Placing wireless receivers in an metal rack, can it cause interfierance?

[Martinez011101]

Hey!
So just like the title says. I have a couple of sennheiser g3 receivers in plastic cases but I'd like to move them to a rack with caters on ir however I'm concerned about the metal rack causing interference or other problems, should I be worried?

 

[RonHebbard]

Since no one else has replied yet, I'll give you my thoughts. Anytime I've rack mounted wireless gear, I've included optional rack mount diversity antennas on a 1RU rack panel on either the front or rear of the rack depending upon the application. On the plus side, this puts the antennas outside the rack with the metal of the rack possibly providing an improved ground plane for the antennas. On the negative side it adds a couple of cables and connectors which likely waste a little RF.
Toodleoo!
Ron Hebbard.

 

[AudJ]

I'll second Ron. From experience, I'll say you don't want antennas buried inside the back of a rack. The G3 rack ears allow front mounting the antennas, with optional cables. If is only 2 units, that is probably all you need, but much more than that you would want distro and better antennas.

 

[RonHebbard]

Or, if you're only dealing with two or three receivers, at least Sennheiser's appropriate antenna distro and the same antennas if you're line of sight and not going very far through the air.
** Edit: Be certain you're using the correct cable(s). From memory, I'll say 52 ohm, not 75 ohm.
Toodleoo!
Ron Hebbard.

 

[Silicon_Knight]

Or, if you're only dealing with two or three receivers, at least Sennheiser's appropriate antenna distro and the same antennas if you're line of sight and not going very far through the air.
** Edit: Be certain you're using the correct cable(s). From memory, I'll say 52 ohm, not 75 ohm.
Toodleoo!
Ron Hebbard.

If you are going to remote mount the antennas, ensure that you are using at least half-wave ducks (1/4 wave antennas require a ground plane and are trickier to remote mount). Also, the antennas should be physically separated by at least 1/4 wavelength (ideally a full wavelength). (For 400-700Mhz, the wavelength is ~40-75cm, so antennas should be at least 10-18cm apart).

For the antenna cable, yes you should be using 50 ohm cable. RG8/U (Belden 9913) is ideal (but expensive). For short runs (~20ft), RG58C/U (Belden 8262) is sufficient. However, if you have very short runs (~1ft), 75 ohm cable can work in a pinch; however, the impedance mis-match could lead to issues due to poor RF signal quality.

This is an example of a Shure half-wave antenna:

This is an example of a Shure quarter-wave antenna:

 

[Chris15]

Folks, we've been here before.

On receive systems, using 75 ohm cable causes a mismatch loss of ~ 1dB. 75 ohm cable will give you less attenuation for the same outer diameter and lower cost. Your receive antennas are frequency variable in their impedance, they are not generally 50 ohms.

75 works, is stable and is much easier to lay hands on.
You just gotta remember 3 rules, only change nominal impedance once, 50 ohm once you hit an active stage (powered splitter for example), and 50 ohm ONLY when transmitting (eg. IEM applications).

 

[Silicon_Knight]

Folks, we've been here before.

On receive systems, using 75 ohm cable causes a mismatch loss of ~ 1dB. 75 ohm cable will give you less attenuation for the same outer diameter and lower cost. Your receive antennas are frequency variable in their impedance, they are not generally 50 ohms.

75 works, is stable and is much easier to lay hands on.
You just gotta remember 3 rules, only change nominal impedance once, 50 ohm once you hit an active stage (powered splitter for example), and 50 ohm ONLY when transmitting (eg. IEM applications).

Good input. I hadn't thought about receive/passive being much more forgiving than the issues with SWR under transmit. Thanks for your response!