Wireless Power & antenna distribution for 20+ RF mic receivers

[mbenonis]

Yes and no... TV antenna runs on 75 ohm coax and for best results, RF should be running on 50 ohm. And so you will have an impedance mismatch when connecting to a 75 ohm splitter and / or amplifier... There's 6dB gone...

Not necessarily. Two things come into play with regard to impedance in your coax/antenna/receiver circuit. First, neither the antenna or the receiver is really 50Ω, and in fact they might deviate quite a bit from this ideal rating. Second, the impedance mismatch causes 0.18 dB of loss total, independent of cable length. See Jim Brown's article here for the gory details.

So I say it's just fine to use 75Ω coax cable for your mics, if you use good connectors and good cable with it. 75Ω splitters...yes, if they're high quality (probably not the rat shack variety).

 

[Chris15]

For the time being I'll take your word Mike... The numbers I heard from my source were bigger...

It's 0.18dB (to use your number) per mismatch. So to assume a 50R antenna and a 50R receiver, there's 0.36dB. Now while the antenna itself might not have a very fixed value, would not an active antenna's preamplifier have a more stable output impedance?

Now this also relies on using 50R connectors on 50R cable and likewise with 75R. Otherwise you can get a mismatch each time there...

We use 50 ohm coax exclusively for RF and likewise 75 ohm coax exclusively for MADI, Word Clock, AES, and if were were forced to deal with video, video.
But when you are running in some cases 50 metres of coax, every portion of a dB matters... Especially on a transmit line... (That would be on Times LMR- 600 extraflex for what little it might be worth, with only 4.9dB loss at 900 MHz)

Alas, you can normally get away with 75 ohm yes, but when you have the option, why would you risk it?

 

[mbenonis]

You are absolutely correct - it is 0.18 dB per mismatch. That said, this assumes a perfect mismatch. I would argue that the antenna probably doesn't look like 50Ω at all - it could be higher (if it's a dipole, it's going to be closer to 73Ω assuming you're looking at it at the exact center frequency) and if it's something else it could be anywhere between a few ohms and a few kOhms. The input of the receiver (or i/o of the preamplifier) is going to be a lot closer to 50Ω, but I would say it could range anywhere from 40 to 75Ω or so, depending on frequency in use. The bottom line is that what you should use depends on your unique situation. If you have good signal to begin with and have a limited budget, go for the RG6 - it's lower loss than RG58 and much less expensive. But if your SNR is getting low, or you need to run longer distance, buy the LMR400 or equivalent cable just to be safe.

If you're TRANSMITTING, all of the above goes out the window! You should absolutely use 50Ω cable when transmitting, and also verify that your antenna is tuned for a low VSWR (Voltage Standing Wave Ratio). This is to protect the transmitter and to ensure minimal losses.

 

[Chris15]

I agree I'd take RG6 over RG58 for anything longer than an in rack patch any day... The lack of copper in the shield on some RG58 I've seen makes me wonder why they even bothered...

I think that anywhere where you have both transmit and receive it it important to use proper 50 ohm cable, because we have all seen people do the swapsies thing...

Mike and some others may find it interesting to note (where they were not previously aware of it) that the copper strip in the middle of a PWS helical is there solely to set up the correct impedance of the antenna...

And these things are, let's be honest, a bit like speaker impedances, at what frequency are you talking about. They all have a complex impedance that is frequency dependent, as Mike has noted...

Also, if you are transmitting, particuarly if an amplifier is involved, then do whatever you can to protect against dimwits unplugging the antenna, not only because one loses transmission, but because the backfeed from lack of load on the amplifier can kill it... (Let's just say that putting the antenna on a accessible press / camera riser maybe wasn't such a great idea...)

 

[avare]

Chris and Mike:

It is an interesting direction you are taking this topic. If you are going to mix the two impedances, just use one impedance matching bridge at the interface.

Andre

 

[mbenonis]

Chris and Mike:

It is an interesting direction you are taking this topic. If you are going to mix the two impedances, just use one impedance matching bridge at the interface.

Andre

Yes, you could absolutely do that. Though it would add cost to the project, depending on what bridge you used...

 

[Wolfgang]

Now switchmodes...

But as far as you need to know, that's supremely irrelevant. You take a rack case and mount a pair of supplies like this, except one that is relevant to your voltage and current needs. Chances are someone else sells them at a more reasonable price.

So I've been looking for 5 amp switch mode power supplies, without much luck. Each receiver needs .2A of current, so 5 amps seems to be a good rating to go with. Any suggestions? Or should I be using 2 power supplies, each running half the receivers.

Also, how do you run those parallel power supplies you referred to earlier? Having that kind of redundancy seems to be a good idea, just in case.

Thanks,
Wolfgang

 

[waynehoskins]

Alas, you can normally get away with 75 ohm yes, but when you have the option, why would you risk it?

At the risk of causing more discussion or argument:

The impedance of the transmission line should be matched to the nominal feedpoint impedance of the antenna. The receiver's front-end is very high impedance, or at least used to be, so they don't care so much.

A dipole should be fed not with 52-ohm coax, but 72-ohm coax. And in fact, not coax but balanced line since it's a balanced antenna. But what do we do? We feed a dipole with 52-ohm coax because it's readily available.

So, a TV antenna is typically a nominally-72-ohm antenna at its feedpoint. The impedance of the receiver is unimportant; it's high. Feeding a 72-ohm antenna with 75-ohm cable will also result in theoretical loss of some tiny fraction of a dB.

And to using a 75-ohm cable on a 52-ohm antenna, it's not that bad. If your 52-ohm antenna is in absolute free space, and you're able to measure its feedpoint impedance at the frequency of interest, you might find that it's 52+j0 ohms. Probably it's not. Sure, things in the near field tend to lower the impedance rather than raise it, but it's a receiver feedline. It doesn't care.

 

[Chris15]

So I've been looking for 5 amp switch mode power supplies, without much luck. Each receiver needs .2A of current, so 5 amps seems to be a good rating to go with. Any suggestions? Or should I be using 2 power supplies, each running half the receivers.

Also, how do you run those parallel power supplies you referred to earlier? Having that kind of redundancy seems to be a good idea, just in case.

Thanks,
Wolfgang

To run the paralleled supplies, connect the ground terminal of supply A to the ground of supply B and to the ground of the outputs. Do likewise for the +ve connections... It is that basic...

I think in a way, the redundancy becomes a touch more important when running a single supply. Switchmodes, in the normal course of events, fail. If you lose one radio mic receiver that isn't good, but you can probably work around it. Your single supply dies, you have bigger issues...