standing waves in wireless sys
in [Live Sound]
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From: Phil Allison on 26 Jun 2010 20:38 "alex" > yes, tried with and without antenna distibution system and moving antennas > back and forth. Almost no change whatever i done. I tried also to move the > receiver from the desk position to stage area. Still no effect. changing > brand of system (thus freq range) solved. Outside the church no problem > even at 70 meters distance. That sounded to me like a phase related > problem on the carrier, ** Completely absurd thinking. Phase related problems are always VERY sensitive to the exact position of mics and receivers - plus diversity antennas are very effective ant removing the issue. YOU hade another kind of problem entirely - comes under the title of RFI. ..... Phil
From: alex on 26 Jun 2010 22:25 Il 27/06/2010 2.32, Phil Allison ha scritto: > "alex" > Phil Allison >> "alex" >>>> >>>> can two parallel walls generate a standing wave on the carrier of a >>>> wireless system and affect the reception of the signal, just like in >>>> sound? >>>> >>> >>> ** The reflectivity of a solid wall to RF signals in the VHF /UHF band is >>> not nearly so strong as it is for sound waves - so the behaviour is not >>> the >>> same. >>> >>> Certainly, a radio mic's signal can bounce off a wall and arrive at the >>> receiver out of phase with the direct signal from the same transmitter >>> causing reception to almost disappear - diversity receivers using twin >>> antennas and RF stages are designed to defeat this phenomenon. >>> >>> >>> >> thanks phil, >> but in the situation described above ... > > > ** Looks to me that you described nothing. > > > > ... Phil > > > > > look better Phil. this was the first answer to Ron, third message in thread. bye alex
From: Phil Allison on 26 Jun 2010 22:43 "alex" >> >>>> >>> thanks phil, >>> but in the situation described above ... >> >> >> ** Looks to me that you described nothing. >> >> > look better Phil. ** I have. You have described nothing in any detail. The devil is in the detail ( famous saying). .... Phil
From: geoff on 27 Jun 2010 06:59 Phil Allison wrote: > "alex" >> >> can two parallel walls generate a standing wave on the carrier of a >> wireless system and affect the reception of the signal, just like in >> sound? >> > > ** The reflectivity of a solid wall to RF signals in the VHF /UHF > band is not nearly so strong as it is for sound waves - so the > behaviour is not the same. > > Certainly, a radio mic's signal can bounce off a wall and arrive at > the receiver out of phase with the direct signal from the same > transmitter causing reception to almost disappear - diversity > receivers using twin antennas and RF stages are designed to defeat > this phenomenon. Maybe not 'defeat', but certainly 'reduce'. RF is reflected off any metalic object, including hidden ones such as reinforcing in concrete, booms, lighting rigs, cables, candlesticks, organ pipes etc. It's a minefield out there.... geoff
From: alex on 27 Jun 2010 07:01
Il 27/06/2010 4.43, Phil Allison ha scritto: > "alex" >>> >>>>> >>>> thanks phil, >>>> but in the situation described above ... >>> >>> >>> ** Looks to me that you described nothing. >>> >>> >> look better Phil. > > > ** I have. > > You have described nothing in any detail. > > The devil is in the detail ( famous saying). > > > > ... Phil > > > > Santa Maria del fiore (Florence cathedral) http://maps.google.com/maps?client=firefox-a&hl=it&ie=UTF8&ll=43.773193,11.256107&spn=0.001718,0.004292&t=k&z=19 Ok, things happened some 6-7 years ago, in winter, just before christmas, inside the church there are a lot of well known pieces of art, so RMI from a weird security or control system, was the first thing i thougt (under the main floor there is an underground museum, with a lot of electronic protection). So i asked a friend, with much more knowledge than me in radio transmissions, to discover if actually was RMI. His frequency scanner tell us that was nothing affecting the band used for microphones, but we noticed that transmission from a single bodypack lead to a very weak reading on the scanner in opposite to the reading from the same bp outside the church. Moving the scanner around the church the signal stenght varied a little, never reaching the level measured outside, even very close to the transmitter. He told me that, in his opinion, a lot of bouncing with different timing leaded to an heavy cancellation in phase, wich sounded ok for me due the comparision with sound waves. Another brand of system was apparently unaffected by anything and worked well for the show. --- Something very similar happened to me in another church in Florence last year (second job in my life inside a church). Santa Maria del Carmine http://maps.google.com/maps?client=firefox-a&hl=it&ie=UTF8&ll=43.773193,11.256107&spn=0.001718,0.004292&t=k&z=19 They call me because the pa system doesn't worked well, they hearded big noises coming from the speakers, like a pickup scaratched over a record surface. Due to "art protection authority" new laws, the system was wireless (you cannot use cables) for the audio signals and wired for power supply (pwr outlets where placed all around the church before the law). 3 Transmitters and 6 receivers, from dB technologies, showed the same behaviour: big drops and weak signal even at small distance. After asking the old audio company responsible of the system setup, i discover that they used originally a very powerful radio signal amplifier for the trasmitter with a directional antenna, but had to remove it beacuse people in the near buildings was no more able to watch TV! So they carefully find an antenna position (the regular antenna) to minimize drops, but apprently after 4 years she has moved so i get the call. Even in this second case my suspect go to the cancellation of the carrier due to reflections because the room size (both are very large churches) and the very complex internal geometry. For now my "poor man experience" says: don't use wireless system in churches! bye alex |