RF Minimum Separation Transmitter/Receiver...
 

Hi All,

I can use some input from the Pro Wireless RF folks out there. We are currently running a show at a local theatre, and the resident theatre tech is insisting on stacking a VHF wireless intercom transceiver directly on top of our UHF Lectrosonics Venue receivers. The antennas of the 2 systems are maybe 3 inches apart, maximum!

Is this small separation between a RF source and a wireless receiver something I should be concerned about (my gut instinct is yes, yes, yes!)? There is a plenty of bandwidth separation, but at that physical distance I can't help but think we are asking for trouble.

/BILLW

Hi Bill.............
 

Yep, asking for trouble at that range.

Depending on the output power of the transceiver, every press of the PTT switch could (temporarily) kill the front ends of the other receivers at 10 paces or even more.

I once took my brand new car fitted with a new fangled UHF remote locking and engine disableing system up to the top of a mountain to observe close up how they were coming on with the new VHF TV transmission tower.

Parked it a good 200 yards from the tower(s), the locking/ disableing system timed out and set, one very dead car and no amount of pressing the remote button would get it to unset.

Had to borrow a mobile to phone the missus to bring me up the override key to bypass the system, 'cos that VHF had drowned it completely.

They eventually put up BIG signs telling people not to park within 500 yards of the tower as it became quite a problem with new cars.

Giving it a test will be the proof, one way or another.


CS

Yes it could be a problem but may not be. I have one Lectrosonics 411 which doesn't like having a transmitter within 2 feet of it but the other 3 I have don't. I haven't sent it in for repair yet but it is on a list of things to do. Should not do any permanent damage to see if it causes problems. The question is how much time do you have to determine if it is causing minor interference or limiting the distance the mics work well.

Lectro is saying that having them this close will "de-sense" the receivers, which I take to mean reduced operating range and increased chances of drop outs. So far I haven't observed any problems, but if we're running at reduced sensitivity then it may be just a matter of time before the stars (and multipath nulls) align and we start seeing intermittent issues. This, by definition, will most likely occur on the day I need to record.

Daniel - the 411 has the tracking front end, so maybe this helps (at least 3 out of 4 times)? We have a few VRT's in our rack, but mostly VRS receivers.

I think I'm going to buy a couple of good, short (10') cables and move the antennas away from the intercom unit. This should at least give me some peace of mind.

Sounds like a good idea to be able to move the Antennas but the proximity of the receivers to the transmitter may still give you problems if you have them

I have used "well Packed" units many times without problems as long as your shark fins are mounted properly.

All the Best!

David, what do you mean by well packed? And how far away do you place your shark fins?

It generally makes no difference if the metal receiver boxes are right next to (or on top of) each other, cheek-by-jowl. What makes the difference is the proximity of their antennas.

Note that "shark fin" antennas are rather directional while the little wire "whip" antennas are omnidirectional and more sensitive to proximity of transmitters.

Putting a transmitter antenna anywhere near a receiver antenna is never a good idea, even when the devices are in different bands (VHF vs. UHF). Where I have had the option, I prefer to get any transmitting antenna at least 15-20 feet away from the wireless mic receivers.

Probably a bigger problem is cell phones. They emit a harsh digital signal very similar to the kinds of jamming signals that were used by the Evil Empire USSR behind the Iron Curtain back in the bad old days of the Cold War. I have stopped using wireless mics in some places because there is no way to get the audience to turn off all their cell phones, PDAs, etc.

Interesting that you are having issues with cell phones/pda's - I thought they were using frequencies far above the sub-700mhz band employed by wireless mics. As long as there is decent distance between the receiver and the cell phone I would have thought that any good wireless front end filter would easily reject the off band signal. Now, placing the cell phone next to the receiver antenna may be another story completely!

Are you hearing the cell phone digital "chatter" in your recording? That happened to me in one instance about 6 months ago and I thought it might be coming in through the wireless. We were getting a multi-channel feed from the soundboard into a multitrack recorder, and it turns out the Sound Tech would leave her iPhone on (in silent mode), directly on top of the board near one of the fader strips we were taking a feed from. We had her turn off the phone and the problem went away (and there were tons of folks in the audience with phones turned on happily texting away during the slow parts).

The biggest problem with cell phones (and similar devices) is not particularly that they are in a different band, but that they emit sharp digital pulses which are very much like the broadband hash that jamming stations deliberately "broadcast". Perhaps most people here are too young to remember the communist jammers back in the bad old days of the cold war.

If you are using two receivers with their attached antennas - then they should be at least a metre to a meter and a half apart.

Otherwise you get problems with blocking and back-emf affecting the other receiver.

If he is trying to put a transceiver there (ie: includes a transmitter) I would keep it about 5-metres away (and at least 8MHz away in frequency).

I think the OP said the other unit was VHF, so that shouldn't be anywhere near so bad as umpteen Watts of RF in the same band. The only sang would be if there's a mathematical relationship between the VHF and UHF - so if, for example the UHF system is on close to 4 times the VHF frequency, then the harmonics could possibly de-sense the UHF receiver.