Radio Mic issue

[erik10_4]

I'm currently lighting a show (with a colleague operating sound some distance away). During the show (on two nights) there has been intermittent booming vocals from one of the performers radio mic. When I spoke to the colleague tonight he demonstrated that the UHF belt pack being used by that particular performer (transmitting on 863.700 MHz) was being received on a VHF Radio mic receiver (173.800 MHz). Whilst, on the surface, this seemed to be the case, I would not have thought this possible with the respective frequencies being so far apart. I did not have the opportunity to discuss or investigate at length tonight as the venue was being locked up. This is something I have never experienced before. Is it physically possible to get crossover from a UHF system to a VHF system?

[Lamplighter]

Is it physically possible to get crossover from a UHF system to a VHF system?

 

The short answer is yes. It shouldn't normally be a problem if all is well, however if the UHF pack is faulty it could be generating some strange outputs, best looked at with a spectrum analyser. It is also possible, if there are multiple frequencies in use, that it could be an intermodulation issue. The frequency separation will not prevent the problem if the maths is right and a suitable non-linear medium is available. I've seen it happen more than once.

Brian

[erik10_4]

Is it physically possible to get crossover from a UHF system to a VHF system?

 

The short answer is yes. It shouldn't normally be a problem if all is well, however if the UHF pack is faulty it could be generating some strange outputs, best looked at with a spectrum analyser. It is also possible, if there are multiple frequencies in use, that it could be an intermodulation issue. The frequency separation will not prevent the problem if the maths is right and a suitable non-linear medium is available. I've seen it happen more than once.

Brian

 

Thanks Brian, I was beginning to think we were imagining things http://www.blue-room.org.uk/public/style_emoticons/default/smile.gif Last night of the show tonight, so I think we'll try swapping out the UHF set in question and hope that resolves the issue.

 

Regards

Erik

 

 

 

[Bobbsy]

I once--about 30 years ago--saw a similar problem. A director brought in a rather cheap and nasty radio mic (Nady from memory) she wanted used for some reason and, whenever it was turned on, it splattered over the entire VHF range and all the way up to Channel 69 on which we had 12 channels. I'm afraid I just dumped the mic rather than trying to trouble shoot.

[paulears]

It does strike me that the transmitter on 863 is almost exactly 5 times the VHF frequency - so could just be unfortunate that the multiplier source is close to the VHF unit. If you add in the IF frequency in the VHF receiver - probably 455KHz then mix products could easily bring this into the receiver window. What make is the transmitter? People like Sennheiser and Shure (Trantec) who made VHF kit too took the trouble to make sure this crossover didn't happen. It's possible that a manufacturer only interested in UHF gave little thought to similar receive spec systems working in VHF bands? Unlikely, but possible. Looking at some old marine band radio kit I was attempting to sort out today confused me with a huge spike that interfered with my repeater on 453Mhz, and there was enough up at 900MHz to be easily visible on the analyser screen display.

[erik10_4]

It does strike me that the transmitter on 863 is almost exactly 5 times the VHF frequency - so could just be unfortunate that the multiplier source is close to the VHF unit. If you add in the IF frequency in the VHF receiver - probably 455KHz then mix products could easily bring this into the receiver window. What make is the transmitter? People like Sennheiser and Shure (Trantec) who made VHF kit too took the trouble to make sure this crossover didn't happen. It's possible that a manufacturer only interested in UHF gave little thought to similar receive spec systems working in VHF bands? Unlikely, but possible. Looking at some old marine band radio kit I was attempting to sort out today confused me with a huge spike that interfered with my repeater on 453Mhz, and there was enough up at 900MHz to be easily visible on the analyser screen display.

 

Fascinating. Interestingly, the VHF receiver is a Trantec and we have never had any problem with it working alongside the (including Trantec) UHF sets. On this occasion, very similar to Bobbsy's tale, the Director had asked that we use a couple of Citronic UHF's. I suggested that we take the Citronic UHF out of the picture and guess what; problem solved for the final night. Not knowing much, if anything, on the science of intermodulation I was just incredulous that frequency bands so far apart could conflict. I've started reading up on the subject now and, to be frank, it's blowing my mind http://www.blue-room.org.uk/public/style_emoticons/default/laugh.gif

[paulears]

RF behaves much like audio where multiple tones interfere and interact with each other - like the overtones piano tuners use to do their job - and the sequence of amplifiers and multipliers in radio devices is rarely in the books. I remember just a few months ago the man from OFCOM's radio investigation branch here in Lowestoft trying to find out why cars on a public car park were all refusing to unlock for their owners. The local Ford main dealer across the road had an entire day of angry Ford owners coming in complaining. The keys all work in the 430MHz band - and they eventually tracked it down to a radio location device in the harbour nearby that should have been working up in the microwave band, but was putting out a huge signal just where the car fobs work - totally swamping the cars receivers. Even made the news - the black hole in town where your car would refuse to let you in! I suppose plastic cases don't really help.