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Author Topic: MPD Digital RG8X cables? //check my setup?  (Read 2562 times)

Henry Cohen

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #10 on: January 04, 2018, 08:21:50 pm »

I'm still not completely convinced remote mounting the antenna (closer to stage) with associated cable loss is better than having antenna at FOH with less cable loss. Though I guess the logic here is that air losses is greater than coax losses.

Coaxial cable losses at the distances we're working with will always be less than than free space path loss.
 
Pick a coax cable, any coax cable (and a frequency): http://www.qsl.net/co8tw/Coax_Calculator.htm. For safe measure, add 2dB to account for worn connector losses.

Now check the path loss at that same frequency (use "0" for transmitter and receiver gain): https://www.pasternack.com/t-calculator-fspl.aspx.

This is why it's just about always better to get the antennas as close to the transmitters as possible.

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Henry Cohen

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Russell Ault

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #11 on: January 05, 2018, 05:48:56 pm »

Get in the habit of doing path loss calculations over free space and cable.

Now check the path loss at that same frequency (use "0" for transmitter and receiver gain): https://www.pasternack.com/t-calculator-fspl.aspx.

In the spirit of Merlijn van Veen ("if your mental estimate is within a decibel, it's close enough"), you can do quick-and-dirty Free Space Path Loss calculations for UHF microphones (etc.) in your head using the following equation:

FSPL = 20log(d) + 27

where 'd' is the distance between antennas in meters. For VHF devices, replace the 27 in that equation with an 18. This should get you within 2 dB of the Pasternack calculator; if you need more accuracy than that, you can use the following:

FSPL = 20log(d) + 20log(f) - 27.5

where 'd' is the distance between antennas in meters and 'f' is the frequency in MHz. This equation is not frequency dependent, and should get you well within a decibel every time.

(If you're wondering how "quick-and-dirty" and "20log(d)" go in the same post together, just remember that it's exactly the same mathematical operation you'd use for figuring out point-source SPL drop over distance.)

-Russ
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Henry Cohen

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #12 on: January 07, 2018, 05:17:42 pm »

FSPL = 20log(d) + 20log(f) - 27.5

where 'd' is the distance between antennas in meters and 'f' is the frequency in MHz. This equation is not frequency dependent . . .
[Emphasis added]

How is this equation not frequency dependent?
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Henry Cohen

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Russell Ault

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #13 on: January 08, 2018, 04:37:11 pm »

How is this equation not frequency dependent?

Sorry, I should have been clearer: the second equation can be used for any given frequency, whereas in the first equation the constant used (and therefore the equation itself) changes with frequency; to my way of thinking this makes the first equation itself frequency-dependant, whereas the second equation merely has a frequency-dependant result (as do all FSPL calculations).

-Russ
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Henry Cohen

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #14 on: January 08, 2018, 07:04:44 pm »

Sorry, I should have been clearer: the second equation can be used for any given frequency, whereas in the first equation the constant used (and therefore the equation itself) changes with frequency; to my way of thinking this makes the first equation itself frequency-dependant, whereas the second equation merely has a frequency-dependant result (as do all FSPL calculations).

No problem. I haven't used RPN since college  ;D
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Henry Cohen

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Joseph D. Macry

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #15 on: January 09, 2018, 12:26:05 pm »

One reason for using low loss coax (braid over foil, double braid or corrugated shield) within a rack is for RFI immunity. Remember that a standard single braid shielded coax is not just lossy, meaning RF energy escapes, but that same porosity that let's RF energy out lets unwanted RF energy into the coax and can contribute to a number of maladies including IMD, gain stage saturation and receiver desense.

For a typical wireless mic receiver rack, low loss coax won't make a substantive difference, unless the rack is also loaded with a significantly "noisy" digital equipment, IEM transmitters with less than stellar shielding, or other potential RFI/EMI sources.

I thought the term "low loss" referred to the heavier inner conductor, not the completeness of the shield.
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Don Boomer

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #16 on: January 09, 2018, 01:19:05 pm »

One reason for using low loss coax (braid over foil, double braid or corrugated shield) within a rack is for RFI immunity. Remember that a standard single braid shielded coax is not just lossy, meaning RF energy escapes, but that same porosity that let's RF energy out lets unwanted RF energy into the coax and can contribute to a number of maladies including IMD, gain stage saturation and receiver desense.

For a typical wireless mic receiver rack, low loss coax won't make a substantive difference, unless the rack is also loaded with a significantly "noisy" digital equipment, IEM transmitters with less than stellar shielding, or other potential RFI/EMI sources.

I just wanted to add to this for those that are new to digital wireless systems.  Digital transmitters are sending data (ones and zeros) so there is no dynamic range component to the signal (in standard terms).  The transmitters are always running at full power compared to analog systems.  There is likely much more energy running through your jumper cables and so a much bigger potential for interference.

For this reason you should use higher grade cables (RG8x or LMR-195) when connecting to your distro.  It's also a best practice to NOT neatly lace all the cables together.  A rat's nest will likely work better.
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Nathan Riddle

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #17 on: January 16, 2018, 10:31:34 am »

I just wanted to add to this for those that are new to digital wireless systems.  Digital transmitters are sending data (ones and zeros) so there is no dynamic range component to the signal (in standard terms).  The transmitters are always running at full power compared to analog systems.  There is likely much more energy running through your jumper cables and so a much bigger potential for interference.

For this reason you should use higher grade cables (RG8x or LMR-195) when connecting to your distro.  It's also a best practice to NOT neatly lace all the cables together.  A rat's nest will likely work better.

Thanks for this golden nugget :)

I'm a bit OCD when it comes to cable management so my RF rack looks spic'n'span; I'll have to go clutter it up :(
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Nathan Riddle

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Re: MPD Digital RG8X cables? //check my setup?
« Reply #18 on: January 16, 2018, 10:38:24 am »

LMR-400 UF (basically what you quoted) Is what we use for the majority of our RF cables..... Well, what we purchase at least for between antennas and receivers/distros. 

We also use LMR-240, usually it is what most of the manufacturers/distributers are sending out now with large RF orders (or RG8). The few extra dB loss per run for the majority of our applications, it doesn't make a considerable difference between the two (<50ft).

However, for longer runs (100's of feet) LMR-400 is the way to go (or bigger, or fibre for 1000's of feet).

But do realize, that with the larger numbers (LMR-400), it is as thick as a 15A AC cable, and not nearly as flexible. You can fit a few hundred feet (probably a thousand+ feet) of coiled RG8 in the same space as 100ft of coiled LMR-400.

Get in the habit of doing path loss calculations over free space and cable. Find out what your receiver needs for signal strength and figure out what cable best suits your needs. If LMR-240 fits the bill, save some money and get that. But if you really need LMR-400, get the LMR-400.

BRad

Thanks for simplifying all this for me and pointing out the specifics!

Coaxial cable losses at the distances we're working with will always be less than than free space path loss.
 
Pick a coax cable, any coax cable (and a frequency): http://www.qsl.net/co8tw/Coax_Calculator.htm. For safe measure, add 2dB to account for worn connector losses.

Now check the path loss at that same frequency (use "0" for transmitter and receiver gain): https://www.pasternack.com/t-calculator-fspl.aspx.

This is why it's just about always better to get the antennas as close to the transmitters as possible.

I played with both; I wasn't sure about the gain at first but once you stated using "0" it worked itself out. Thanks!

Makes a ton of sense now to use coax to get the antenna closer than leaving the antenna further away.

I need to figure out how to calculate the dB loss of couplers, connections, & cable differences [a rule of thumb].
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I'm just a guy trying to do the next right thing.

This business is for people with too much energy for desk jobs and too much brain for labor jobs. - Scott Helmke
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