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Author Topic: Antenna splitters  (Read 10333 times)

Russell Ault

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Re: Antenna splitters
« Reply #10 on: February 21, 2016, 11:55:34 pm »

You could contact the Sennheiser or Shure engineers and ask them.
I know they want to sell gear but think of this, if the CATV splitter works, why don't you see it in any wireless mic racks?

The way I tend to look at it (and, to be sure, this is from the bottom looking up) is that there are many factors that go into choosing the right tool for a job: cost is always a factor (although its importance starts to diminish slightly as the cost of a particular tool becomes a smaller and smaller fraction of the overall budget of the project), as is the relative importance of being able to rely on a tested, major-manufacturer product (which starts to become more important as budgets go up and having a "no one was ever fired for buying IBM"-style company attached to gear that failed may be the thing that saves your job when the lead singers mic goes dead in front of 60k people).

Most (but, admittedly, not all) of the wireless racks I've seen that use proper distro are on shows where jobs are on the line if something fails, and I'm simply not there yet. I'm still at the level where, if only one mic (out of seven) decides to randomly pop during a show, that was a good show. The group-owned RF gear I'm dealing with wasn't what I would have spec'd, and now I'm trying to make the best of a bad situation while saving as much as possible to buy decent gear down the road, so for me, the cost of the tool is far more important than it's brand-name reputation: if it works, I'll take it (something almost has to be better than nothing at this point), and if we need better down the road, it won't cost too much to replace.

As you've surmised, the primary issue will be the quality of the amplifier. What are its 1dB compression and IP3 (saturation) specifications? The amplifier needs to be able to handle the composite power levels expected. The other aspect is the port to port isolation and the actual power level balance at each output port. In other words, a cheap splitter will not provide the same quality and performance as a higher priced, better quality unit.
The amp in a good CATV splitter should be of excellent quality.  It's a huge market, and RF performance matters a lot more than in our audio applications.

The splitter I've been looking at is the PCT-VC-9U. The only specification (of the ones Henry suggested) that I can decipher is port-to-port isolation, which is probably less than would be ideal (>22dB vs Shure's >30dB). Of course, part of the problem of knowing whether this splitter is up to the job is the lack of full specifications on Shure's and Sennheiser's products, at least that I can find, which makes comparison very difficult. Judging by the information here (which I fully expect isn't enough to make any real conclusions) is there a chance that this might work?

A home-grade TV splitter may be mechanically packaged cheaply.  A good CATV splitter will probably be very durably packaged, but this packaging may not be optomised for audio use.

I really don't know the CATV splitter market well enough to even guess what end of the spectrum this one would fall on. Obviously not rack-mountable, but a shelf will fix that, and it doesn't look like it's going to fall apart...

The fact that the splitter is 75 ohm is somewhat moot. I would only suggest that you use a coax - and proper impedance connectors - of matching impedance. 75 ohm coax of equivalent size will actually have slight less attenuation than a 50 ohm coax. And, a bit of history: Sennheiser RX splitters of 70's and 80's vintage used all 75 ohm CATV parts.

I was planning on using RG6 for that very reason. From what I've been seeing on various attenuation charts, it'll beat LMR-240 in our UHF band and is less than a quarter the price. Astonishing what volume production can do...

Thanks everyone!

-Russ
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Lyle Williams

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Re: Antenna splitters
« Reply #11 on: February 22, 2016, 12:24:57 am »

That one would seem to do.  Are you going to use two for the diversity antennas?
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Keith Broughton

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Re: Antenna splitters
« Reply #12 on: February 22, 2016, 06:20:25 am »

It looks like it's an inexpensive item so give it a try and let us know how it goes. :)
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Russell Ault

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Re: Antenna splitters
« Reply #13 on: February 22, 2016, 09:20:09 am »

That one would seem to do.  Are you going to use two for the diversity antennas?

That's the plan. And two of those is an order of magnitude cheaper than 8 channels of Sennheiser or Shure distro...

It looks like it's an inexpensive item so give it a try and let us know how it goes. :)

I'll be sure to report back with my results! :)

Thanks all!

-Russ
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Jason Glass

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Re: Antenna splitters
« Reply #14 on: February 22, 2016, 10:51:05 am »

For building up small systems (4-8 receivers) used in relatively rural areas what down sides are there to using standard/cheap CATV splitter in place of a more expensive splitter?

Thanks

Philip

There are some details about antenna splitting (and amplification, in the case of unity gain splitters) that should be mentioned in this discussion to assure that we all understand why pro audio splitters are optimal for their job, and why cheaper alternatives might work but also might not.

Efficient and effective antenna splitting is more complex than simply compensating for losses, impedance mismatch, and port isolation.

Professional audio grade splitters usually contain filtering that passes frequencies of interest while blocking undesirable out of band signals.  When these signals enter an active RF stage, the lowest noise floor and IMD products are generated when out of band signals are suppressed.

Wideband cable TV splitters include the FM radio broadcast band, which contains very strong transmission signals that can contribute to internal IMD in your receiving equipment.  This is why some cable systems insert "FM traps" into in-home cabling that includes active distribution.  These systems also allow high power two-way radio signals in the commercial 450-470MHz band to pass, along with other undesirable carriers across their entire pass band.  This is a non-issue in most residential installations, but can be a major issue in industrial applications like entertainment venues where all kinds of strong radio transmissions are in use.  It's worth noting that residential locations that have HAMs for neighbors can experience TV interference due to out of band transmissions from the local HAM shack, even in a closed circuit like cable TV.  This is a similar scenario to a 5W walkie stomping on your mics even though they are operating in different frequency bands.

As most readers here know by now, impedance mismatch causes a predictable loss of signal due to signal reflection.  This can be manageable.  However, some passive bandpass and band blocking filtering inside your receiving equipment relies on a specific range of impedance in the transmission path to determine the filter's corner frequencies, slope, band blocking depth, and pass band insertion loss.  When the impedance of the line is out of tolerance, these filter characteristics may also shift out of tolerance.  This impedance mismatch also makes it difficult to use relatively inexpensive off-the-shelf insertable filters to solve specific problems that might pop up, which pro RF techs often do.

All of this info is not intended to discourage experimentation, because it just might work fine for a given scenario and save some $.  I just want to help clarify what's going on inside those pricey black boxes.

Lyle Williams

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Re: Antenna splitters
« Reply #15 on: February 23, 2016, 01:06:37 pm »

While the pro-audio splitters probably roll off VHF, I'd be suprised if any notched out any bands within their specified frequency range.

The price of these splitters is highway robbery (for what they really contain, incl design effort)
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John Sulek

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Re: Antenna splitters
« Reply #16 on: February 23, 2016, 05:34:19 pm »

While the pro-audio splitters probably roll off VHF, I'd be suprised if any notched out any bands within their specified frequency range.

The price of these splitters is highway robbery (for what they really contain, incl design effort)

No notch filters, but two of the expensive options (Axient AXT630 and Sennheiser ASA3000) can do 60Mhz bandpass filters that cover their respective transmitter frequency ranges.
Front panel switch on the Axient, swap-able hardware modules on the Sennheiser. The Axient also has an unfiltered out for cascading.
Good for noisy environments.
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Jason Glass

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Re: Antenna splitters
« Reply #17 on: February 23, 2016, 09:50:32 pm »

While the pro-audio splitters probably roll off VHF, I'd be suprised if any notched out any bands within their specified frequency range.

The price of these splitters is highway robbery (for what they really contain, incl design effort)

I wasn't referring to notches within the specified band.  I was referring to stop bands and sufficient filter slopes at the edges of the pass band.  These significantly reduce the noise level in the pass band generated by subsequent active stages.  Wideband cable TV splitters and DA's typically pass <54MHz to >800MHz without any filtering at all.

Keith Broughton

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Re: Antenna splitters
« Reply #18 on: February 24, 2016, 08:28:51 am »

Here is a question about splitters.
John Sulek and I worked with a band that has (correct me John if wrong) 12 mic receivers wired like this...
Antennas into the first active splitter that fed 3 receiver and the forth out cascaded to a second splitter...and so on.
Despite scanning a nice clean RF space, we had a hard time finding "clean" freqs for the last receivers. They were showing RF activity on pretty much all of the available channels.
Seems to me the first splitter should feed the 3 other splitters and they in turn feed 3 groups of 4 receivers.
Or, a passive 3 split feeding the 3 active splitters.
Thoughts?
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Jason Glass

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Re: Antenna splitters
« Reply #19 on: February 24, 2016, 09:25:02 am »



...the first splitter should feed the 3 other splitters and they in turn feed 3 groups of 4 receivers.
Or, a passive 3 split feeding the 3 active splitters.
Thoughts?

Yes, these are your best options.  The scenario you described was behaving exactly as one would expect with too many cascaded active components.

Jason Glass
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ProSoundWeb Community

Re: Antenna splitters
« Reply #19 on: February 24, 2016, 09:25:02 am »


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