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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => Wireless and Communications => Topic started by: Brad Harris on May 11, 2016, 01:20:36 pm

Title: A view from the Rx
Post by: Brad Harris on May 11, 2016, 01:20:36 pm
Sometimes you will see or hear mention of "Always look from the standpoint of the receiver" in regards to antenna and downstream devices.


Here is a quick example of why.




This is taken during a show. The first one from the LPDA antenna grid feeding a few racks of Rx, the other, a Dipole in the same physical location (just 16" lower in height) as the grid antenna.






BRad
Title: Re: A view from the Rx
Post by: Nathan Riddle on May 11, 2016, 03:51:30 pm
Hey Brad,

Could you help me understand more about your images. I'll admit I know little about RF (just what my EE classes taught me, basics), but I'd like to know more.

Are each of the single line peaks (looks like impulse, but because of scale of your chart is really just a narrow width wireless system freq) a wireless system being used?

Are you trying to document/explain that 'looking' at what the Rx antenna gear receives is a good idea so that you can 'see' extra noise. Rather than looking at whatever test equipment you have 'sees'

Thanks,
Nathan
Title: Re: A view from the Rx
Post by: Mac Kerr on May 11, 2016, 07:45:18 pm
Hey Brad,

Could you help me understand more about your images. I'll admit I know little about RF (just what my EE classes taught me, basics), but I'd like to know more.

Are each of the single line peaks (looks like impulse, but because of scale of your chart is really just a narrow width wireless system freq) a wireless system being used?

Are you trying to document/explain that 'looking' at what the Rx antenna gear receives is a good idea so that you can 'see' extra noise. Rather than looking at whatever test equipment you have 'sees'

The quieter scan shows how much spurious RF the LPDA (Paddle) antenna rejects compared to a simple whip.

Mac
Title: Re: A view from the Rx
Post by: Kevin Graf on May 12, 2016, 07:28:26 am
Is it a half wave dipole or a quarter wave whip?
Title: Re: A view from the Rx
Post by: Brad Harris on May 12, 2016, 09:00:14 am
Here is another scan I did this morning to hopefully clear up the image a bit




This Dipole is not a whip antenna (like you would normally find on the front/back of a receiver), rather a wide band omni antenna (in this case a Shure UA860SWB). Most RF manufacturers make a similar antenna (Sennheiser, Telex, etc).


The lower noise floor is from the LPDA which is a directional antenna, whereas the higher noise floor is from the omni antenna. What this signifies, is always look at RF from the receivers point of view (i.e., the antenna that your receiver systems is using whenever possible).


Some exceptions are IFB/IEM as most are 'built in' and can't be 'tapped into', where you would require a separate antenna to monitor what is happening in the spectrum at that position.




BRad
Title: Re: A view from the Rx
Post by: Kevin Graf on May 12, 2016, 12:57:14 pm
A dipole is far from an omni antenna. Viewed from the ends, it has a null.

******************
That's a real nice frequency read-out.
Title: Re: A view from the Rx
Post by: Brad Harris on May 12, 2016, 01:09:56 pm
A dipole is far from an omni antenna. Viewed from the ends, it has a null.


Yes, if one wasn't talking about antennas. What you are talking about is a theoretical Isotropic antenna.

https://en.wikipedia.org/wiki/Omnidirectional_antenna


BRad
Title: Re: A view from the Rx
Post by: Dan Currie on August 04, 2016, 10:45:13 am
Really cool stuff!!  What kind of attenuator did you use to protect the scanner from the high level signal of the distribution amplifier?


Sometimes you will see or hear mention of "Always look from the standpoint of the receiver" in regards to antenna and downstream devices.


Here is a quick example of why.




This is taken during a show. The first one from the LPDA antenna grid feeding a few racks of Rx, the other, a Dipole in the same physical location (just 16" lower in height) as the grid antenna.






BRad
Title: Re: A view from the Rx
Post by: Brian J Holder on August 04, 2016, 11:45:40 am
Brad
I know that curiosity killed the cat, but what device did you capture the images from?
Brian
Title: Re: A view from the Rx
Post by: Karl Winkler on August 10, 2016, 10:54:17 am
Sometimes you will see or hear mention of "Always look from the standpoint of the receiver" in regards to antenna and downstream devices.

Interesting images, Brad, and a good illustration of what you are talking about.

Another reason to "see the world from the receiver's perspective" is that receivers, active splitters, active antennas, etc. will all generate intermods where a simple antenna into an analyzer will not. So, in effect, you can see something opposite than your images show, specifically more IM frequencies.

When compared to analyzers, wireless mic receivers are more sensitive but generally not real time and not as wide band. So of course both tools have their place.
Title: Re: A view from the Rx
Post by: Samuel Rees on August 10, 2016, 12:05:52 pm
I mostly replicated this experiment at a gig during some downtime yesterday. Very interesting. Thanks for posting!
Title: Re: A view from the Rx
Post by: Keith Broughton on August 10, 2016, 01:37:55 pm


Another reason to "see the world from the receiver's perspective" is that receivers, active splitters, active antennas, etc. will all generate intermods where a simple antenna into an analyzer will not. So, in effect, you can see something opposite than your images show, specifically more IM frequencies.


Whenever possible, I connect my analyser to the antenna splitter output.
Title: Re: A view from the Rx
Post by: Lyle Williams on August 10, 2016, 04:55:11 pm
Most receivers have more sensitivity than you need.  It's all about signal to noise.
Title: Re: A view from the Rx
Post by: Brad Harris on October 02, 2016, 11:32:51 pm

Been very busy in the trenches lately ....
Really cool stuff!!  What kind of attenuator did you use to protect the scanner from the high level signal of the distribution amplifier?

None, there usually is a small level gain with UA84x units, but not enough to warrant padding the inputs.

BradI know that curiosity killed the cat, but what device did you capture the images from?Brian

RF Venue Vantage (Mac) with a RFExplorer unit.




BRad