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Combining antennas from multiple studio locations


LarryF

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<<Question: I need to setup multiple receiver antennas in four separate studios as well as outdoor antennas for "man in the street" interviews. I want to have eight receivers in a central equipment rack to keep my life simple and I want to combine the remote antennas into as few coaxial cables as possible to save on cable costs and installation. Can I do all this with antenna combiners, splitters and amplifiers?>>

We get questions like this about once a month at Lectrosonics. Combining receiving antennas from multiple locations is not trivial. Most of the multiple antenna systems are "saved" by the fact that the distances from talent transmitter to the receiver antenna are small and, since the signals are strong, a lot of compromises can be overcome.

First, I'd like to list some of the problems, some subtle some not:

1. Contrary to what intuition would say, RF amplifiers inserted between the antenna and the receiver rarely increase range or performance. All professional receivers have more gain than they can use. Generally receivers have so much RF and IF gain that they clip the RF signal or its equivalent even when only low level RF noise is present let alone a stronger desired signal. More gain is just wasted and can cause RF intermod or overload. Extra RF gain is only necessary when there is loss in the antenna system caused by long cable runs or caused by combiners and splitters.

2. When combining receiving antennas you will have loss proportional to the number of antennas you are trying to combine into one signal. A 2 into 1 RF combiner will give you 3 dB of loss (1/2 of the original signal), a 4 into 1 will give you 6 dB of loss (1/4 of the signal), etc. This will always be true if either the amplitude, phase or frequency of the antenna signals are different. They always are. So you have unavoidable losses just from the act of combining signals. If this is a problem, it can be addressed with amplifiers at each antenna. Amplifiers present other problems but at least it is theoretically possible to overcome the combining losses. Note that trying to use a single amp after the combiner (after the loss) does you no good. Once you have a combining loss, your signal to noise ratio has deteriorated and you can't get it back with amplification after the loss. You have to use an amplifier at each antenna to compensate perfectly for combining losses. Fortunately, perfection is rarely needed.

3. When combining receiving antennas, each antenna may add additional desired signal but it will also always (!) add additional noise. At best you can break even. Here's an example. If you have 4 antennas, all picking up roughly equal signals (and noise) and you then connect them to a 4 into 1 combiner, you will have a 6 dB loss (1/4) in the combiner. You now have an output signal equal to a single antenna with the noise of one antenna. You have broken even. However, if each antenna is in a different room with the talent only being near one of the antennas, you now have 1 antenna's worth of signal and 4 antennas worth of noise. After the combiner loss of 6 dB (1/4), you will have one fourth of the original antenna signal combined with one full antenna's worth of noise. Your signal to noise ratio is now 6 dB worse. Putting amplifiers at the four antennas does not help the signal to noise ratio because they amplify the noise just as well as the desired signal.

If this all sounds nebulous, simply imagine four microphones in 4 slightly noisy rooms with the talent only in one room. Mixing the four mics into one output gives you one mic worth of talent and four mics worth of noise. Amplifying all four mics equally does not improve the talent to noise ratio. The talent level comes up but so does the room noise.

Above are the negatives. Below are some positives.

1. Since the antennas are usually placed in each room, that means the antennas are usually placed close to the talent such as 25 feet away typically. Having the antennas that close to the talent generally overcomes a lot of combiner shortcomings.

2. It may be possible, depending on the room placements to put a centrally located antenna that will not be too far away from the talent in any room. For instance, for four rooms arranged in a square around a central point would allow an antenna at the center to pick up all the rooms. If, however, the four rooms are in a straight line, antennas placed at the common wall of the first and second room and a second set placed at the common wall of the third and fourth room could work well. That is still better than four sets of antennas.

3. If the rooms are not used at the same time, it may be possible to use RF switches or remote antenna relays to connect antennas as they are needed. This can eliminate a lot of complexity. This is particularly handy if one set of antennas needs a lot of range such as the outside "man on the street" scenario. The relays are several hundred dollars and can be controlled with a simple on-off 12 Volt DC signal.

4. Omnidirectional antennas in the center of a room will probably work better than a directional gain antenna at the end of the room. Directional antennas come into use when you must have the antenna at one edge of the talent movement area and have longer distances.

5. Getting the antennas overhead, if possible, will generally always help bring up signal levels, helping with both range and combiner losses.

So, let's try constructing a system using the parameters of the original question at the beginning of all this. We'll make it tougher by specifying hard walls with metal sheathing between the 4 studios so we can't reuse common antennas even though the rooms are close to one another. We will require the "man in the street" scenario to cover pretty good distances, and we will require that all 8 receivers be in a common control room or closet and no antenna switching will be allowed because the users aren't up to reconfiguring the systems, even with access to detailed pictures and crayons. Oh, and all the studios may be used at once. The distance from each the studio rooms to the central equipment rack will be 100 feet more or less. This was a real setup by the way. The system will be discussed as a single antenna per location basis. In reality, two antennas would be used at each location for diversity but that just doubles everything.

First, for the outdoor location, we will use an ALP650 amplified antenna set for 12 dB of gain. The talent would use an SMQv transmitter set for 250mW for maximum range. If it is a handheld application, the HH would be used and set at 100 mW. Away from the body, that would be equivalent to a 250 mW belt pack near the body. Using Belden 9913 cable, we can run 100' to an 8 way combiner (#ZFSC843) with only a few dB of loss. The four rooms would each have an SNA600 antenna in the center of the ceiling. In a 40 foot square room, there would be less than 25 feet from the transmitter(s) to the antenna so the received signal would be very strong particularly with 250 mW belt packs. Again, we will run up to 100 feet of 9913 coax with only a few dB of loss to get to the 8 way combiner. We will do the same with the other three studio rooms. We have used 5 of the 8 input ports on the 8 way combiner but since 5 way combiners are specials, this is our best (only) choice. We now run a 100 feet of 9913 cable to the input of the UMC16b, a dual channel 1 to 8 amplified splitter. The 8 outputs now go to an input on each of the 8 receivers in our central location. All of this, except the UMC16b and receivers, will be doubled for the necessary diversity reception.

Something seems wrong here since, we have a combiner (#ZFSC843) with 10 dB of loss and no make up amplifiers for four of the antennas. First, let's track the signal from the outside antenna, the ALP650 to the receivers. We have +12 dB of gain from the built in antenna amplifier less various losses in cables and combiners. The 100 foot cable from the ALP650 to the 8 way combiner has 3 dB of loss typical, the 8 way combiner has 10 dB of loss and the 100 foot cable from the combiner to the amplified 8 way splitter has 3dB of loss. The UMC16b 8 way splitter has unity gain. From the ALP650 antenna port terminals (before the built in amplifier) to the receiver input, the gain loss numbers in sequence are +12dB -3dB -10dB -3dB -0dB = -4dB. This -4dB is not quite on the edge of needing an additional amp for the best possible range. We are helped by the fact that the ALP650 has 4.5dBd of antenna gain even before the built in amp and the 250mW transmitter has 4 dB more output than a 100mW transmitter. We should have plenty of signal to overcome a moderate 4dB loss and have very good range.

What about the signal from the 4 inside rooms? Assuming the same losses from cables and the 8 to 1 combiner, we have a loss from the central antenna to the receiver of -16dB. Since we would use an antenna like the dipole SNA600 in the center of the 40ft x 40ft room, we have no antenna gain. So we have the full 16dB of loss as added up above. However, the antennas are only 25 feet or less away from the transmitters. If we set the SMQv's for 250mW, then the 16dB of loss makes it seem as if the transmitters are 6 mW units at the same 25 feet since 16dB is a power factor of 40. However, 6 mW is plenty of power for a 25 foot distance.

If 6 mW is enough, why does Lectrosonics push 100 mW and 250 mW units as being necessary (or at least desirable) in today's RF environment. It is because of RF "noise" pollution. It sometimes takes strong signals to overcome the other garbage that is on the receiver's frequency such as TV stations a 100 miles away that aren't producing a signal strong enough to register on the display (1 uV or -107 dBm) but are still enough to cut the range in half. If the noise from the environment is greater than the front end noise of the receiver, the receiver sensitivity doesn't define potential range; the noise does, along with the transmitter power. Again, the RF noise at your location usually sets the limits of your range. In today's world, we can assume some amount of environmental noise in any urban setting. And guess where most studios are located.

The antennas will not only pick up the desired signal but also a fair amount of this environmental RF noise. The good news is that the loss in the cables and the combiner reduce this environmental RF noise by 16 dB at the same time as they are reducing the transmitter signal by 16 dB. You could do all kinds of complicated calculations but if we assume the environmental noise, though bad, is less than 16 dB above the receiver input noise floor, then the 16dB of loss applied to the noise makes it seem as if the antenna system and the receiver are operating in a perfectly quiet location. In a perfectly quiet location, that 6 mW equivalent (250 mW with 16 dB of loss) is more than enough for a 25 foot distance.

To make sure there is no confusion, I am not recommending adding attenuators to antenna signals to reduce the noise and somehow improve reception. I am just saying that in a noisy environment, attenuating both noise and a desired signal the same amount may very well make no difference in reception. Remember that the important thing at a receiver's antenna terminals is that the RF signal to noise ratio be high enough to provide good reception. It doesn't make a real difference if the noise is at -100 dBm and the signal is at -80 dBm or if the noise is 16 dB higher at -84 dBm and the signal is 16 dB higher at -64 dBm. The receiver will perform the same and perform well in either case, since the SNR is 20 dB in both cases. A high power transmitter can overcome higher levels of noise than a low powered transmitter and will be more able to overcome losses in the antenna system.

The example given was lucky enough to have short distances for four of the studios and only one long distance pickup, the "man in the street". If all 5 pickups had to be at long distance, the setup would have been much more complicated with amplification at all 5 antennas or the use of RF relays or more RF cables from the studios back to the receivers. If all the situations were at long distances and all had to perform at their very best, then more receivers would be the solution with much simpler audio mixing used to arrange what went where.

Finally, I hope the "straw man" example showed some of the subtleties of antenna systems. If it is clear as mud, let me know and I'll try to rewrite the offending section before I make an FAQ out of it.

Finally, finally: other good equipment besides Lectro could provide the same sort of system. I've used Lectro equipment because I know it.

Best Regards,

Larry Fisher

Lectrosonics

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Larry, thank you so much for taking the time to write this. I'm still learning as a soundman and coming to terms with exactly how RF issues work, so having this kind of detail spelled out for the layman (me) is invaluable.

The Lectro user manuals have long been my go-to source for RF and wireless knowledge, though a good 50% of it is still way over my head. I'm just thankful that there are folks around willing to put it out there for the sake of giving a hand up.

Thanks again.

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I forgot one important fact, that the range of the outdoor, long range, amplified antenna is mostly unaffected by the un-amplified antennas, since the outside antenna's signal is raised well above their noise contribution. I'll add this:

<<Back at the beginning of this, I said there would be some subtleties. One of them is that the outside antenna, which needed the best range, is the only one that is amplified. When the amplified outside antenna is combined with the four inside antennas, the noise from those inside antennas has only a small effect on the outside antenna's range because the outside antenna has been amplified by 12 dB. Let's look at the numbers. Assume the received environmental noise (before amplification) of the outside and inside antennas is the same. Amplifying the outside antenna noise by 12 dB increases the signal power by 16 as well as the noise power by 16 (12 dB = power factor of 16). After amplification, the outside antenna has 16 units of noise power. Add to that the noise from the four inside antennas which is 4 units of noise power and we have 20 units of noise power. In decibels, that is 13 dB. But we started out with 12 dB of noise power from the outside antenna after amplification anyway. So our noise power has only gotten 1 dB worse from 12 dB to 13 dB. One dB more noise would reduce the outside antenna range by about 11%, which is not bad. You could also calculate this in power units as range = the square root of 16/20, which again is 0.894 or about 11% less range. If we had amplified all five antennas, then the total noise power measured in our noise power units would be 16+16+16+16+16 = 80. In decibels this is 19 dB or 7 decibels worse than our original noise power 12 dB. Whether we figure it out in decibels or noise power, the effective outside range is cut to half of what it would be with un-amplified inside antennas.

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  • 1 month later...
  • 1 month later...

[snip]

Could I possibly milk your goodwill a little further by explaining a theoretical or case study where the situation might be all 5 pick-ups at relatively long distances.

That is covered by number 3 in my post above with the added difficulty that you need even better antenna performance because of the long distances. Directional antennas would help by giving you improved signal to noise ratios since it is the same SNR dB improvement as the antenna gain. However, with 5 antennas combined into one signal you will have 8 dB of combiner loss, which is border line for needing an antenna amplifier at each antenna. No matter what you do, you still have the fundamental problem that you have the combined noise of 5 antennas added to the desired signal from each distant transmitter. You have actually described the worst case scenario for antenna combining.

Probably, not all antennas are required at one time so an antenna switch would get rid of the combination problems. A switch is really the best solution. If they are all required simultaneously, separate antennas is a solution also. More expensive plus more cabeling but as good as the switch. Past that, high output transmitters may be able to overcome the noise buildup problems and give you acceptable range.

Best,

Larry F

Lectro

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  • 9 months later...

Hi Jay,

MiniCircuits has mechanical SPDT relays at 12V and 200 mA. They are terminated in SMA connectors though.

 

Pasternack has manual units here:

  http://www.pasternack.com/2-port-n-mechanical-coaxial-switch-surge-protection-dc-1.3-ghz-1500watts-pe7139-p.aspx

 

They are terminated in N connectors but the N to BNC adapters are plentiful and rugged.

Best Regards,
Larry Fisher
Lectrosonics

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