead on frequency? If not, this could be a process of elimination - try retuning your third and fourth packs to the first two frequencies (switch the other two off of course) and see if the problem remains. If all is well, it suggests that one of the first two packs is misbehaving. If the problem is still there, it might be a receiver at fault. Try then retuning the third & fourth receiver to the first & second frequencies and see what happens there.

 

Before doing that though, with only one pack switched on (try A then B), do both receivers A & B indicate receiving a signal?

[SoLiEn]

With only one pack switched on only the correct receiver is getting a signal. and unfortunately, both mics do need to be on simultaneously! and yes, those are the freq's. and no, unfortunately I do not have a freq meter!