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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Russell Ault on June 22, 2015, 05:09:45 pm

Title: RX Antenna Cables of Unequal Length?
Post by: Russell Ault on June 22, 2015, 05:09:45 pm
(I'm a long-time reader/first-time poster.)

I feel like I've read most of the wireless-related posts in the LAB, but I haven't seen an answer to what I'm sure is a more difficult question than I realize: will using unequal lengths of coax between a diversity wireless microphone receiver and its antennas cause problems?

I'm thinking of a scenario where the receivers are on stage left and the antennas are in (for example) the downstage corners: you might need 100' of coax to reach stage right, but you may only need 10' for stage left. Even using LMR-400 that'd be a ~3dB difference in line losses between the two antennas at UHF microphone frequencies. Will that cause (or lead to an increased likelihood of) reception issues?

Thanks!

-Russ
Title: Re: RX Antenna Cables of Unequal Length?
Post by: Mac Kerr on June 22, 2015, 05:42:28 pm
will using unequal lengths of coax between a diversity wireless microphone receiver and its antennas cause problems?

I'm thinking of a scenario where the receivers are on stage left and the antennas are in (for example) the downstage corners: you might need 100' of coax to reach stage right, but you may only need 10' for stage left. Even using LMR-400 that'd be a ~3dB difference in line losses between the two antennas at UHF microphone frequencies. Will that cause (or lead to an increased likelihood of) reception issues?

No. You should use the shortest cable that will reach to where you want to put your antenna. Having and additional 3dB of loss through one of the cables may mean the receivers "favor" that antenna, but think of the alternative, you use 100' cables on both and the best reception you can hope for is 3dB down.

The first priority of antenna placement is to get full coverage of the performance area, the next priority is diversity reception. You can also take into account things like known interference that you can reduce of by placing your antennas so the interference is in the antenna null.

Mac
Title: Re: RX Antenna Cables of Unequal Length?
Post by: Russell Ault on June 23, 2015, 01:14:58 am
The first priority of antenna placement is to get full coverage of the performance area, the next priority is diversity reception. You can also take into account things like known interference that you can reduce of by placing your antennas so the interference is in the antenna null.

Mac

I think this quote should be the preface to every wireless product user manual; it dispels so much wireless voodoo in two sentences. Thank you so much!

-Russ