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[Russell Ault]

{...} Attached are images of pre-transmitter and post. {...}

Is your SDR's RF input overloading (etc.)? I wouldn't have expected the TV broadcast on channel 18 to look that different between those two scans. Also, you mentioned 6 IEM TXs, but the scan appears to be showing 12 carriers (or, rather, six consistently-spaced pairs of carriers), so something definitely seems fishy.

-Russ

[Henry Cohen]

Is your SDR's RF input overloading (etc.)? I wouldn't have expected the TV broadcast on channel 18 to look that different between those two scans. Also, you mentioned 6 IEM TXs, but the scan appears to be showing 12 carriers (or, rather, six consistently-spaced pairs of carriers), so something definitely seems fishy.

Agreed.

OP:
Lower the RF power on the PSM's to 10mw and move the SDR and it's antenna at least 30' from the transmitter antennas.
What are you doing for TX antennas; 6 separate antennas or a transmitter combiner?

[Cliff Fuller]

Is your SDR's RF input overloading (etc.)? I wouldn't have expected the TV broadcast on channel 18 to look that different between those two scans. Also, you mentioned 6 IEM TXs, but the scan appears to be showing 12 carriers (or, rather, six consistently-spaced pairs of carriers), so something definitely seems fishy.

-Russ

I don't believe the input is overloading, I lowered the gain until there was little or no change in the noise floor. There was maybe a minute between scans. When looking at CH 18 at home I see a fair amount of fluctuation in the signal. I suppose the averaging may be such that we see that fluctuation in these scans. If one looks close enough there are actually 6 pairs of carriers and 2 singles. I find as I turn scans on and off in WWB that there are residual representations.

Channel 18 is a LPTV station here, WDTJ.

My curiosity was raised only because of the differences in what I was seeing in the 2 scans that I initially posted. I don't hear anything on the receivers other than low level noise that is similar between all 6 of them.
 

Thanks Russ...

[Cliff Fuller]

Agreed.

OP:
Lower the RF power on the PSM's to 10mw and move the SDR and it's antenna at least 30' from the transmitter antennas.
What are you doing for TX antennas; 6 separate antennas or a transmitter combiner?

I will lower the RF power and get some space between the DUT and my SDR when I'm in the venue next, this Saturday.

Unfortunately, the antennas are the stock 1/2 waves mounted on a 5 space panel, 2 rows of 3, at a little higher than knee level. No, no combiner although I wish there were. I agree that many transmitting antenna in close proximity to each other will cause problems but would it also cause what I'm seeing in the individual channels without other transmitters being powered (as in the initial posting)? Is it likely that the rack panel is reflecting back onto the antenna and causing the noise?

I experienced an issue years ago where 1/4 wave antennas were mounted on a single space panel. That was not pretty...

[Russell Ault]

{...} If one looks close enough there are actually 6 pairs of carriers and 2 singles. {...}

Just to be clear, each TX only produces a single carrier, so seeing pairs of carriers in your scan means something isn't working properly (most likely with the scan itself).

Do you have any other devices that you can use to perform scans with (e.g. a network-attached wireless microphone RX)? It might be informative to see how scans from your SDR compare.

-Russ

[Cliff Fuller]

Just to be clear, each TX only produces a single carrier, so seeing pairs of carriers in your scan means something isn't working properly (most likely with the scan itself).

Do you have any other devices that you can use to perform scans with (e.g. a network-attached wireless microphone RX)? It might be informative to see how scans from your SDR compare.

-Russ

I was able to use a QXLD receiver as my scanner. I lowered the transmitter gains and saw an appreciable difference in the amount of noise that I'm seeing. I still haven't exactly answered my original questions regarding the difference in the single channels that I was seeing, I'll look into that again at another time.

I've attached several images; baseline and comparison to the SDR scan, a comparison of all transmitters on at 100 mw and 50 mw, and seperate 100 mw and 50 mw scans.

The transmitter frequencies are as follows: 474.050, 486.375, 488.775, 490.825, 504.575, and 504.975.







I've learned several things; a $30 USB device does not give the same results as a $600 device although valuable for a quick idea as to what is in the air, a higher gain is not always a better thing, and the more I learn the more I realize how much there is yet to be learned.

Thanks all for reading and responding...

[Henry Cohen]

I was able to use a QXLD receiver as my scanner. I lowered the transmitter gains and saw an appreciable difference in the amount of noise that I'm seeing. I still haven't exactly answered my original questions regarding the difference in the single channels that I was seeing, I'll look into that again at another time.

I've attached several images; baseline and comparison to the SDR scan, a comparison of all transmitters on at 100 mw and 50 mw, and seperate 100 mw and 50 mw scans.

The transmitter frequencies are as follows: 474.050, 486.375, 488.775, 490.825, 504.575, and 504.975.







I've learned several things; a $30 USB device does not give the same results as a $600 device although valuable for a quick idea as to what is in the air, a higher gain is not always a better thing, and the more I learn the more I realize how much there is yet to be learned.

There's a reason professional spectrum analyzers start at 4 figures. No two SA's will produce the same results unless all configuration parameters, antenna, coax and placement relative to RF source are identical.

It still appears that either the front end of your SDR is in saturation, the PSMs are generating IM due to the close proximity of the antennas, or a combination of both.

[Cliff Fuller]

It still appears that either the front end of your SDR is in saturation, the PSMs are generating IM due to the close proximity of the antennas, or a combination of both.

All of the most recent image postings were done with a QXLD mic receiver save the 2nd image, the green trace is that of the SDR. Most probably the IM is due to the close proximity of antennas, something I won't be able to change without jumping through some major hoops.

Thanks Henry....

[Russell Ault]

All of the most recent image postings were done with a QXLD mic receiver save the 2nd image, the green trace is that of the SDR. Most probably the IM is due to the close proximity of antennas, something I won't be able to change without jumping through some major hoops.

The QLXD RX is almost certainly suffering some desense as well, given how how hot those IEM signals are measuring; are your microphone RX antennas located very close to your IEM TX antenna farm?

That IMD is also pretty brutal, and it's so close to the edge of the band I'd almost be worried about causing issues with LMRs in the area (not to mention rendering much of the spectrum less than useful for wireless microphones); a proper combiner is the only real solution.

-Russ

[Cliff Fuller]

The QLXD RX is almost certainly suffering some desense as well, given how how hot those IEM signals are measuring; are your microphone RX antennas located very close to your IEM TX antenna farm?

That IMD is also pretty brutal, and it's so close to the edge of the band I'd almost be worried about causing issues with LMRs in the area (not to mention rendering much of the spectrum less than useful for wireless microphones); a proper combiner is the only real solution.

-Russ

Close proximity?  LOL  The IEMs are the left side of the dual rack and the QLXD is on the right side. Certainly raised my eyebrow when I first saw it.

There's a ton of video displays, large video display panels upstage, and all kinds of whirly-gig slot machines in the environment. This gambling house evidently likes the odds for the house.  ;-)   Slim to no chance for a combiner.

BTW, LMR would be "Land Mobile Radio"?

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