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Author Topic: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?  (Read 5128 times)

Ed Hall

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I’m new to wireless mics having just bought two Shure ULXP units. I was looking for a rack mount for the antennas when I came across this

ebay listing for dual remote antenna kit

In the description the seller says that 75 ohm coax is better for this use. Shure specs 50 ohm cable and connectors. I don’t know a lot about this so I thought I’d ask the experts here.

Does this make sense or is it BS?

The relevant section is below


Ed



“What's the difference between 50 and 75 ohm coaxial cable?
Coaxial cable is comprised of three main components.  In the middle of the coaxial cable is what is known as the center conductor.  It can be made of either solid or stranded wire and is typically a mix of Aluminum and Copper.  Surrounding the center conductor is something called the dielectric.  The dielectric acts as a buffer of sorts to keep the center conductor isolated and straight. It usually is comprised of some blend of plastic and/or foam. Finally, on the outside of the dielectric is the coaxial cable’s shield, which is usually a combination of Copper and Aluminum foil and/or wire braid.  The shield is then coated by something like PVC to insulate it from the environment.

Now, not all coaxial cable is created equal and that is where the coaxial cable impedance comes into play.  It is the coaxial cable’s physical characteristics that will determine its impedance. According to Wikipedia, “The characteristic impedance of the coaxial cable (in Ohms) is determined by the dielectric constant of the inner insulator and the radii of the inner and outer conductors.” The cutaway drawing above is helpful in visualizing these characteristics.  With these details in mind, over time, the industry settled on two characteristic coaxial cable impedances for the vast majority of applications (>90%): 50 Ohm and 75 Ohm.

With 50 Ohm Coaxial Cables being the best compromise solution, practically any application that demands high power handling capacity, i.e. 100 watts or more, will use 50 Ohm Coaxial Cable.  A good rule of thumb is that any device that functions as a transmitter or transceiver tends to use 50 Ohm Coaxial Cable.  This includes devices such as CB/Ham Radios, Broadcast Radio/TV Transmitters, Wi-Fi and Cellular Phone Repeaters and 2-Way Radios (Walkie Talkies).

However, not every case warrants high power handling, so 50 Ohm Coaxial Cable is not appropriate for every application. When the objective is to ensure that the signal gets through the cable in the most efficient way possible, losing very little signal strength in the process, 75 Ohm Coaxial Cable is the way to go.  A good rule of thumb is that if the device being connected via coaxial cable is a receiver of some kind, 75 Ohm Coax is ideal.  This includes devices such as Satellite and Cable TV Receiver Boxes, High Definition Televisions, AM/FM Radio Receivers and Police Scanners. 

This combination of low attenuation and capacitance effectively make 75 Ohm Coaxial Cable the cable of choice for practically all types of digital audio, digital video and data signals.  This is why every cable TV company uses 75 Ohm coax for distributing its digital video channels as well as its broadband internet data signals.  Direct broadcast satellite dishes and over-the-air HDTV antennas also require 75 Ohm Coaxial Cable to ensure that all of the digital channels transfer down the cable with the lowest loss and distortion possible. “
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Keith Broughton

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Re: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?
« Reply #1 on: January 22, 2018, 04:54:36 am »

I'm sure one of the RF gurus will expand on why, but here is a short answer.

75 ohm is not "better" for this!
Will it work? Yes
Can you get away with short  75 ohm jumpers? Yes.
Is it optimum? No.
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Henry Cohen

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Re: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?
« Reply #2 on: January 22, 2018, 09:22:49 am »

To re-hash a prior post; this is a common misconception as it pertains to real world performance for low power wireless microphones, coms, IEMs and IFB's.

First, remember that an antenna is a transducer, which means it's actual impedance varies with frequency (just like a speaker). With wideband antennas marketed to cover in excess of 200MHz, their impedance will vary anywhere from about 37Ω to over 80Ω, being 50Ω roughly only in the center of the band.

Second, as it regards the front end of the wireless mic receiver, it too varies in impedance, somewhere between 50Ω & 75Ω with frequency due to parts choices and tolerances, and circuit design.

Third, whereas the output of a transmitter is closer to 50Ω across its tuning range, there's still the matter of the antenna and its wide impedance range.

In the end, using 75Ω coax to the antenna will be no better or worse than using 50Ω coax. What will matter is the shield construction of the coax: A low loss braid over foil dual shield construction (e.g. LMR series, Belden 9913F7, PWS S9046), having 100% coverage, will exhibit far less attenuation and permeability to RFI than a single shield style such as standard RG58, RG8X. RG8, and RG213 having only 95% - 96% shield coverage.

There can be some merit however to matching impedance when connecting passives (splitter/combiners, filters, isolators, hybrids, etc) together if they're being being used within their optimal performance range, but the mismatch loss is so small (less than .2dB) it's insignficant in RX applications and only marginally an issue in most TX situations. Now, if you're deploying a large distributed antenna system, with a lot of passive components throughout, the cumulative mismatches could add up enough to impact TX performance (but then your gain structure map would reveal that).
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Lyle Williams

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Re: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?
« Reply #3 on: January 24, 2018, 12:00:46 am »

+1
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Ed Hall

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Re: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?
« Reply #4 on: January 24, 2018, 05:14:50 pm »


Thanks for the replies.

I didn't think that it would make a big difference because it is a receive only application. I'm only talking about 2 feet of cable, just enough to get the antennas to the rear of the case. I have 1/2 wave antennas and will have the receiver stage side. The mic will never be more than about 30 feet or so from the receiver.

I just like to know the why behind the what.
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Lee Douglas

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Re: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?
« Reply #5 on: January 24, 2018, 08:39:09 pm »

Pre-made 50 ohm/RG-58 cables are pretty inexpensive. Check out the link below.  Look on the right for some right angle options.  By the time you figure a couple of compression connectors and a little bit of RG-6 cable for a roll your own, your not too far off from getting the right cable for the job.

http://www.cablesondemand.com/Library/InfoManage/Items.asp?product=CO-058BNCX200&gclid=EAIaIQobChMIyJC04fvx2AIVyIF-Ch3eaAKcEAQYAiABEgJe0PD_BwE
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Ed Hall

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Re: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?
« Reply #6 on: January 24, 2018, 08:59:16 pm »

Pre-made 50 ohm/RG-58 cables are pretty inexpensive. Check out the link below.  Look on the right for some right angle options.  By the time you figure a couple of compression connectors and a little bit of RG-6 cable for a roll your own, your not too far off from getting the right cable for the job.

http://www.cablesondemand.com/Library/InfoManage/Items.asp?product=CO-058BNCX200&gclid=EAIaIQobChMIyJC04fvx2AIVyIF-Ch3eaAKcEAQYAiABEgJe0PD_BwE

That’s what I did. I found another panel with the bulkhead connectors and bought the 50 ohm cables sized to fit.
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Re: 50 ohm vs 75 ohm coax for wireless mic antenna - does this make sense?
« Reply #6 on: January 24, 2018, 08:59:16 pm »


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