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Antenna Distribution: PSC RF multi & diversity fin signal troubleshooting and questions


B_Van_Deusen

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Hi,

 

I have been using a PSC RF multi (distro function on, amplifier function off) with the Diversity fin (dipole & sharkfin combo, non-powered) for several years with mostly flawless results.  However i'm on a job now where one of the 6 wireless is play is giving me frequent drop outs due to the perfect combo of that talent being just far enough with the transmitter on their back, with their body directly in line between the raised antenna and the transmitter.  There was no way to adjust it on the fly today (and the problem was minor enough), but I'm planning on fixing it before we go tomorrow and butting up against my knowledge of antenna distribution / signal loss / cable runs.  So i have a few questions:

 

-I've heard running non-powered antennas (same meaning as "passive"?) more than 70 feet is where you will get signal loss, but if you are willing to accept that loss, is that not a big deal? Or does that mean you will get more drop out because you are losing RF signal? My assumption that the .1 dB of signal loss per foot was in audio signal rather than RF signal.

 

-If i connect two same resistance BNC cables (50 ohm) with a BNC barrel to make my cable run longer is that a bad idea?

-I have another diversity fin and was planning to use the two sharkfin components (leave out the dipole) in different areas of the room to widen my spread.  If one cable is longer than the other, can that create issues? Or is there a threshold where it doesn't matter?

-Does the length of a coiled cable count as much as cable thats been run out? (i know electricity does things with coils in a specific way, but not enough to understand how it might affect this)

 

Sorry if questions like these have been answered elsewhere, i did search but didn't find these specific questions very easily.

Thanks in advance for any advice, i'm also happy to view any articles or resources that explains antenna distribution more thoroughly if anyone has a link!

 

Best,


Brett

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Answering because I'm here and can be quick ... others probably have more knowledge than I do.

I think running a second sharkfin to a second location to give you spread is a good plan.  Different cable lengths shouldn't matter — your receiver should pick whichever signal is strongest, so optimize each antenna for what's best for the antenna placement.

Coils *probably* don't matter ... until they do.  I wouldn't sacrifice safety for signal unless you know you're in a situation where you need to try it.

Yes, longer runs mean more signal loss ... for your antenna runs, the signal in the cable is RF, not audio, so you are losing reception, not audio amplitude (though, those things can be related sometimes depending on the modulation in your wireless).  I can't say off the top of my head when you need signal amplification, but I recall somewhere around 100', so your 70' guess seems reasonable.  I've definitely run 100' passive antenna runs, so while it may be suboptimal, it will work, and from the sound of things, in your situation, you'll benefit from having two separate signal paths even if one of them isn't 100% perfect.  If the issue truly is that the RF isn't going through your actor, having a different line on the transmitter is what you need rather than trying to wring every last dB of SNR out of both antennas.

Those are my thoughts for now.  Hopefully other people can chime in.

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9 hours ago, B_Van_Deusen said:

-I've heard running non-powered antennas (same meaning as "passive"?) more than 70 feet is where you will get signal loss, but if you are willing to accept that loss, is that not a big deal? Or does that mean you will get more drop out because you are losing RF signal? My assumption that the .1 dB of signal loss per foot was in audio signal rather than RF signal.


You will get loss with any length of cable - here's a handy calculator to see how much loss you're getting: https://www.timesmicrowave.com/calculator/
If you're running 70ft of let's say RG58 coax on blk21 540mHz, that's giving you 9.4dB loss or 14% efficiency, which is not very good. As I understand it the dB loss is not audio volume but RF gain loss. So you'd benefit from an amplifier or powered antenna here.
It's worth noting that passive sharkfins add gain - a typical shark fin reduces the pickup pattern to 120 degrees and adds 7 dB of gain.
 

Quote

-If i connect two same resistance BNC cables (50 ohm) with a BNC barrel to make my cable run longer is that a bad idea?


It's not optimal - you can add around another 3dB loss for each RF interconnect.

 

Quote

-I have another diversity fin and was planning to use the two sharkfin components (leave out the dipole) in different areas of the room to widen my spread.  If one cable is longer than the other, can that create issues? Or is there a threshold where it doesn't matter?


Your RX will still just pull from whichever antenna has the best signal.
 

Quote

-Does the length of a coiled cable count as much as cable thats been run out? (i know electricity does things with coils in a specific way, but not enough to understand how it might affect this)


Yes.

 

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I appreciate the help that some posters have given but I want to correct two numbers:

1.   BNC interconnects have much less than a decibel of loss at wireless frequencies. We ran tests with a Network Analyser on BNC barrels (tying two BNC cables together) and found almost no loss and minor reflections at up to 800 MHz. Losses due to the barrels were less than a quarter decibel. This means if you have two 25 foot cabels and need a 50 foot cable, tie them together with a barrel and go onward with no worries. This can simplify the lengths of cables in your inventory. A 100 foot cable can be made of two 50 foot cables.

2.   A typical shark fin antenna (log periodic, LPDA) does have commonly 7 dB of isotropic gain. However isotropic gain is kind of a theoretical number since isotropic antennas don't commonly exist. The better comparison is dB referenced to a dipole or 1/4 wave antenna (whip) which have a isotropic gain of 3 dB. Therefore, an LPDA gives you a 4 dB advantage over the most common low gain antennas. 

 

One last comment: If you have high loss in cables, splitters, etc., remember to apply the gain before the lossy elements since applying gain after losses just gives you an amplified, noisy signal.

Best Regards,

Larry Fisher

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17 hours ago, LarryF said:

I appreciate the help that some posters have given but I want to correct two numbers:

1.   BNC interconnects have much less than a decibel of loss at wireless frequencies. We ran tests with a Network Analyser on BNC barrels (tying two BNC cables together) and found almost no loss and minor reflections at up to 800 MHz. Losses due to the barrels were less than a quarter decibel. This means if you have two 25 foot cabels and need a 50 foot cable, tie them together with a barrel and go onward with no worries. This can simplify the lengths of cables in your inventory. A 100 foot cable can be made of two 50 foot cables.

2.   A typical shark fin antenna (log periodic, LPDA) does have commonly 7 dB of isotropic gain. However isotropic gain is kind of a theoretical number since isotropic antennas don't commonly exist. The better comparison is dB referenced to a dipole or 1/4 wave antenna (whip) which have a isotropic gain of 3 dB. Therefore, an LPDA gives you a 4 dB advantage over the most common low gain antennas. 

 

One last comment: If you have high loss in cables, splitters, etc., remember to apply the gain before the lossy elements since applying gain after losses just gives you an amplified, noisy signal.

Best Regards,

Larry Fisher


Love to learn! Thank you for the corrections Larry.

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  • 2 months later...

Hi I just wanted to cirlce back here and thank everyone for their input, I managed to improve the signal loss on that job quite a bit from the tips I recieved here and i'm thirsty to learn more about antenna distribution in general.  I did try several things before resolving to 2 sharkfins w/ short BNC runs (20') mounted 10' or so apart higher up to get the best results. 

 

One of the options that i did try before that was having one fin on the short run, and one fin maybe 100' or so to move it much closer the main talent hidden behind a panel.  This created one the strangest wireless phenomena I've ever heard: It was like a fuzzy, hissy noise floor that was very loud, it came in and out, sometimes touching the RF multi would cause it go away briefly. Very strange ghost in the machine.  I don't think I have a recording unfortunately.  I did use high impedance cable (same ohm though) with splitters to get it there.  After quickly resetting back to 2 fins, short runs, I got the best results.

 

Furthermore, yesterday I also experienced a strange buzz on one channel of wireless coming through the RF multi system (this time the Diversity fin + Diapole combo).  In troubleshooting we first switched the mic pack (SMQV), ran the Diversity fin closer, and finally minimized it by switching the Lav mic itself (cos11).  As we continued on I did notice some of the same artifacts (slight buzz attached to the peaks of some of the dialogue) burried in there occationally across several of the channels, not just the same one.  I had to listen at much higher volume to find it, but i am a little perplexed here.  Is it possible the RF multi is the culprit? Or the diapole/ sharkfin combo itself? 

 

I remember reading somewhere that Lectros combine signals from both antennas when creating its output signal, could the diapole / sharkfin combo not mesh well with this process and create artifacts?

 

Has anyone else ever experienced a buzz at the tips and peaks of dialogue when using antenna distro? esp the RF Multi?  I will try to fish out a section from the recording later on today.

 

Thanks very much in advance.

 

Best,

 

Brett

On 12/12/2022 at 2:50 PM, LarryF said:

 

One last comment: If you have high loss in cables, splitters, etc., remember to apply the gain before the lossy elements since applying gain after losses just gives you an amplified, noisy signal.

Best Regards,

Larry Fisher

 

Also Larry i am a curious exactly what you mean here, is this if you were to put a booster in line with the antenna? 

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This is not exactly the topic from above but since I see that LarryF has been here, I'll go ahead and ask it: Would Lectrosonics Srcs in a PSC rf 6 pack or similar rf multi type situation have similar range to a venue? Is one method of antenna combining (maybe wrong word here) more efficient than the other?  

Here's my situation: I have a friend soundmixer whom I work with often who gets amazing range from his venue 1 and passive sharkfins.  I want to emulate his system and success but I have a PSC 6 pack.  Should that work as well if all other parts of the system are equal?

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6 hours ago, BAB414 said:

Not Larry but the SR's (not sure about the new DSR4) don't have the front-end tracking (or filtering) that the Venues or 411's have.

I looked this up to find an explanation here: https://ftp.lectrosonics.com/wireless-faq/general-questions/what-is-a-tracking-front-end.html

According to this, the new DSR4 does have front-end tracking: https://www.lectrosonics.com/phocadownload/QSG_DSR4.pdf

 

On 3/1/2023 at 1:18 PM, B_Van_Deusen said:

 

Furthermore, yesterday I also experienced a strange buzz on one channel of wireless coming through the RF multi system (this time the Diversity fin + Diapole combo).  In troubleshooting we first switched the mic pack (SMQV), ran the Diversity fin closer, and finally minimized it by switching the Lav mic itself (cos11). 

mic artifact_1.wav - These artifacts that were being generated even with perfect reception, on multiple SMQV's, on multiple frequencies, and was resolved by switching the COS 11.  The COS11 in question does not appear to have any connection issues.  I will replace the connector on it anyhow, but curious if anyone has an explanation for this.

 

On 3/1/2023 at 1:18 PM, B_Van_Deusen said:

As we continued on I did notice some of the same artifacts (slight buzz attached to the peaks of some of the dialogue) burried in there occationally across several of the channels, not just the same one.  I had to listen at much higher volume to find it, but i am a little perplexed here.  Is it possible the RF multi is the culprit? Or the diapole/ sharkfin combo itself? 

mic artifact_2.wav - This clip is much shorter but is an example of the more buried buzz that I noticed infrequently after fixing the issue from mic artifact_1.wav, on a different channel.

 

I think both of these clips were recorded from a Lectro SRb that was recently serviced from Lectro.  Its worth noting that I use this system all the time with the whips with no weird issues, which leads me to believe it's something to do with the RF multi, the Diversity fin (sharkfin + dipole), or the combination of the 2.   Could this have something to do with the absence of the front-end tracking thats showing up in the RF multi, but not without it? could there be a bit of RF overlap from another frequency that is causing a blip of interference? 

 

Thanks for any input in advance.

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I'm not familiar with lectro, but have you played with squelch settings?

 

Not used COS11 either, but I remember reading that older models were susceptible from high frequency EM interference. If you can't get it sorted playing with settings, maybe renting a dpa to test...

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On 3/3/2023 at 6:47 AM, Kambor said:

I'm not familiar with lectro, but have you played with squelch settings?

 

Not used COS11 either, but I remember reading that older models were susceptible from high frequency EM interference. If you can't get it sorted playing with settings, maybe renting a dpa to test...

 

I have not ever noticed a squelch setting, but i'll take another look at my lectros tomorrow.

 

High frequency EM interference is interesting re: older COS11s, upon rewiring this one, the connector did look really old and a bit rusty.  It also had a little resistor wired into it (photo attached), which i've seen, but never used myself, i'm not exactly sure what this contributes.  Otherwise i have recently switched to DPAs, but didn't have enough of them for number of wireless needed for this job.

IMG_0745.JPG

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