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Author Topic: IEM FFT scan  (Read 225 times)

Russell Ault

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Re: IEM FFT scan
« Reply #10 on: Yesterday at 05:25:47 PM »

{...} 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
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Henry Cohen

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Re: IEM FFT scan
« Reply #11 on: Yesterday at 07:16:58 PM »

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?
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Henry Cohen

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Cliff Fuller

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Re: IEM FFT scan
« Reply #12 on: Yesterday at 07:34:06 PM »

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...
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Cliff Fuller

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Re: IEM FFT scan
« Reply #13 on: Yesterday at 07:43:59 PM »

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...

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Re: IEM FFT scan
« Reply #13 on: Yesterday at 07:43:59 PM »


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