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jonathan chiles

RF Multicouplers - Wide or Narrow Band?

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 Assuming wide band antennas like the Betso Sharkies I am using (470 - 850mHz) I have always gone for RF multicouplers with as narrow a pass band as possible (such as Lectro UMC16A which is 2 blocks wide only) and avoid anything wide band. Real world use... is there a valid reason to avoid something like a PSC RF Multi wideband (470 - 870mHz)?? I am feeding Lectro SRb and 411A and am looking for an RF combiner that is baggable and can power antennas. I know the filters on the receivers do a good job (especially 411A) but I worry about amplifying a lot of out of band noise which is then passed on by the wideband multicpupler and then possibly overloading the receiver front ends and thereby neutralizing any gains from the directional active antennas. 

 

Thoughts? I know Larry F is near :)

 

 

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The PSC MultiSMA doesn’t have an RF amp, only to make up for its own loss. So it shouldn’t boos any signal more than if you didn’t have a distro. Nonetheless, the output from my MultiSMA is passed through a Lectro UFM50 amd then onto the receivers 

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On 4/14/2019 at 11:14 AM, jonathan chiles said:

 Assuming wide band antennas like the Betso Sharkies I am using (470 - 850mHz) I have always gone for RF multicouplers with as narrow a pass band as possible (such as Lectro UMC16A which is 2 blocks wide only) and avoid anything wide band. Real world use... is there a valid reason to avoid something like a PSC RF Multi wideband (470 - 870mHz)?? I am feeding Lectro SRb and 411A and am looking for an RF combiner that is baggable and can power antennas. I know the filters on the receivers do a good job (especially 411A) but I worry about amplifying a lot of out of band noise which is then passed on by the wideband multicpupler and then possibly overloading the receiver front ends and thereby neutralizing any gains from the directional active antennas. 

 

Thoughts? I know Larry F is near :)

 

 

Avoid is kinda strong. There are good reasons for using narrower filters in a multicoupler, particularly as new cell phone usage starts up in the 600 to 800 Mhz bands. Your post pretty well gives the reasons for narrower filters. The 411A does a better job than most of front end filtering but that advantage is somewhat over ridden by any multicoupler, more so by wide ones with weak amplifiers. In a well designed multicoupler not only will the filters be only as wide as necessary but the internal amp will be low distortion at high input levels in order to not produce RF intermod products. This spec is commonly left out by some manufactures, as it is hard (expensive) to accomplish. Instead they will quote amazing noise figures which are easy (read cheap). Ideally the amp in a multicoupler will have low gain, low noise, and low distortion (a high third order input intermod number). As usual with things RF, these desirable traits are not easy to attain simultaneously. A really strong, low intermod RF amp can make up for wide band input filters, but the ideal is narrow filters and strong amps.

Watch out for quotes of output intermod number. These values are always higher than the input intermod value and make for better numbers. What really measures the performance in a multicoupler is the input intermod value. A high gain, low power amplifier can have good output numbers but weak input numbers since the input number is the output value MINUS the amplifier gain, i.e., high gain leads to poor input intermod values but usually excellent noise figures.


One way to improve the performance of a wideband multicopler, is to use antennas with built in filtering or inherent narrow band response. For instance, an SNA600 dipole has about a 30 MHz bandwidth. That is equivalent to having a 30 MHz filter at the input of the multicoupler. A Yagi antenna would be an even narrower bandwidth. Sharkfins (log periodics) have wide response so are not good "filters".  Powered sharkfins with built in filters can help.

Another way to protect a wideband unit is to put a low loss inline filter in front of the wideband multicoupler input and then swap out the inline filters depending on what bands you are operating in. As an easy example, the Lectro PF25 is a one block wide passive filter and the PF50 is two blocks wide.

 

What the user would like to have is a wideband antenna system and a wideband multicoupler that does not introduce spurious signals (low intermod) and is usable for all possible wireless frequencies.  As in most RF compromise, as the airwaves become more congested this dream is going to become a little bit of a nightmare or at least a nightpony.

Best Regards,

Larry Fisher

 

On 4/14/2019 at 2:02 PM, Constantin said:

The PSC MultiSMA doesn’t have an RF amp, only to make up for its own loss. So it shouldn’t boos any signal more than if you didn’t have a distro. Nonetheless, the output from my MultiSMA is passed through a Lectro UFM50 amd then onto the receivers 

All very true, though I would say, if it has an amp at all, then input intermod values need to be considered. I would like to see PSC measure and publish third order input intermod numbers rather than just an excellent noise figure. See discussion above.

Best Regards,

Larry Fisher

 

Edited by LarryF
Added words" bandwith" for clarity

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Just checking back after a crazy week.. Larry thanks so much for the detailed reply. I have read you before mentioning these 3rd order input intermod figures and that those are the ones that matter. Also thanks for reminding me that a SNA600 is only 30mHz wide and thus could be a better choice sometimes. I think I am going to invest in some PF25s for now and see how things go.

 

Best wishes from Cape Town!

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30 minutes ago, jonathan chiles said:

Just checking back after a crazy week.. Larry thanks so much for the detailed reply. I have read you before mentioning these 3rd order input intermod figures and that those are the ones that matter. Also thanks for reminding me that a SNA600 is only 30mHz wide and thus could be a better choice sometimes. I think I am going to invest in some PF25s for now and see how things go.

 

Best wishes from Cape Town!

👍

LEF

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This topic has raised a couple of questions in my mind:

 

* There seems to be conflicting specs on the bandwidth of the SNA600.  I have seen the 30MHz figure, but I have also seen the figure of VSWR <= 2:1 over a 100MHz range, and the comment that the SNA600 fully extended will operate satisfactorily from 440MHz to 600MHz.  Does it come down to how we define bandwidth in a particular case?  For instance I wonder if the 30MHz spec is -3dB where the 2:1 VSWR might correspond more with -6dB?  (and VSWR is a spec more suited to transmitting anyway, correct?)

 

* My personal experience includes feeding block 26 receivers with a pair of SNA600's fully extended with no problems, so I'm doubtful they would offer much protection from new 600MHz cellular interference.  In this particular case it was also at the end of a long chain - (2) VRWB Lo, (2-3) VRWB Mid, a two-block UMC16 and a two-block UMC200d with the block 26 receiver in it.

 

* And, would there be any benefit in converting a "chain" of VRWB into a "star" configuration where each VRWB is fed from a wideband UMC16a?  It seems there might be slightly S/N for the downstream receivers but I'm not sure whether it would matter real world.

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1 hour ago, Matthew Steel said:

* There seems to be conflicting specs on the bandwidth of the SNA600.  I have seen the 30MHz figure, but I have also seen the figure of VSWR <= 2:1 over a 100MHz range

Hi Matthew,

The answer is simple. I was wrong. RTFM, Larry.

 

The listed 3 dB bandwidth for the SNA600 is indeed 100 Mhz. In the design of that antenna, I tried to broadband it as much as possible and apparently I succeeded. You'd think I'd remember the specs.

 

You are correct that the  VSWR number is important for transmitters because it indicates how much of the transmitter's power is reflected back into the transmitter where it can burn up the output of unprotected, high power outputs. Not a problem with 50 mW devices. You can have infinite VSWR (100 % reflection) and not hurt the output stage of a lavaliere wireless transmitter.

 

Typically, good  VSWR ratios are on the order of 2:1.  For a VSWR of 2:1 the antenna output is factored (reduced)  by 1 minus ((SWR-1)/(SWR+1)) squared. So for SWR=2 it is 1 minus 1/3 squared or 1 minus 1/9 or 8/9 or 88.9% of the power is delivered to the antenna and 11% is reflected back to the transmitter. The 2:1 number is considered good because a 100 Watt transmitter would have 11 Watts reflected back from the antenna and any typical transmitter can absorb that.

 

The star configuration is better but does require an additional device. For two receivers, the inline is very slightly better than the star since the first Venue receiver has only one amplifier (input) and the second Venue receiver has two: the first receiver's input plus the input of the second one. For three Venue receivers, the star is slightly superior, since all three receivers have two amplifiers at the input. However, the last Venue receiver of the inline has three in series. We don't usually recommend 4 inline Venues, though with good signals it would be satisfactory. In sum, star or inline are usually very close in performance until you get more than three Venues. 

Best Regards,

Larry Fisher

 

 

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What about a passive 2-way split vs. running two venues in-line? 3dB? loss for both but maybe better than having two amp stages for the second venue? Of course, I know there’s plenty of rigs out there with 2-3 venues daisy chained without any practical issues. Just curious. 

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1 hour ago, Derek H said:

What about a passive 2-way split vs. running two venues in-line? 3dB? loss for both but maybe better than having two amp stages for the second venue? Of course, I know there’s plenty of rigs out there with 2-3 venues daisy chained without any practical issues. Just curious. 

Hi Derek,

Curiosity coached the cat.

If you have good RF and/or excellent antennas, the 3 dB of loss is negligible in any area.

 

If you have a high noise floor in an RF busy area, the 3 dB loss is meaningless since even though you may be dropping the desired RF by 3 dB, you are also dropping the interference and/or noise floor by 3 dB. In any system, analog, hybrid or full digital, the signal to noise ratio at the receiver input is the critical number. A solid 100 uV signal will do you no good if you also have 50 uV of interference.

 

The reason really weak signals cause loss of transmission is that the incoming RF is too weak to overcome the inherent noise of the input stage. In noisy environments, the external noise may be 10 to 30 dB higher than the receiver front end noise so that the ratio of desired signal to environmental noise is the deciding factor rather than the level of the desired RF. So the only time a 3 dB splitter loss will hurt reception is if you are in very quiet RF environment (Central Australia) trying to pick up a 100 mW transmitter that is just a speck in the distance. Or a reasonable facsimile.

Best Regards,

Larry Fisher

 

 

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On 4/25/2019 at 10:55 AM, LarryF said:

...We don't usually recommend 4 inline Venues, though with good signals it would be satisfactory. In sum, star or inline are usually very close in performance until you get more than three Venues. 

Best Regards,

Larry Fisher

 

Thanks Larry!

 

I'll have to put a wideband multicoupler on my list of possible improvements, since one of our buildings has 5 inline Venues at this point.  Thankfully we haven't seen any reception issues so far.  In fact our last show was our biggest ever and we had an even longer antenna chain without troubles: SNA600 -> VRWBL -> VRWBL -> VRWBM -> VRWBM -> UMC16 25/26->(VRWBM, UMC200D, UMC200D).  The show had 49 transmitters (all Lectrosonics) and the only transmitter issues were a few noiseups on the ancient VHFs and a couple of LMb battery doors coming open.

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Hi Matthew,

Obviously you have good signals (and frequency planning). Five inline venues and 49 transmitters is pushing the real world envelope. What kind of distances were you operating at?

 

I kind understand the noise ups on the older VHFs. They had about 1/3 the range of the later UHF gear, partially because they were 50 mW and some were not diversity receivers.

 

Did you get the battery door problems resolved? That concerns me a bit.

Best Regards,

Larry Fisher

 

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We have a couple of things going for us as far as channel count - we are indoors, in a building that uses a wire mesh as a base for plaster.  Also, we are in a small/medium city and so the TV stations are not totally crammed in.  We routinely operate at 200-250 feet between transmitter and antenna but not much farther.

 

I use Wireless Workbench for coordination.  I found the WW default settings for Lectrosonics gear to be a bit too conservative and so I messed around with Wireless Designer to see what intermod spacings it uses, then I set up custom device profiles from that info.  And, it helps that of the 49 channels there were 38 UHF and 11 VHF.

 

Personally I think the diversity has more to do with the VHF noiseups than anything.  And the fact that these are 30 year old units, some of which have never been sent back for a tuneup ever.  I doubt the 50mW had as much to do with it since some of our UHF were also 50mW.  Although we had 100mW stuff for that show it was our older 600mHz stuff, and with the 600MHz transition I have moved to 50mW or less across the board.  This is mainly because I don't believe we meet all the FCC requirements for part 74 licensing and I prefer us to be compliant.

 

As far as the LMb battery doors, I think it was confined to a small number of transmitters, possibly even only one.  I was able to adjust catch for the door a bit tighter.  We'll see how things go after that.

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3 hours ago, Matthew Steel said:

 

 We routinely operate at 200-250 feet between transmitter and antenna but not much farther.

LEF- Farther than I would have guessed.

 

I found the WW default settings for Lectrosonics gear to be a bit too conservative. 

LEF- Very true. Conservative R us.

 

Personally I think the diversity has more to do with the VHF noiseups than anything.

LEF- Ok.

 

As far as the LMb battery doors, I think it was confined to a small number of transmitters, possibly even only one.  I was able to adjust catch for the door a bit tighter. 

LEF- What the factory would have done.

 

Thanks for the update and additional info.

Best Regards,

Larry Fisher

 

 

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16 hours ago, LarryF said:

 We routinely operate at 200-250 feet between transmitter and antenna but not much farther.

LEF- Farther than I would have guessed.

 

Just for the record, I just paced it off and we're more in the range of 110-170 feet.  Considerably closer than my mental estimate.

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1 hour ago, Matthew Steel said:

Just for the record, I just paced it off and we're more in the range of 110-170 feet.  Considerably closer than my mental estimate.

Very reasonable.

LEF

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