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LarryF

Question about passive splitter for bag use

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This a question on Facebook about using passive splitters to feed three receivers in a bag. The user wanted to reduce the number of antennas from 6 to 2 and was questioning whether he would have to amplify the antennas which would complicate the bag setup. My reply for what it is worth:

 

Since you don't need extra range, the small loss in a passive splitter won't hurt your results. Passive splitters may reduce the transmitted RF signal but they also reduce the RF noise in most urban locations by the same amount ,so quite often it is a wash. Getting the antennas up out of the bag will give you extra signal and will probably more than make up for the splitter loss. Also, and I can't emphasize this enough, it will get your antennas away from any RF noise sources in your bag. This could be a very big improvement by itself.

Best Regards,

Larry Fisher

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Where would you say the tipping point is for when you need to add an amp to makeup the splitter loss? 4 receivers? I have a few of the mini-circuits 4-way splitters but I'm always hesitant to use them without amps. 

 

 

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1 hour ago, Derek H said:

Where would you say the tipping point is for when you need to add an amp to makeup the splitter loss? 4 receivers? I have a few of the mini-circuits 4-way splitters but I'm always hesitant to use them without amps. 

 

 

Hi Derek,

To quote a famous philosopher, "It depends."

 

You will have 6 dB of loss so your range will be cut in half in perfect RF conditions. In Flat Rock, Montana, shooting far distant filmscapes, an amplifier might be helpful. However, in LA, with your signal squeezed between two TV stations, you probably can't tell the difference for the reasons mentioned above. An amplifier is just going to amply the interferences also, weak though they may be.

 

Yet another way of looking at it: 

If you are getting a barely dependable 100' due to other RF noise, the splitter is not going to reduce your range to 50'. You will still get 99'. An amp is not going to give you 200 feet. It will just amplify the ambient RF noise.

If you are getting a solid 1/4 mile in obviously quiet RF conditions the splitter is going to reduce your range to an eighth mile. If you need the long range, this is where you need and can gainfully use an amp.

Best Regards,

Larry Fisher

 

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Hello Larry,

 Do you have any thoughts about how the recent incorporation of aluminum framed structures in sound kit bags, and the resulting near proximity of the sheet of metal to both receivers and sources of emissions, may effect radio transmission and reception?

 

 Thank you.

 

 

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If the framing is open, that is the bag has struts that have distances between the struts of a 1/4 wavelength or more, it probably will make little practical difference to range, i.e., if the struts are mostly 6 inches or so apart, they will have small effect on the signal. 

 

If the antennas are closer than three inches to a large piece of metal, you will start getting some loss of range. More than three inches of separation and the differences will be small. At a 1/4 wavelength of distance you will get 3 dB of gain for instance. Basically you don't want the antennas in a solid metal box or a closed box made of metal struts all closer than 3" inches to each other. A wire screen, for instance, is just about as effective as a solid metal plate for screening. The metal parts will cause reflections, but all the other recorders, wire, power supplies, etc., in the bag reflect signals anyway. What's a few more reflections.

 

Of more possible concern, is in band RF noise put out by other devices in the bag. As some of you remember, this was quite a problem with some recorders a few years ago. Fortunately, this is mainly resolved. It is something to look out for though. If your receiver has an RF display, you can turn devices off and on and look for low level signals. Even the smallest (a pixel or two) on the display will greatly mess up your range. Some camera hop transmitters can be quite noisy for instance.

Best Regards,

Larry Fisher

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Been meaning to add a passive splitter to my bag as well but wasn't clear about a few things, this discussion helped thanks Larry and others.

 

CS

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On 3/13/2018 at 2:04 PM, LarryF said:

To quote a famous philosopher, "It depends."

Please don't think for a second that this line went unnoticed or unappreciated!

 

<HAT>

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So I just wanted to follow up here real quick and make sure I understand some of this correctly. My company has a little extra money to get some audio stuff and I was looking into getting some shark fins, as we've had a few issues with signal strength and range with out Lectro SSM's and SRc's (I'm guessing its the SSM since it can only output 50mW). I called Trew to get their input into the case and the person I spoke with told me that one part of the chain has to be active (i.e. either the antenna has to be active or the splitter), however what I'm gathering form this thread is that is not the case. I can put a passive antenna into a passive splitter and that will work just fine? 

 

The other questions I have is can 1 antenna feed 2 SRc's? They are A and B blocks respectively if that impacts anything. Again Trew told me that I should get two fins to have a larger coverage area but I don't know if we have the budget for that at this time. I'm really just looking for something to help us get a little stronger signal when we are a little too far from talent with the SSM. 

 

Thanks! 

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You are better off with a good antenna into a two way splitter than a week antenna into a splitter with gain. Getting the antennas up and also away from other gear will do more than some gain from an amplifier. Use a mini circuits or Lectro two way splitter. A four way splitter needs gain by the way.

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Understood, this is the set up I was thinking: Lectrosonics ZFSC41 with a PSC UHF Log Periodic Skeletal Antenna. Now based on your last comment there I guess this won't actually work because it's a 4 way splitter, which also answers my last questions about 1 antenna feeding into multiple SRc's, since the 4 way needs power I take it that won't work. 

 

I'm guessing this would work better for the splitter: Lectro ZSC24?

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5 minutes ago, Erob said:

Understood, this is the set up I was thinking: Lectrosonics ZFSC41 with a PSC UHF Log Periodic Skeletal Antenna. Now based on your last comment there I guess this won't actually work because it's a 4 way splitter, which also answers my last questions about 1 antenna feeding into multiple SRc's, since the 4 way needs power I take it that won't work. 

 

I'm guessing this would work better for the splitter: Lectro ZSC24?

The 4 way has a little over 6 dB of loss. That is more or close to what you will gain with better antenna position. Your range probably won't improve. The 2 way has 3 dB of loss and that can be overcome with better antennas and position.

Best Regards,

Larry Fisher

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Something Larry mentioned bears repeating (or should I say "bares repeating" since there's another thread on mixing naked).  Getting your antennas high enough that there is not anything (people, etc.) between the transmitter and receiving antennas will make a big difference.  That, together with  good antennas makes more difference than pretty much anything else.

 

As far as transmitter power -- that is an often misunderstood specification.  The difference between 50mW and 100mW is only 3dB and in the real world, unless distance is truly a limiting factor, it makes even less impact than the 3dB figure would have you believe.  For instance, a Comtek transmitter outputs only 10mW yet has admirable range.

 

The biggest limiting factor with a bodypack transmitter is that it's typically fastened to a large bag of water (person) and if the antenna isn't high enough, the signal must permeate many other large bags of water.  Then, there's multipath to take into account, where sometimes lower power can work better than higher power.  In other words, way too many people obsess over transmitter power numbers when there are much more pertinent things to consider.

 

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This all makes a lot more sense now. Thank you both for going further in depth. I'll go with the two way splitter and then just try to get the antenna up high to give a better line of sight (if you will) to the transmitter which will hopefully help with the signal we are getting. 

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The four-way splitter will work but just keep in mind you’re dealing with a significant signal loss. 

 

John’s right, take a careful look at transmitter placement on your talent. For example if you’re filming a walk and talk from the front putting the transmitter at the center of their back is not a good spot. The side or ankle would be a better choice. Less body material in between the transmitter and receiver is better. Skin contact with the antenna should be avoided too if you’re having trouble. Try stabbing the transmitter antenna through a makeup sponge to maintain some space from the skin. 

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16 minutes ago, Derek H said:

The four-way splitter will work but just keep in mind you’re dealing with a significant signal loss. 

 

John’s right, take a careful look at transmitter placement on your talent. For example if you’re filming a walk and talk from the front putting the transmitter at the center of their back is not a good spot. The side or ankle would be a better choice. Less body material in between the transmitter and receiver is better. Skin contact with the antenna should be avoided too if you’re having trouble. Try stabbing the transmitter antenna through a makeup sponge to maintain some space from the skin. 

+1

LEF

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With great success, I use the 3-way passive splitter with 3 x 411a receivers on a process trailer - using 25ft of high loss 50 ohm BNC cable.

 

The factor which gives me success is putting the antennas on the back of the truck, only 15ft or so from the car with the actors in it. But I have also used this rig with free driving, or on a mini cart. In all cases, the fact that the antennas were clear of as many obstacles as possible was enough to compensate for the loss in the system.

 

There's a lot of science to all of this, and the biggest misconception is increasing transmitter power will improve range. I can't tell you the number of mixers who struggle with reception, and are convinced they're doing everything they can by using powered antennas and running their transmitters at 250mW. There are other mixers who have no issues at all running SSMs at 50mW.

 

My experience has been to only add cable if it will significantly improve line of sight, enough to compensate for line loss. I have never used powered antennas, and almost always run at 100mW. The few times I've gone to 250mW seems to have made virtually no difference.

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16 hours ago, RPSharman said:

With great success, I use the 3-way passive splitter with 3 x 411a receivers on a process trailer - using 25ft of high loss 50 ohm BNC cable.

 

 

Now are you going into all of the receivers? The only thing I'm still a little confused about is if one antenna can feed multiple receivers. 

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1 hour ago, Erob said:

 

 

Now are you going into all of the receivers? The only thing I'm still a little confused about is if one antenna can feed multiple receivers. 

Yes. One antenna can feed multiple receivers through either an active or passive splitter. A passive splitter will have loss equal to the number of output ports. i.e., a 4 way splitter will feed each port (receiver) 1/4 of the incoming signal (-6dB). A 2 way spltter will feed each 1/2 the signal (-3dB). The loss may not be important depending on circumstances and may be negated by better or closer antenna placement.

Best Regards,

Larry Fisher

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16 minutes ago, RPSharman said:

Unless it isn't clear. Two antennas. Two splitters. Two antenna inputs on each receiver.

OK. Each diversity antenna input on a receiver needs to come from different antennas. On the SRc, for instance, each input on the SRc needs to connect to a different antenna or different antenna splitter. Feeding each input with an identical signal would defeat the operation of diversity input (no diversity in the inputs).

So, antenna A goes to splitter A and one splitter output from splitter A goes to an SRc. The other input on the same SRc is fed by antenna B through an output port from splitter B.

 

Am I answering the question raised?

Best Regards,

Larry Fisher 

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