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Old 5-Sep-2020, 8:05 PM   #1
eclipsme
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Join Date: Aug 2020
Posts: 20
What are these spectrum?

RABBIT77 introduced me to the wonders of Software Defined Radio (SDR). (Thanks Rabbit!)

I have downloaded and installed sdrsharp, a free app that gives a live display of a 2 Mhz slice of a 6Mhz station live

I have also downloaded and installed rtld scanner which displays a snapshot of a range of frequencies.

Both can be found by googling and require a $25 capture dongle.

So, I have been scanning the TV spectrum using my DB8e antenna on my roof. Can anybody help identify what is happening in the following 3 scans?

First is a scan of 620-638Mhz, channels 39, 40, and 41. What is that from about 622-627 mhz, the upper part of ch 30 and the lower of 31?
and what is that around 634 mhz in ch 41?

Next is 566-578 Mhz, Ch 30 and 31. The pilots look like they might be in the correct locations for the channels, and we do have a local ch 30 but I have no idea what is happening on 31. Is this all noise or week signal?

And finally is a scan of the Hi-VHF band. The DB8e is notoriously bad at VHF. Does any of this look like possible stations that I might be able to get with a different antenna?

Thanks a lot for looking!
Attached Images
File Type: jpg Scan 620.0-638.0MHz-chan-39-40+41.jpg (58.6 KB, 94 views)
File Type: jpg Scan 566.0-578.0MHz-chan-30-31.jpg (48.2 KB, 89 views)
File Type: jpg Scan 174.0-216.0MHzHiVHF.jpg (48.6 KB, 80 views)
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Old 5-Sep-2020, 9:10 PM   #2
Tower Guy
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The signals that you see on the VHF scan are second harmonics of the FM band. The strongest signal is twice the frequency of WGYL (93.7) I’d assume that something is overdriven and/or the gain is too high.
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Old 6-Sep-2020, 1:34 AM   #3
rabbit73
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Location: S.E. VA
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Quote:
Originally Posted by eclipsme View Post
I have also downloaded and installed rtlsdr scanner which displays a snapshot of a range of frequencies.

So, I have been scanning the TV spectrum using my DB8e antenna on my roof. Can anybody help identify what is happening in the following 3 scans?
The gain setting is too low. You must increase the gain until the noise floor just starts to rise, and then stop there.









Attached Images
File Type: jpg RTLSDR Scanner Screen2_1.jpg (73.5 KB, 170 views)
File Type: jpg RTLSDR Scanner Screen1_1.jpg (98.9 KB, 168 views)
File Type: jpg VHF-High Scan Gain 25.4 dB6.jpg (144.3 KB, 163 views)
File Type: jpg VHF-High Scan Gain 48 dB5.jpg (125.3 KB, 167 views)
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Last edited by rabbit73; 6-Sep-2020 at 2:23 AM.
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Old 6-Sep-2020, 3:24 AM   #4
rabbit73
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Quote:
Originally Posted by eclipsme View Post
First is a scan of 620-638Mhz, channels 39, 40, and 41. What is that from about 622-627 mhz, the upper part of ch 30 and the lower of 31?
and what is that around 634 mhz in ch 41?


There are no more high power TV signals in channels 39, 40, and 41, so it is probably cellular signals. There are, however, a few low power transmitters left after Repack.
Quote:
Next is 566-578 Mhz, Ch 30 and 31. The pilots look like they might be in the correct locations for the channels, and we do have a local ch 30 but I have no idea what is happening on 31. Is this all noise or week signal?


What you see are the tops of CH 30 and 31. The gain is set too low.
Quote:
And finally is a scan of the Hi-VHF band. The DB8e is notoriously bad at VHF. Does any of this look like possible stations that I might be able to get with a different antenna?


I'm not certain what that is; the gain is set too low. Tower Guy is probably correct.
http://www.fmfool.com/modeling/tmp/a...0/Radar-FM.png



You might need an FM filter.
Attached Images
File Type: jpg eclipsmeTVFscan620-638MHzCh39-41_1.jpg (70.7 KB, 168 views)
File Type: jpg eclipsmeTVFscan566-578MHzChan30-31_1.jpg (57.2 KB, 175 views)
File Type: jpg eclipsmeTVFscan174-216MHzHiVHF_1.jpg (76.4 KB, 166 views)
File Type: jpg eclipsmeTVF FM report.jpg (177.4 KB, 162 views)
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Last edited by rabbit73; 6-Sep-2020 at 4:06 AM.
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Old 6-Sep-2020, 11:58 AM   #5
eclipsme
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Thanks for the pointers.

Since the scanner is not real time, the only way I see to set the gain is through trial and error - try a setting, rescan, try another. Am I missing an easier way?

In any event, here in the Hi-VHF and ch 30 and 31 at higher gain.

Thanks again.
Attached Images
File Type: jpeg Scan 174.0-216.0MHz20.7db.jpeg (55.6 KB, 82 views)
File Type: jpeg Scan 566.0-578.0MHz20.7db.jpeg (45.4 KB, 79 views)
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Old 6-Sep-2020, 6:50 PM   #6
rabbit73
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Quote:
Originally Posted by eclipsme View Post
Thanks for the pointers.

Since the scanner is not real time, the only way I see to set the gain is through trial and error - try a setting, rescan, try another. Am I missing an easier way?
You are doing it just the way I do it. I don't know of an easier way. It's not possible to see a change in the scan as you change the gain because it takes time for the scan to assemble the individual separate parts of the scan that make up the total scan. The dongle isn't able to do more than 2 MHz at a time.

If you want a scanner that will show changes of gain in real time, you would have to buy the Airspy R2 SDR ($200) and use the Spectrum Spy software that comes with the SDR# software download.

Video (too long)
https://www.dropbox.com/s/ci56eknzhh...32-00.mp4?dl=0

https://www.dropbox.com/s/ci56eknzhh...32-00.mp4?dl=1
Quote:
In any event, here is the Hi-VHF and ch 30 and 31 at higher gain.

Thanks again.


What I see in the image above are the 2nd harmonics (freq x 2) of strong FM signals and the top of a weaker TV channel 10 (192-198 MHz).

http://www.fmfool.com/modeling/tmp/a...0/Radar-FM.png



WGYL FM is 93.7 MHz x 2 = 187.4 MHz



You will probably need an FM filter to keep the strong FM harmonics from causing interference to VHF-High reception.



I only see the tops of channel 30 and 31 because the gain is set a little too high. The dongle can only show a dynamic range of 50 dB because of the limitations of its inexpensive 8 bit ADC (Analog to Digital Converter).

Try reducing the gain a little to move the 50 dB "window" down to see the noise floor.

The gain setting for my dongle is usually at 25.4 dB, but when I use a preamp it's set at about 7.7 dB.



Attached Images
File Type: jpg eclipsmeTVFscan174-216MHz20.7db_2.jpg (87.5 KB, 170 views)
File Type: jpg SDR Comparisons2.jpg (126.3 KB, 157 views)
__________________
If you can not measure it, you can not improve it.
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http://www.megalithia.com/elect/aeri...ttpoorman.html

Last edited by rabbit73; 6-Sep-2020 at 8:36 PM.
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