IEM antenna combiner

[mkc]

We are using 7 Sennheiser G3 IEMs currently all with their own stubby aerials and often suffer from drop outs and "whooshing". We place the transmitters on stage but there is another non-Sennheiser IEM (still on ch70) and a number of guitarists using 2.4GHz radio gear which ought not to interfere. To improve things I am getting 2 Sennheiser AC3 antenna combiners. The recommended antenna for use with these is the Sennheiser A2003. This is passive so I assume there would be no improvement on signal strength. For around the same price we could get the RF Venue CP Beam antenna which claims a 9dB gain as well as helical polarization. Does anyone have any experience of these, and know whether they are compatible and any better? We cannot afford to go for PWS before anyone suggests that route!! Thanks

[paulears]

Apart from the physical benefit of just one aerial, IEM combiners are pretty much a waste of time, as they are passive. Functional, but actually less efficient. Sennheiser actually claim 0dB loss, which is a bit strange because combiners usually exhibit some loss - even tuned devices, but I guess they add a bit of gain before the split. However, getting a simple dipole connected to it up high will improve things quite a bit, but don't go for gainy aerials - they need aiming - which is difficult with people spread across the stage. The whoosh noises are usually at the receiver end when the body is between the source and destination. The usual small telescopic aerials are a bit naff really, as they should have a proper ground plane to make them operate properly. A passive dipole would be my favourite, connected to each AC3. A sleeve dipole would be quite suitable too. I'd look to making sure the receiver aerials are in the open, and experimenting. You can get a whoosh and virtual dropout even when the distance is quite short, when the very strong signal ought to be bomb proof, but the nulls can be very deep.

 

I'd be wary of using the helical beam - they need to be quite a way away from the performing area, as they are quite narrow, but they do provide circular polarisation, which means two 'normal' aerials at right angles don't create a null.

[mkc]

Thanks, confirmed my suspicion that using a combiner is a convenience rather than an improvement! Any dipole aerials you could suggest?

 

The RF antenna claims to be 70degrees, it would be offstage by monitor board somewhere and band do not leave the stage so wouldn't expect that to be too much of an issue. The Sennheiser Ac2003 that was recommended to me says is it 50degrees! Another show I've come across use a dome aerial (don't know which) that I assume is pretty directional.

 

 

 

 

 

[paulears]

The main benefit of the log periodic aerial (that inside the paddle) is that it has high bandwidth and decent gain, and a good front to back ratio. You can jiggle this around a bit and come up with what looks like a very 'gain' aerial. If you are using a system with the new add on ch69 frequencies at the bottom of the band, plus maybe ch70 at the top, they're about the only single aerial design that can do both, but the real advantage is that they throw the RF in the performers direction. The front useful lobe is quite wide, so by not being sensitive at the rear very much, they're quite useful. I've not seen the dome types in use for quite a while, but they're omnidirectional, have zero gain, but might be worth trying, although I doubt it.

 

I doubt that the real problem lays in your lack of clever stuff. At the frequencies we're talking about here, I'd be very tempted to get a couple of cheap vehicle mount taxi aerials, and cut them for the centre frequency you are using and then mount them on a couple of aluminium plates. Drill a mic stand size hole, and stick them up in the air on a couple of boom stands. I would absolutely not spend money on the really clever aerials, because they're much less forgiving. If I had a couple of the sennheiser passive paddles, I'd be happy with them. I'd not want the helical unless I could mount it up out the way in a truss, or other high position.

[mkc]

Thanks, all the stuff I'm interested in sorting is on new ch38 frequencies, the other is very much a spare (sometimes used by the guitar tech). Sounds like we're best to stick with the Sennheiser kit.

[Mr.X]

Apart from the physical benefit of just one aerial, IEM combiners are pretty much a waste of time, as they are passive. Functional, but actually less efficient. Sennheiser actually claim 0dB loss, which is a bit strange because combiners usually exhibit some loss - even tuned devices, but I guess they add a bit of gain before the split.

The Sennheiser AC-3 is active, which is why it exhibits 0db loss.

 

I'd be wary of using the helical beam - they need to be quite a way away from the performing area, as they are quite narrow, but they do provide circular polarisation, which means two 'normal' aerials at right angles don't create a null.

Helicals are the best way to get the RF signal where it needs to go and nowhere that it doesn't. The nature of it's spiral means the receiver's antennas will pick up better even when they are at changing angles, as you will have when someone moves around on stage. Additionally, it helps counteract multipath that you can sometimes when using omni antennas.

[paulears]

I did mention they add the gain - but unity gain is a rather odd concept - a lot of electronics for the benefit of a single antenna? I realise you can't up the power more than the legal limit, but as in most cases the single aerial would be remote, then 3dB or maybe a bit more would offset the feeder loss? Just odd to me to not make an expensive device do a little bit more?

 

I can't agree with helicals used on short paths, because they exhibit some quite odd lobes in the radiation pattern , and work best where the path to the performing area is a little longer. You see paddles stage side, or a bit higher up, and their radiation pattern gives good coverage. Helicals are, for me, just a bit too narrow.

[mackerr]

I did mention they add the gain - but unity gain is a rather odd concept - a lot of electronics for the benefit of a single antenna?

 

Yes. The isolation in the combiner will be better than the isolation between individual antennas unless they are all remotely mounted and spaced at least a meter or 2 apart. This will reduce the intermod interaction due to each transmitter leaking back into its neighbors. There are other combiners than the Sennheiser that do offer gain, but it is the user's responsibility that the radiated power doesn't exceed local regulations. The nicest that I have used is the Radio Active Designs TX8 that can set the output of each transmitter to 25mW, 50mW, 100mW or 250mW before combining. It also will tell you if your output antenna cable is not up to spec as well as having signal present lights on the inputs.

 

Mac

[paulears]

That's rather a nice piece of design, and quite useful facilities. I'd like to see some RF analyser plots of these devices, from the passive versions to the complex ones, to see how clean the amplification and combining stages are.