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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: Jamin Lynch on May 26, 2022, 05:09:33 PM

Title: Antenna Cominer Question
Post by: Jamin Lynch on May 26, 2022, 05:09:33 PM
Could I use this passive antenna splitter/combiner for 2 Sennheiser wireless IEM units along with one paddle antenna instead of a powered antenna combiner system?

Thanks
Title: Re: Antenna Cominer Question
Post by: Paul Johnson on May 26, 2022, 05:33:31 PM
Short answer - no. The thing with antenna combiners is that despite the IEM transmitters being quite low power, RF transmitters are quite sensitive to reflections coming back up the feeder from an antenna. VSWR is the ratio of of oomph going out compared to it coming back up. Normally, it's just less than ideal antenna systems that don't match well and produce this reflected power. In something like a Sennheiser combiner, each input connector is fed to a buffer stage before being combined, so this active buffer presents each transmitter with a constant proper load.

A passive splitter is a combiner too, so if you stick power into it, some of it will appear at the other input terminal. Some transmitters might not care, but others could be damaged. The buffer is pretty important. It might work, but it could produce some very strange results - possibly even odd mixing and spurious outputs. Transmitters need careful treatment.
Title: Re: Antenna Cominer Question
Post by: Rui Lisboa on May 27, 2022, 05:57:33 AM
Short answer - no. The thing with antenna combiners is that despite the IEM transmitters being quite low power, RF transmitters are quite sensitive to reflections coming back up the feeder from an antenna. VSWR is the ratio of of oomph going out compared to it coming back up. Normally, it's just less than ideal antenna systems that don't match well and produce this reflected power. In something like a Sennheiser combiner, each input connector is fed to a buffer stage before being combined, so this active buffer presents each transmitter with a constant proper load.

A passive splitter is a combiner too, so if you stick power into it, some of it will appear at the other input terminal. Some transmitters might not care, but others could be damaged. The buffer is pretty important. It might work, but it could produce some very strange results - possibly even odd mixing and spurious outputs. Transmitters need careful treatment.


This is a resistive combiner and not a Wilkinson divider so there is less isolation from port to port. A resistive unit also has more insertion loss.
StillÖ shure has one of these on their PA combiners upfront to link units. So Iím guessing they might work ok. If a passive device should be used and you can bear with the inherent loss I would choose the mini circuits or similar wilkinson divider type.
Title: Re: Antenna Cominer Question
Post by: Keith Broughton on May 27, 2022, 06:56:29 AM
Could I use this passive antenna splitter/combiner for 2 Sennheiser wireless IEM units along with one paddle antenna instead of a powered antenna combiner system?

Thanks
It would be better to just use 2 separate antennas.
This is a passive device and would not work well for what you want.
Title: Re: Antenna Cominer Question
Post by: Paul Johnson on May 29, 2022, 11:45:56 AM
Antenna combiners in the commercial RF comms world only get used when there is a real reason - such as the cost of running separate antennas up a tower, where the cost of combiners and the small loss they introduce. It's just so much easier to use separate antennas.
Title: Re: Antenna Cominer Question
Post by: Don Boomer on May 29, 2022, 05:45:47 PM
Antenna combiners in the commercial RF comms world only get used when there is a real reason - such as the cost of running separate antennas up a tower, where the cost of combiners and the small loss they introduce. It's just so much easier to use separate antennas.

Pretty big hit to the budget.  CP antennas arenít cheap.
Title: Re: Antenna Cominer Question
Post by: Rui Lisboa on May 30, 2022, 12:09:01 PM
Pretty big hit to the budget.  CP antennas arenít cheap.
Agreed. But they are a priceless investment and will last forever with lesser care than comparable antennas.
Title: Re: Antenna Cominer Question
Post by: Henry Cohen on May 30, 2022, 12:38:32 PM
Antenna combiners in the commercial RF comms world only get used when there is a real reason - such as the cost of running separate antennas up a tower, where the cost of combiners and the small loss they introduce. It's just so much easier to use separate antennas.

Not entirely accurate, at least in the US, though I'm fairly certain the same physics applies in the other ITU Regions ( ;) ). Combining base station antennas in the commercial world is often the case so as to minimize the number of antennas on a tower: Not because of transmission line costs, but rather to reduce wind and weight loading on the structure, and reducing the numbers of antennas in each others' near fields, especially when talking about omni or near-omnidirectional antennas. (We can ignore highly directional mobile services sector antennas for the purpose of this discussion.)

Once in the commercial 'real' world of RF, combining is accomplished via high Q filters and isolators, precisely tuned, and kept in tune by being in temperature and humidity controlled spaces. Insertion losses can be an 1/8th to 1/2 that of a resistive (Wilkinson) or hybrid networks.

Even in our little world of production wireless, the physics remains the same with respect to multiple antennas in each others' near fields. If one has the luxury of time, spatial separation and protection of longer coaxial cable runs, deploying two TX antennas is not an issue. But when the real world intrudes and both time and space are at a premium, having a proper combining network to reduce the number(s) of antennas is by far the better option (and has proven such in countless deployments).

 
Title: Re: Antenna Cominer Question
Post by: Jesse Stern on June 12, 2022, 03:20:40 PM
I did this once out of necessity with 2 PSM900s when I didn't have enough antennas, and one of the IEM packs kept dropping out despite being on what looked like a clean, coordinated frequency in workbench and having solid bars of RF.  At one point it started picking up interference that sounded like FM radio, which I didn't understand since it was in UHF band.  I never got to the bottom of what the actual problem was but suspect the passive combiner.  Any guess what the "FM radio" sounding interference could have been? 

Short answer - no. The thing with antenna combiners is that despite the IEM transmitters being quite low power, RF transmitters are quite sensitive to reflections coming back up the feeder from an antenna. VSWR is the ratio of of oomph going out compared to it coming back up. Normally, it's just less than ideal antenna systems that don't match well and produce this reflected power. In something like a Sennheiser combiner, each input connector is fed to a buffer stage before being combined, so this active buffer presents each transmitter with a constant proper load.

A passive splitter is a combiner too, so if you stick power into it, some of it will appear at the other input terminal. Some transmitters might not care, but others could be damaged. The buffer is pretty important. It might work, but it could produce some very strange results - possibly even odd mixing and spurious outputs. Transmitters need careful treatment.
Title: Re: Antenna Cominer Question
Post by: Henry Cohen on June 12, 2022, 10:08:22 PM
I did this once out of necessity with 2 PSM900s when I didn't have enough antennas, and one of the IEM packs kept dropping out despite being on what looked like a clean, coordinated frequency in workbench and having solid bars of RF.  At one point it started picking up interference that sounded like FM radio, which I didn't understand since it was in UHF band.  I never got to the bottom of what the actual problem was but suspect the passive combiner.  Any guess what the "FM radio" sounding interference could have been?

Wavelength at 100MHz is 3 meters or just under 10'. 1/4 wavelength is about 30". How long were the jumpers interconnecting the splitter/combiners, and were they single braid coax? Congratulations; you have an FM band antenna.
Title: Re: Antenna Cominer Question
Post by: Jesse Stern on June 13, 2022, 05:10:17 PM
Each jumper to the split was about 12" long, then the run to the antenna was 25'.  I was using a Shure UA860SWB omni wideband antenna to transmit.  I don't know if it was single-braided; is it standard for 50ohm coax to be double-braided?



Wavelength at 100MHz is 3 meters or just under 10'. 1/4 wavelength is about 30". How long were the jumpers interconnecting the splitter/combiners, and were they single braid coax? Congratulations; you have an FM band antenna.
Title: Re: Antenna Cominer Question
Post by: Jesse Stern on June 13, 2022, 05:13:27 PM
And even if the cables could pick up the FM frequency as a 1/4-wave antenna, how does the signal actually come through the ears since the pack receiver is tuned to a UHF frequency?
Title: Re: Antenna Cominer Question
Post by: Henry Cohen on June 18, 2022, 12:30:52 PM
Each jumper to the split was about 12" long, then the run to the antenna was 25'.  I was using a Shure UA860SWB omni wideband antenna to transmit.  I don't know if it was single-braided; is it standard for 50ohm coax to be double-braided?
50 ohm coax is far too generic a term to describe the construction of a particular spec coax. Both double and single shielded designs are prevalent. RG58 by definition is single 95% braided shield.
Title: Re: Antenna Cominer Question
Post by: Henry Cohen on June 18, 2022, 12:35:35 PM
And even if the cables could pick up the FM frequency as a 1/4-wave antenna, how does the signal actually come through the ears since the pack receiver is tuned to a UHF frequency?
The FM broadcast carrier is picked up by the earphone cable where the induced current is fed back into the final PA of the earphone amp, demodulated by the amplifier circuit, and then amplified out to the ear piece. Was likely a rather strong FM signal at your location.