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Author Topic: Cable? transmitter combiner out to LPDA  (Read 1371 times)

Tim McCulloch

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Cable? transmitter combiner out to LPDA
« on: January 02, 2019, 12:49:05 pm »

All my work with RF in pro audio has been on the wireless mic receiver side.

We've added Senny G4 eW300 IEM systems and the cost of the official Sennheiser cabling is staggering.

The idea of RG-58 isn't sitting well with me but the boss likes that price.  Is LMR-240 acceptable or overkill?  Any supplier have better prices than others?
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Scott Helmke

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Re: Cable? transmitter combiner out to LPDA
« Reply #1 on: January 02, 2019, 12:55:54 pm »

You can use all the same cables you use with wireless mic systems.
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Tim McCulloch

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Re: Cable? transmitter combiner out to LPDA
« Reply #2 on: January 02, 2019, 01:14:15 pm »

You can use all the same cables you use with wireless mic systems.

Yeah, but those stay with the mic systems.  As this is new gear & packaging the boss wants it to have dedicated cabling so re-purposing existing inventory won't be done.  He doesn't see why CB radio coax isn't a proper choice and I don't have any *evidence* or recommendations by RF professionals to refute his desires.  My gut says "no, move up the food (price) chain".
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

Nathan Riddle

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Re: Cable? transmitter combiner out to LPDA
« Reply #3 on: January 02, 2019, 03:28:34 pm »

Obviously, I'm not an RF Guru like others here.

But in my readings, I had some thoughts regarding your post.

Use the foam/flex version of cabling (the dB loss is slightly more than the other kind, but it is less susceptible to kinks which cause permanent damage). Unless you have the appropriate tools to TDR the cable regularly.

According to this calculator, RG213 is equivalent to LMR-240 and RG9913 is equivalent to LMR-400
https://www.qsl.net/co8tw/Coax_Calculator.htm

So buy whichever is the cheapest...

FWIW I've had good success with my generic LMR-400 cable which can be had for much less than real LMR-400. But then again I use it a handful of times per year and presumably you'll be using it once a week.
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Rob Spence

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Re: Cable? transmitter combiner out to LPDA
« Reply #4 on: January 02, 2019, 04:40:24 pm »

Tim, is this for internal cables for the rack or rack to antenna outside of rack?


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Russell Ault

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Re: Cable? transmitter combiner out to LPDA
« Reply #5 on: January 02, 2019, 05:27:25 pm »

He doesn't see why CB radio coax isn't a proper choice and I don't have any *evidence* or recommendations by RF professionals to refute his desires.

The simple answer is to this question is that CB radio (~27 MHz) operates at a lot lower frequency than the Sennheiser IEMs, and lower frequency = lower cable loss.

Let's use a 100' length of RG-58 as an example. In that cable, a CB transmission at 27 MHz would lose roughly 2 dB to cable attenuation. In the same cable a 550 MHz IEM transmission would lose over 10 dB to the cable, meaning that of your 50 mW output less than 5mW will show up at the antenna.

To get cable attenuation at 550 MHz down to less than 2 dB per 100' you'd have to use LMR-600 (or something like it), which is over 1/2" thick. Obviously shorter cables mean less loss (so you can use thinner, cheaper cable), but unless your TX antenna is mounted to the front of the rack (which is probably a bad idea) you can see why you have to spend a bit to keep your cable losses at UHF frequencies reasonable.

-Russ
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Nathan Riddle

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max dB loss for a given distance?
« Reply #6 on: January 02, 2019, 07:12:14 pm »

To get cable attenuation at 550 MHz down to less than 2 dB per 100'

Is this the general rule of thumb for long distances (greater than 100ft)?

IE. What is a typical suggested max dB loss for a given distance?
Or I guess a better question would be, how much signal does the TX antenna need to still work, which really asks how much signal above the noise floor does the RX antenna need?

I guess that just leads to 'do the math'.

30mW TX @ 50ohms = 15dBm gain
LPDA  TX = 6.5dBi gain (dBi ~= dBm -2.15) = ~8.5dBm

100ft @ 550mHz = 56dB loss
https://www.pasternack.com/t-calculator-fspl.aspx

= -32.5dB max signal at rx
= -52.5dB for noise floor with 20dB separation

Using the following spec from the manual and the power converter we get an incredible sensitivity.
Sensitivity (with HDX, peak deviation) < 4 μV, typ. 1.6 μV for 52 dBArms S/N
https://www.pasternack.com/t-calculator-power-conv.aspx
-102.91 dBm

Am I doing this right?

Maybee I should make my own thread...
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Rob Spence

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Re: Cable? transmitter combiner out to LPDA
« Reply #7 on: January 02, 2019, 07:25:29 pm »

Is this the general rule of thumb for long distances (greater than 100ft)?

IE. What is a typical suggested max dB loss for a given distance?
Or I guess a better question would be, how much signal does the TX antenna need to still work, which really asks how much signal above the noise floor does the RX antenna need?

I guess that just leads to 'do the math'.

30mW TX @ 50ohms = 15dBm gain
LPDA  TX = 6.5dBi gain (dBi ~= dBm -2.15) = ~8.5dBm

100ft @ 550mHz = 56dB loss
https://www.pasternack.com/t-calculator-fspl.aspx

= -32.5dB max signal at rx
= -52.5dB for noise floor with 20dB separation

Using the following spec from the manual and the power converter we get an incredible sensitivity.
Sensitivity (with HDX, peak deviation) < 4 μV, typ. 1.6 μV for 52 dBArms S/N
https://www.pasternack.com/t-calculator-power-conv.aspx
-102.91 dBm

Am I doing this right?

Maybee I should make my own thread...

Every cable model has its spec for loss over distance for specific frequencies.

The different models of similar cable trade off things like diameter, flexibility, loss, resistance to interference, dialectical properties etc.


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Henry Cohen

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Re: Cable? transmitter combiner out to LPDA
« Reply #8 on: January 02, 2019, 09:27:59 pm »

All my work with RF in pro audio has been on the wireless mic receiver side.

We've added Senny G4 eW300 IEM systems and the cost of the official Sennheiser cabling is staggering.

The idea of RG-58 isn't sitting well with me but the boss likes that price.  Is LMR-240 acceptable or overkill?  Any supplier have better prices than others?

Russ provided a good overview, but I think a more clear way to view this is to understand the relative losses involved:
If one has, for example, 6dB of loss due to the coaxial cable, this is a somewhat negligible amount for a typical receive system where the RF power at the receiver, in the most general of terms, needs to be between -85dBm and -70dBm for reliable performance. 6dB is a relatively small percentage of power loss. On the other hand, 6dB is a significantly large percentage of loss for a transmitter that's only putting out +15dBm to +20dBm and will spell the difference between reliable operation and an upset artist.

Rule of thumb: Use the largest, lowest loss coax for transmit antennas, even if only 25'. LMR400, LMR400-UF, 9913F7, CQ106, PWS S9046 and S3018 are all very good candidates. And if you have the wallet for it, I use LMR600 for 100' - 200' runs. (For those of you that still use it for anything other than rack jumpers, RG58 and RG213 should be sent to the scrap yard.)
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Henry Cohen

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Henry Cohen

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Re: max dB loss for a given distance?
« Reply #9 on: January 02, 2019, 09:48:47 pm »

Is this the general rule of thumb for long distances (greater than 100ft)?

IE. What is a typical suggested max dB loss for a given distance?
Or I guess a better question would be, how much signal does the TX antenna need to still work, which really asks how much signal above the noise floor does the RX antenna need?

I guess that just leads to 'do the math'.

30mW TX @ 50ohms = 15dBm gain
LPDA  TX = 6.5dBi gain (dBi ~= dBm -2.15) = ~8.5dBm

100ft @ 550mHz = 56dB loss
https://www.pasternack.com/t-calculator-fspl.aspx

= -32.5dB max signal at rx
= -52.5dB for noise floor with 20dB separation

Using the following spec from the manual and the power converter we get an incredible sensitivity.
Sensitivity (with HDX, peak deviation) < 4 μV, typ. 1.6 μV for 52 dBArms S/N
https://www.pasternack.com/t-calculator-power-conv.aspx
-102.91 dBm

Am I doing this right?

Sort of.  First, be consistent with terms. "dB" is a relative measurement unit, akin to "foot" or "pound" and is not an absolute value. Absolute value is dB referenced to a standard, be it volt (dBV), SPL (dB-SPL), watt (dBW), etc. In the case of RF the standard is milliwatt and expressed as dBm. So an RF device's loss or gain is expressed as [-]XdB, but the actual RF output level of the transmitter will be expressed as [-]XdBm.

Secondly, no need to apply dBi to dBd conversion here as all measurements are relative to one, which is the isotropic antenna.

Now the math . . .

Let's take the wireless mic as an example.
Transmitter RF output = 50mW                      +17dBm
TX antenna mis-match loss                             -03dB
Salty water bag absorption loss                     -20dB
Free space path loss, 100' @ 550MHz            -57dB
RX antenna gain (dBi)                                    +06dBi
Coax loss                                                        -04dB  (a published spec by the coaxial cable OEM.)
Other insertion losses within rack                  -02dB
Total RF energy at receiver antenna port =    -63dBm

Jason Glass has a nifty Android app for this exact calculation, but IIRC his calculator also accounts for gain stage noise figure and RF environmental noise floor.
« Last Edit: January 02, 2019, 10:01:44 pm by Henry Cohen »
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Henry Cohen

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Radio Active Designs   www.radioactiverf.com

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Re: max dB loss for a given distance?
« Reply #9 on: January 02, 2019, 09:48:47 pm »


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