ProSoundWeb Community

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 March 02, 2016, 06:55:04 pm

Title: 75 ohm Coax for RX
Post by: Russell Ault on March 02, 2016, 06:55:04 pm
Hi all!

In other places on this forum the concept of using 75 ohm coax for RX applications has been discussed, and I'm curious about what kind of 75 ohm cable to use, especially for portable applications. A cost/benefit analysis pushes me toward RG6, but even there I wonder what I should be looking for, since it seems like there are as many RG6 shielding options as there are uses for it!

What should I be looking for in 75 ohm coax? Is RG6 going to be acceptable for portable use? If so, is the standard 100% foil 60% braid sufficient, or should I spend the extra money for a more complete braid? What about quad-shielding? Is there a particular RG6 that will stand up to portable use better?

Thanks!

-Russ
Title: Re: 75 ohm Coax for RX
Post by: Henry Cohen on March 02, 2016, 07:40:47 pm
In other places on this forum the concept of using 75 ohm coax for RX applications has been discussed, and I'm curious about what kind of 75 ohm cable to use, especially for portable applications. A cost/benefit analysis pushes me toward RG6, but even there I wonder what I should be looking for, since it seems like there are as many RG6 shielding options as there are uses for it!

What should I be looking for in 75 ohm coax? Is RG6 going to be acceptable for portable use? If so, is the standard 100% foil 60% braid sufficient, or should I spend the extra money for a more complete braid? What about quad-shielding? Is there a particular RG6 that will stand up to portable use better?

Shielding is a double-edged sword, regardless of the type or impedance coax. More comprehensive shielding is always better from the perspective of less attenuation and greater immunity to outside RFI getting in, but the downside is it will have a shorter lifespan due to the physical breakdown of the foil portion (or corrugated copper, if really masochistic) with repeated coiling/uncoiling. And this brings me to a point I stress constantly: Coax in a touring or one-off environment is a perishable. Not as bad as gaffer tape, but certainly not as long term as an SO power cable.

In the end, buy the best combination of shielding (92%-95% braid over foil) and flexibility you can. At the end of each tour, or about every six months of heavy use, measure the response on a VNA. For 75Ω coax, the Belden "Brillance" line with the neporene(?) jacket is hard to beat. Just ensure all the connectors and passives are true 75Ω.
Title: Re: 75 ohm Coax for RX
Post by: Keith Broughton on March 03, 2016, 06:23:57 am
]Coax in a touring or one-off environment is a perishable[/i][/b]. Not as bad as gaffer tape, but certainly not as long term as an SO power cable.


Good point Henry!
 Now I just have to convince one the procuction houses I work with.
Title: Re: 75 ohm Coax for RX
Post by: John Rutirasiri on March 09, 2016, 08:58:35 pm
I'm curious about what kind of 75 ohm cable to use, especially for portable applications.

I have used Belden 1694A for years for live events, and it has been 100% reliable.  It's heavy enough that it almost lays flat, but not as flexible.

BTW I'm sure you know that wireless mic RF use 50 Ohm cables, not 75 Ohms (video and AES/EBU.)

John R.
Title: Re: 75 ohm Coax for RX
Post by: Russell Ault on March 10, 2016, 01:04:09 am
I have used Belden 1694A for years for live events, and it has been 100% reliable.  It's heavy enough that it almost lays flat, but not as flexible.

Good to know! That stuff looks great!

BTW I'm sure you know that wireless mic RF use 50 Ohm cables, not 75 Ohms (video and AES/EBU.)

I'm going to spare Henry Cohen the trouble of chiming in and suggest this link (http://forums.prosoundweb.com/index.php/topic,95680.msg881459.html#msg881459).  :)

Thanks!

-Russ