How much of a problem is IM really?

[Hockeybod]

I understand the theory behind IM and frequency planning, and I'm used to using frequency planners such as Wireless Designer, but I'm faced with the problem of needing to cram 6 Sennheiser W500 G2 mics into 830 to 832MHz, and another 6 into 863-865MHz. For clarity, those are the only frequency ranges that the Ofcom licences and packs will allow.

The max I can co-ordinate is four mics to each frequency range, but if I run another two how likely am I to run into real life problems? They will be used indoors with line of sight paddle antennae, so potentially very good signal strength.

Am I wasting my time even trying?

[sandall]

1 hour ago, Hockeybod said:

Am I wasting my time even trying?

Probably😥

[J Pearce]

That'd be a very bold bit of frequency planning. I'd be looking at licensing some additional spectrum - it's not silly money and isn't hard to do.

[Hockeybod]

1 hour ago, J Pearce said:

That'd be a very bold bit of frequency planning. I'd be looking at licensing some additional spectrum - it's not silly money and isn't hard to do.

That's the problem, there aren't any other frequencies available from Ofcom that fall within the range of the packs/receiver

[Hilary Watts]

I may well be burnt at the stake as a heretic for posting this but amateur shows in two local theatres seem to get away with 16 or possibly even 18 channels crammed into Channel 38 using EW100 G3 systems with absolutely no regard for the recommended Sennheiser frequencies and IM doesn't seem to be a significant problem.  I'm not usually there when the system is set up but the procedure seems to be that initially the frequencies are fairly evenly spread across the available spectrum and then if a problem occurs in rehearsal one of the offending microphones is tuned to a different frequency apparently chosen more or less at random.  Paddle aerials are used at the back of the auditorium about 20 metres away from the stage. I don't think the people who set up the system have heard of intermodulation.

I certainly don't recommend it but if you've got rehearsal time in which you can experiment then you could give it a go...

[Hockeybod]

1 hour ago, Hilary Watts said:

I may well be burnt at the stake as a heretic for posting this but amateur shows in two local theatres seem to get away with 16 or possibly even 18 channels crammed into Channel 38 using EW100 G3 systems with absolutely no regard for the recommended Sennheiser frequencies and IM doesn't seem to be a significant problem.  I'm not usually there when the system is set up but the procedure seems to be that initially the frequencies are fairly evenly spread across the available spectrum and then if a problem occurs in rehearsal one of the offending microphones is tuned to a different frequency apparently chosen more or less at random.  Paddle aerials are used at the back of the auditorium about 20 metres away from the stage. I don't think the people who set up the system have heard of intermodulation.

I certainly don't recommend it but if you've got rehearsal time in which you can experiment then you could give it a go...

That's what I was hoping to hear. It's an amateur production so not a disaster and worth a try at rehearsal. Thanks

[sunray]

3 hours ago, Hilary Watts said:

I may well be burnt at the stake as a heretic for posting this but amateur shows in two local theatres seem to get away with 16 or possibly even 18 channels crammed into Channel 38 using EW100 G3 systems with absolutely no regard for the recommended Sennheiser frequencies and IM doesn't seem to be a significant problem.  I'm not usually there when the system is set up but the procedure seems to be that initially the frequencies are fairly evenly spread across the available spectrum and then if a problem occurs in rehearsal one of the offending microphones is tuned to a different frequency apparently chosen more or less at random.  Paddle aerials are used at the back of the auditorium about 20 metres away from the stage. I don't think the people who set up the system have heard of intermodulation.

I certainly don't recommend it but if you've got rehearsal time in which you can experiment then you could give it a go...

The problem with that approach is it will work fine with an empty venue but fill it with 200 mobile phones and right in the middle of an important line...

[sunray]

10 hours ago, Hockeybod said:

I understand the theory behind IM and frequency planning, and I'm used to using frequency planners such as Wireless Designer, but I'm faced with the problem of needing to cram 6 Sennheiser W500 G2 mics into 830 to 832MHz, and another 6 into 863-865MHz. For clarity, those are the only frequency ranges that the Ofcom licences and packs will allow.

The max I can co-ordinate is four mics to each frequency range, but if I run another two how likely am I to run into real life problems? They will be used indoors with line of sight paddle antennae, so potentially very good signal strength.

Am I wasting my time even trying?

I always think I'm lucky if I get away with 4 in close proximity in 2MHz, however most of my kit is Trantec S4.4 sort of quality

[timsabre]

9 hours ago, Hockeybod said:

That's what I was hoping to hear. It's an amateur production so not a disaster and worth a try at rehearsal. Thanks

RF conditions change from moment to moment so unfortunately working at rehearsal doesn't necessarily mean it'll work the same on shows. You might get away with it... You might not. If you're prepared for that then there's no harm trying it. 

[J Pearce]

If the surrounding spectrum is also full (assuming it may well be as it's non-licensable), don't forget you'll have intermod with that, not just intermod with your own frequencies. As discussed above, transmissions in that spectrum might not be continuous.

You might get something working. You might get something that works just about if you don't listen too closely for intermod artefacts. You might have a horrible mess and have to start cutting mic packs til something starts working again.

WSM is particularly strict with its intermod predictor, and plays things very safe (Shure WWB is the same). You can usually get a few more in than WSM predicts, but I can't really see how anyone gets 18 freqs working well in C38 on analogue radio mics. 12 to a full TV channel (as was...) is just about doable with the higher end models that have better filtering, and that's already very 'edge case'.

It's worth noting that when avoiding intermod you usually want lower radio power levels, not higher. So don't run the packs in high gain mode (I can't remember if G2s have that option), and keep an eye on how much antenna and distro gain you have. You want to keep the wanted frequencies above the squelch, and the unwanted below. Naturally this may also mean adjusting the squelch level on your receivers. Directional antenna may help reduce unwanted noise, but equally I've found I have more intermod issues when running directional antenna.
If you're spacing mics that close in the available bandwidth, watch out for intermod generated by having packs adjacent to each other - either for double mic'ing or at the mic table. The traditional solution at the mic table is foil takeaway containers to shield packs from each other.

[Peter F]

WWB will generate 10 compatible frequencies in those ranges without me having to compromise the compatibility settings too much.

 

Group 1 (10)
EW 500 G2	E		G:-- Ch:--	830.000 MHz
EW 500 G2	E		G:-- Ch:--	830.425 MHz
EW 500 G2	E		G:-- Ch:--	831.075 MHz
EW 500 G2	E		G:-- Ch:--	831.425 MHz
EW 500 G2	E		G:-- Ch:--	831.975 MHz
EW 500 G2	E		G:-- Ch:--	863.000 MHz
EW 500 G2	E		G:-- Ch:--	863.425 MHz
EW 500 G2	E		G:-- Ch:--	864.100 MHz
EW 500 G2	E		G:-- Ch:--	864.450 MHz
EW 500 G2	E		G:-- Ch:--	864.975 MHz

[paulears]

The trouble with all of this is it is to do with so many varying factors. The critical thing is the ratio of wanted to unwanted signals that appear at the receiving antennas for ‘collection’. The various calculators and lists just highlight frequencies that are potentially destructive when combined, but it’s a sort of fruit trifle in reality. Until it crosses the line it is fine but the line crossing can be very quick and yet hard to repeat. One transmitter pack entering the arena can be sufficient to reduce the system to instability. I have had a little success when using too many by considering where people might be on stage. Remember that increasing the receive antenna spacing can make signal strengths in its capture area less IM ‘busy’. Inverse square law works well here, making the strong signals stronger and weak ones weaker. If your two antennas are close then it’s more strong and modest levels making IM products worse than strong and weak, where they annoy more.

[richard_cooper]

16 hours ago, Hilary Watts said:

I may well be burnt at the stake as a heretic for posting this but amateur shows in two local theatres seem to get away with 16 or possibly even 18 channels crammed into Channel 38 using EW100 G3 systems with absolutely no regard for the recommended Sennheiser frequencies and IM doesn't seem to be a significant problem.  I'm not usually there when the system is set up but the procedure seems to be that initially the frequencies are fairly evenly spread across the available spectrum and then if a problem occurs in rehearsal one of the offending microphones is tuned to a different frequency apparently chosen more or less at random.  Paddle aerials are used at the back of the auditorium about 20 metres away from the stage. I don't think the people who set up the system have heard of intermodulation.

The only time I've seen this approach actually work, I'd describe Channel 38 as Channel 38...ish definitely not 100% complying with the license they didn't have....

[sunray]

7 hours ago, richard_cooper said:

The only time I've seen this approach actually work, I'd describe Channel 38 as Channel 38...ish definitely not 100% complying with the license they didn't have....

I was at a function earlier this year they had 24 channels in use between 844 and 865. Operator claimed he didn't know the band had gone and it was all license free.