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Author Topic: Combining RF from 5x IEM Transmitters of Two Bands  (Read 876 times)

Nick Marques

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Combining RF from 5x IEM Transmitters of Two Bands
« on: January 22, 2021, 09:51:09 am »

RF Pros,

I have an install where a (church) client bought themselves Shure P3T IEM systems over the years, but they've just ben sitting on the stage. 4x of them are Shure J13 (566-590 MHz) and 1x is G20 (488-512 MHz). I am going to be actually installing these correctly into their rack but want to correctly get them all onto one transmit antenna. There's only a few active combiners out there that have more than 4x ports but they are very expensive and definitely out of their budget. I can do a Shure PA411, but that leaves me with the 5th transmitter.

Can I do a PA411 for the 4x J13's and then use a UA221 to combine the PA411 with the 5th P3T?

I also thought about using a Mini-Circuits ZBSC-8-82+, and while it is 8-ports, it isn't active.. so it won't have any sort of IMD suppression. It is a lot cheaper though.

I am planning on using a Lectrosonics SNA600 adjustable dipole antenna. The antenna location is directly behind the stage but up about 15ft as the rack in on a sort of mezzanine behind the stage. I think the range is too short for a helical and I think a log periodic may be too narrow. With the SNA600 I can set the length somewhere in between the two bands which should be sufficient.  Maybe around 500 MHz, or a little higher and make sure the G20 is tuned on the upper end. I don't want to run two antennas.

If anyone has other ideas, I'm interested.

Thanks!
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Henry Cohen

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #1 on: January 22, 2021, 12:59:47 pm »

I have an install where a (church) client bought themselves Shure P3T IEM systems over the years, but they've just ben sitting on the stage. 4x of them are Shure J13 (566-590 MHz) and 1x is G20 (488-512 MHz). I am going to be actually installing these correctly into their rack but want to correctly get them all onto one transmit antenna. There's only a few active combiners out there that have more than 4x ports but they are very expensive and definitely out of their budget. I can do a Shure PA411, but that leaves me with the 5th transmitter.

Can I do a PA411 for the 4x J13's and then use a UA221 to combine the PA411 with the 5th P3T?

I also thought about using a Mini-Circuits ZBSC-8-82+, and while it is 8-ports, it isn't active.. so it won't have any sort of IMD suppression. It is a lot cheaper though.

I am planning on using a Lectrosonics SNA600 adjustable dipole antenna. The antenna location is directly behind the stage but up about 15ft as the rack in on a sort of mezzanine behind the stage. I think the range is too short for a helical and I think a log periodic may be too narrow. With the SNA600 I can set the length somewhere in between the two bands which should be sufficient.  Maybe around 500 MHz, or a little higher and make sure the G20 is tuned on the upper end. I don't want to run two antennas.

The PA421 is a wideband unit and can accommodate a frequency range of 470-865MHz. If budget won't permit a proper 8-way active combiner, then simply connect 4 of the transmitters to the four inputs of the 421, then connect the combiner output to one of the front side's 2-way passive circuit inputs (either "A IN" or "B IN"). Connect the 5th transmitter to the other front side "_ IN", and the antenna connects to the "A+B OUT" port. Tip: Connect transmitters in order of ascending or descending frequencies into ports 1, 3, 2, 4. Maximum frequency separation between adjacent ports.

With regards to your comment "I also thought about using a Mini-Circuits ZBSC-8-82+, and while it is 8-ports, it isn't active.. so it won't have any sort of IMD suppression . . .", it actually does in a sense: Since it's passive there'll be about 10dB loss per input so that's about 20dB attenuation of reflected antenna port energy, more than the 4-way's 14-ish dB from the antenna port back into the combiner's internal amplifiers. In the end, if overdriven or insufficient frequency separation, the PA421's amplifiers will have greater chance of saturation resulting in spurious emissions than passive combining, especially if not the newer PA421B.
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Henry Cohen

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Ike Zimbel

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #2 on: January 22, 2021, 01:13:56 pm »

The PA421 is a wideband unit and can accommodate a frequency range of 470-865MHz. If budget won't permit a proper 8-way active combiner, then simply connect 4 of the transmitters to the four inputs of the 421, then connect the combiner output to one of the front side's 2-way passive circuit inputs (either "A IN" or "B IN"). Connect the 5th transmitter to the other front side "_ IN", and the antenna connects to the "A+B OUT" port. Tip: Connect transmitters in order of ascending or descending frequencies into ports 1, 3, 2, 4. Maximum frequency separation between adjacent ports.

With regards to your comment "I also thought about using a Mini-Circuits ZBSC-8-82+, and while it is 8-ports, it isn't active.. so it won't have any sort of IMD suppression . . .", it actually does in a sense: Since it's passive there'll be about 10dB loss per input so that's about 20dB attenuation of reflected antenna port energy, more than the 4-way's 14-ish dB from the antenna port back into the combiner's internal amplifiers. In the end, if overdriven or insufficient frequency separation, the PA421's amplifiers will have greater chance of saturation resulting in spurious emissions than passive combining, especially if not the newer PA421B.
What Henry said: And, if you are looking for the PA421 on e-bay, or anywhere used, bear in mind that you actually need a PA421B as the original PA421 did not have the splitter/combiner on the front. In any case, I would recommend avoiding the older units (both the 421 and the 821) as they did have an issue with ports burning out.
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Brad Harris

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #3 on: January 25, 2021, 06:49:26 pm »

RF Pros,

I have an install where a (church) client bought themselves Shure P3T IEM systems over the years, but they've just ben sitting on the stage. 4x of them are Shure J13 (566-590 MHz) and 1x is G20 (488-512 MHz). I am going to be actually installing these correctly into their rack but want to correctly get them all onto one transmit antenna. There's only a few active combiners out there that have more than 4x ports but they are very expensive and definitely out of their budget. I can do a Shure PA411, but that leaves me with the 5th transmitter.

Can I do a PA411 for the 4x J13's and then use a UA221 to combine the PA411 with the 5th P3T?

I also thought about using a Mini-Circuits ZBSC-8-82+, and while it is 8-ports, it isn't active.. so it won't have any sort of IMD suppression. It is a lot cheaper though.

I am planning on using a Lectrosonics SNA600 adjustable dipole antenna. The antenna location is directly behind the stage but up about 15ft as the rack in on a sort of mezzanine behind the stage. I think the range is too short for a helical and I think a log periodic may be too narrow. With the SNA600 I can set the length somewhere in between the two bands which should be sufficient.  Maybe around 500 MHz, or a little higher and make sure the G20 is tuned on the upper end. I don't want to run two antennas.

If anyone has other ideas, I'm interested.

Thanks!


Yes to the PA411 and the UA221.

The PA421b unit mentioned in the other replies will accomplish the same, at more than twice the price, and twice the space (1u rack space vs. 1/2u)

Brad
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Henry Cohen

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #4 on: January 25, 2021, 07:50:53 pm »

Yes to the PA411 and the UA221.

The PA421b unit mentioned in the other replies will accomplish the same, at more than twice the price, and twice the space (1u rack space vs. 1/2u)

IIRC the 421B was less than a $100.00 price increase over the A when first introduced, but certainly not double. The 421A was the same 1RU form factor as the current B. In fact the metalwork is pretty much the same, including the front side passive splitter.

The UA221 passive splitter should not be used for transmits; it can't handle the RF power. It's meant for receive applications only. The UA221 also has 2dB insertion loss above the 3dB splitting loss. The Mini-Circuits' offerings will have <1dB.


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Henry Cohen

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Brad Harris

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #5 on: January 26, 2021, 11:08:20 am »

IIRC the 421B was less than a $100.00 price increase over the A when first introduced, but certainly not double. The 421A was the same 1RU form factor as the current B. In fact the metalwork is pretty much the same, including the front side passive splitter.

The UA221 passive splitter should not be used for transmits; it can't handle the RF power. It's meant for receive applications only. The UA221 also has 2dB insertion loss above the 3dB splitting loss. The Mini-Circuits' offerings will have <1dB.





OP was asking for a 411, not a 421(b) .... which the 421(b) is more than twice the price over the 411 ....
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Henry Cohen

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #6 on: January 26, 2021, 11:15:11 am »

OP was asking for a 411, not a 421(b) .... which the 421(b) is more than twice the price over the 411 ....

Right you are. I misread the OP.
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Henry Cohen

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Rui Lisboa

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #7 on: January 28, 2021, 03:55:24 am »


it actually does in a sense: Since it's passive there'll be about 10dB loss per input so that's about 20dB attenuation of reflected antenna port energy, more than the 4-way's 14-ish dB from the antenna port back into the combiner's internal amplifiers.

@Henry could you be so kind as to take me thru this math a little bit deeper?
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Henry Cohen

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #8 on: January 28, 2021, 01:15:26 pm »

@Henry could you be so kind as to take me thru this math a little bit deeper?

Math is pretty straight forward. In passive Wilkinson splitter/combiner circuits, each doubling of output/input ports adds another 3dB mathematical loss from input to output, or visa versa since the circuit is bi-directional:
   2-way split/combine = 3db mathematical loss
   4-way = 6db
   6-way =7.8dB
   8-way = 9dB
   . . . etc (see Mini-Circuits application note)

In addition to the mathematical loss, there is also a circuit insertion loss due to the imperfections, resistance and impedance [mis]match of the circuit traces and components at various frequencies. For a typical high quality Wilkinson splitter/combiner, this insertion loss is generally <1dB. Add to that the loss from the interconnect cables to and from the splitter/combiner, and the general rule of thumb is to add 1dB to the splitter/combiner's mathematical loss. Thus, an 8-way splitter can be considered to have 10dB  of insertion loss (9+1).

The primary cause of IMD is when multiple carriers mix in a non-linear stage, in this case either the IEM's or the combiner's power amplifier(s), and create IM products. In the IEM scenario, the most significant factor is when the carriers of the other transmitters in the combing system are reflected back from the combiner output port due to an impedance mismatch down the line (e.g. less than optimum coax, mismatch at the antenna, antenna not having unobstructed far field).

So, from input port to output port of the passive 8-way combiner is 10dB down. From output port [back] to input port is again 10dB down. Thus, any signal making the forward and reverse travel is at least 20dB down (if it's a 100% reflection, which would not be the case as long a working coax and antenna are connected).

And remember that an active combiner is simply an amplifier at each input of an X-way combining circuit to make up for the combing losses (unless it's a Doherty circuit, which is what Lectrosonics is using in their M2C combiner, but that's a subject for another day).
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Henry Cohen

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Rui Lisboa

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #9 on: January 28, 2021, 01:39:36 pm »

Quote
So, from input port to output port of the passive 8-way combiner is 10dB down. From output port [back] to input port is again 10dB down

Thanks Henry. The math is indeed simple and most of it was postulated.
But this "little" detail that I quote you on is where I lost you.
This was enlightening as I never realised reflection of such ports to be considered in a forward and reverse manner taking onto account its consequences. Oddly!, because after so much time trying to VNA everything I use for work I never woke up to this simple fact.
As usual your explanations are both enlightening and revigorating. Now you ignited a new quest: the Doherty circuit!  ;D Let the reading begin...
Bless You
« Last Edit: January 28, 2021, 01:43:57 pm by Rui Lisboa »
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brian maddox

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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #10 on: January 28, 2021, 07:25:25 pm »

Thanks Henry. The math is indeed simple and most of it was postulated.
But this "little" detail that I quote you on is where I lost you.
This was enlightening as I never realised reflection of such ports to be considered in a forward and reverse manner taking onto account its consequences. Oddly!, because after so much time trying to VNA everything I use for work I never woke up to this simple fact.
As usual your explanations are both enlightening and revigorating. Now you ignited a new quest: the Doherty circuit!  ;D Let the reading begin...
Bless You

^^yup
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Re: Combining RF from 5x IEM Transmitters of Two Bands
« Reply #10 on: January 28, 2021, 07:25:25 pm »


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