Radio mic RF help

[samchurchill]

Hi,

 

Sorry that this topic has been covered lots of times, but I've found so many conflicting pieces of advice that, being an experienced sound tech, but a newbie to RF modifications, I thought it best to start from scratch with my own specific questions. Hope you don't mind...

 

Scenario A: a single Sennheiser ew100 G2 ch70 receiver in a rack which cuts out occasionally when there are lots of people in the room as it's too low, but mounting the receiver anywhere else would be quite tricky. I'm wondering about extending the standard antennas on 3m cables on to the top of a mic stand to get over people's heads. No budget for Sennheiser A1031 on this one, I'm afraid. A few questions about this...

1. Some articles I've read say the antennas should be mounted a minimum of 1/4 wavelength apart, but others say the further apart the better, others say not to put them too far apart. Am I correct in assuming I can't go far wrong with the distance that they're set on the back of the receiver?
2. I've read about the need for grounding the antennas, but never really understood it - is this something to worry about?

 

Scenario B: I've got 3 Sennheiser ew100 G2/3 ch38/70 receivers in a rack that is floor mounted. At the moment the standard antennas are just brought onto the front of the rack but, again, they're sometimes cutting out because they're at knee height. I'm thinking of getting an ASA1 to distribute the antenna signals so that we can use external antennas. Can I just get a single Sennheiser A1031 and use one of the standard antennas for the other, or do they both have to be one of the paddles?

 

As you can probably see, we're trying to stretch budget and I had missed the fact the ASA1 didn't come with the paddles!

 

Thanks,

 

Sam

[GR1]

If the receivers are next to the stage/performance area you should have no problems. If they are at the mixer you probably will. If you can site the receivers close to the performers that will solve quite a lot of issues. Just use long XLR cables. I use G2 ch70 with standard aerials directly into the receivers in rooms with audiences of up to 700 and NEVER have RF drop outs because I site the receivers next to the stage,.

[sunray]

<br />Hi,<br /><br />Sorry that this topic has been covered lots of times, but I've found so many conflicting pieces of advice that, being an experienced sound tech, but a newbie to RF modifications, I thought it best to start from scratch with my own specific questions.  Hope you don't mind...

Scenario A:

a single Sennheiser ew100 G2 ch70 receiver in a rack which cuts out occasionally when there are lots of people in the room as it's too low, but mounting the receiver anywhere else would be quite tricky.  I'm wondering about extending the standard antennas on 3m cables on to the top of a mic stand to get over people's heads.  No budget for Sennheiser A1031 on this one, I'm afraid.  A few questions about this...

Some articles I've read say the antennas should be mounted a minimum of 1/4 wavelength apart, but others say the further apart the better, others say not to put them too far apart.  Am I correct in assuming I can't go far wrong with the distance that they're set on the back of the receiver?

The basic premise of diversity is to have the 2 aerials in different areas to improve the chances of a good signal getting to the receiver, so the idea of having both aerials mounted on a tiny receiver is in some ways a bit silly.

On one of my regular jobs the 2 aerials get mounted some 60m apart as they cover an area approx 200x100m and large vehicles are involved in the display, creating big RF shadows.

I've read about the need for grounding the antennas, but never really understood it - is this something to worry about?

If you are using the aerial which was designed to be mounted on the receiver, I'd suggest mounting it in a vaguely similar fashion. In your case I assume your aerials are on a TNC or BNC plug, so a suitable socket mounted on a metal bracket mounted on a metal mic stand should suit well. the easy way to do it is to use a bulkhead female/female connector, an example of such a mounting: https://www.studiospares.com/Microphones/Mics-Wireless/Trantec-S4.16HA-Antenna-Booster_434770.htm As no amplifiers are required a small metal plate with 2 holes and one of these: https://www.ebay.co.uk/itm/10x-TNC-Female-To-TNC-Female-Jack-Bulkhead-Panel-Mount-Connector-Straight-F-F-BK/312498502157?hash=item48c25cba0d:g:-qgAAOSwfiZccYKj:rk:13:pf:0 will be a lot cheaper, or a metre long 'T' bar with 2 of them.

 

Scenario B:

I've got 3 Sennheiser ew100 G2/3 ch38/70 receivers in a rack that is floor mounted.  At the moment the standard antennas are just brought onto the front of the rack but, again, they're sometimes cutting out because they're at knee height.  I'm thinking of getting an ASA1 to distribute the antenna signals so that we can use external antennas.  Can I just get a single Sennheiser A1031 and use one of the standard antennas for the other, or do they both have to be one of the paddles?

As you can probably see, we're trying to stretch budget and I had missed the fact the ASA1 didn't come with the paddles!

Certainly don't need to use identical aerials, do you need a dist amp or could you just mount your 6 aerials higher up, the same as scenario 'A'?

[samchurchill]

Thanks for both replies.

 

Unfortunately moving the receivers in Scenario A any closer to the stage isn't practical (for several long and boring reasons), but the ones for Scenario B are actually on the stage - I think they're cutting out when they're shielded by the person or group speaking, so getting the antennas up in the air would help with that.

 

Sounds like the best plan to provide a degree of practicality and the best reception would be to create a 19" bar that can be screwed onto the top of a mic stand and mount one antenna on each end of it. We'd buy a paddle when we can to replace the standard antennas.

 

Thanks,

 

Sam

[Shez]

the ones for Scenario B are actually on the stage - I think they're cutting out when they're shielded by the person or group speaking

That rather makes me think there's something else going on here. At that range, several people and a brick wall or two shouldn't prevent signal getting through.

 

Questions... Are you 100% certain that they're genuine Sennheiser systems? There are an awful lot of very convincing fakes out there which perform surprisingly well but not as well as the real thing. Are there any other sources of RF (at any frequency) in the vicinity? I've encountered people putting phones, laptops, WiFi access points, IEM transmitters etc right next to the receiver antennae and then wondering why they have RF problems.

[sunray]

I've encountered people putting phones, laptops, WiFi access points, IEM transmitters etc right next to the receiver antennae and then wondering why they have RF problems.

 

I had one job where a customer had god fed up with the original installers trying to get a system running correctly, it was under a year old and they called me in (I'd also quoted for the original installation) with the intention of having me start again with a new installation.

What I found was very poor quality workmanship but the actual problem with the poor performance of the mics was the aerials poking out through the top of the rack, and yes I go mean holes drilled so the standard aerials screwed on the ADA could poke through - hence some of it is inside the rack. The next unit down in the rack was the IEM and its aerial was mounted in the same way (roughly half way between the RX's) and all 5 channels were in the VHF license free band, 4 CH of Trantec S2 + 1CH of (I think) Audio Technica IEM. I pulled the DC plug on the IEM and the mic system burst into life with interference from the multicolour LED light mounted in the top slot of the rack, pulled the plug on that and silence. At that point the system worked, the customer said they didn't want the light so I removed it and moved the ADA up to the top slot which improved the performance to the mics at the back of the hall. I moved the IEM to a vacant spot near the bottom of the rack and extended the coax to the still existing dipole way up in the roof timbers.

 

1/2 a days work resolved nearly a year of anxiety

[Chappie]

1. Some articles I've read say the antennas should be mounted a minimum of 1/4 wavelength apart, but others say the further apart the better, others say not to put them too far apart. Am I correct in assuming I can't go far wrong with the distance that they're set on the back of the receiver?
2. I've read about the need for grounding the antennas, but never really understood it - is this something to worry about?

You can put antennas wherever you want but, generally speaking, to get useful diversity a good separation is preferable, with both at a roughly similar distance from the source. All antennas interact with each other to some extent as well, so keeping them apart will minimise this.

 

When using 1/4-wave antennas, such as the little stubby ones that come as standard with the Evolution series, they need to be directly mounted to a ground plane. This can be any reflecting metal surface roughly the same size as the antenna in at least one dimension (i.e. the unit's chassis, a rack panel, or something else), so if you are using them remotely, they need to be mounted to something that acts as a reflector. A 1/2-wave antenna (such as the A1031) doesn't need to be RF-grounded, so works well remotely and also gives you a theoretical 3 dB gain over the 1/4-wave.

 

Can I just get a single Sennheiser A1031 and use one of the standard antennas for the other, or do they both have to be one of the paddles?

You could quite happily use 1 of each, just understand that with the (potentially) extra gain and better positioning of the 1031 on one input, the tucked away 1/4-wave antenna on the the other might almost become redundant. If you can get the rack with the stubby antenna into an improved location and remotely position the 1031 a sensible distance away, you could achieve good diversity.

[samchurchill]

Thanks very much Chappie, that's really helpful!