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Author Topic: Neophyte to SDR- - question  (Read 5323 times)

Offline tybrad

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Neophyte to SDR- - question
« on: April 07, 2014, 1937 UTC »
Hi all,
I am just now exploring the UTwente site (and others like it) and am enjoying it.  One thing that I cannot figure out is the diagonal signal swaths across wide frequency spectrums.  They appear to be freq sweeps over a period of time- it manifests itself as a diagonal line or sequence of dashes across the waterfall.  What is this phenomenon?
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Offline Token

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Re: Neophyte to SDR- - question
« Reply #1 on: April 07, 2014, 2222 UTC »
These signals are ionosondes, specifically "chirpsondes", there are several different types and they are typically run by universities, militaries, and governments.  They are used to map propagation conditions, or the way the ionoshpere is reflecting / bending / absorbing radio waves.  With this information you can predict / tell how well a specific frequency of operation will work into a specific region of the world.

T!
T!
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Offline tybrad

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Re: Neophyte to SDR- - question
« Reply #2 on: April 08, 2014, 0944 UTC »
Oh yes, also:  What is down in the negative kHz regions shown on webSDR sites?  I see and hear signals, but –kHz is not possible, unless the nomenclature is unfamiliar to me.
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Offline Token

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Re: Neophyte to SDR- - question
« Reply #3 on: April 08, 2014, 1429 UTC »
Oh yes, also:  What is down in the negative kHz regions shown on webSDR sites?  I see and hear signals, but –kHz is not possible, unless the nomenclature is unfamiliar to me.

Can you give a specific example (which specific web SDR including node name, what frequency you are trying to tune to, bandwidth setting, etc)?  And maybe a screen shot of what you mean?  I suspect an incorrect setting or a misunderstanding of what is being presented.  As you said, negative frequencies are not real.  There are a couple of ways to get what appear to be negative frequencies displayed using some SDRs (particularly sound card based ones, like the SoftRock series).

T!
T!
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Offline tybrad

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Re: Neophyte to SDR- - question
« Reply #4 on: April 08, 2014, 1537 UTC »
I am on

http://lindsey.esrac.ele.tue.nl/

View "all bands" and the bottom waterfall shows down to zero kHz and below that.
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Offline Token

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Re: Neophyte to SDR- - question
« Reply #5 on: April 09, 2014, 0037 UTC »
I can't get that site to run for me for some reason, seems to be a Java issue.  However from what does load it looks like that might be a relative frequency display, kind of like an unmarked bandspread on an old school reciever.  By that I mean what you are tuned to is at "0" on the bottom scale, lets say you are tuned to 18100 kHz, then something at "-25 kHz" on the scale would be 25 kHz below you, or at 18075 kHz.

However that is just a guess, as I can't get the page to correctly load and run.  I can see everything, but not make anything function.

T!
T!
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Offline refmo

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Re: Neophyte to SDR- - question
« Reply #6 on: April 09, 2014, 0117 UTC »
Nice link.

With newer versions of Java, I had to add the site to the 'sitelist' in Control Panel --> Java.

The bottom window is a separate receiver.  It appears to be a satellite receiver, with 0 being the center frequency of the satellite passband.
I'm not sure which satellite is being received, so can't be sure of the frequency.  Likely, it is in either the 2 meter or 70 centimeter amateur satellite subbands.
Lots of activity, with euro hams sending their calls and 'grid-square' locators.
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Offline Token

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Re: Neophyte to SDR- - question
« Reply #7 on: April 09, 2014, 0242 UTC »
Ahhhh, that makes since, notice in the upper right corner of the page it lists a satellite and the uplink / downlink frequencies.  A satellite in a pass will have Doppler shift on its signal, and you can predict Doppler if you have software to track the sat.  That way, knowing your uplink freq and the Doppler shift of the sat at that particular time you can predict your downlink shift, generally done in kHz.  So the scale will show you what freq a given uplink can be found, relative to the "center" downlink freq.

So yeah, the bottom scale is a relative display, not true or absolute frequency, and in that case a signal that was 25 kHz below the downlink center would be on -25 kHz on that display.

T!
T!
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Offline tybrad

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Re: Neophyte to SDR- - question
« Reply #8 on: April 09, 2014, 0927 UTC »
Makes sense.  Thanks you two!
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Offline tybrad

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Re: Neophyte to SDR- - question
« Reply #9 on: April 10, 2014, 1856 UTC »
OK, just heard some chatter (Papa alpha seven (repeated).  Thank you satellite" and continues on the repeating) on those –kHz listings.  I am confused again about this waterfall.

Will post a screenshot when I get home.





Also, what could the meandering frequency sweep be?  Ionosonde?  But over such a limited range (I can't even tell what the range is on this waterfall)
« Last Edit: April 11, 2014, 0026 UTC by tybrad »
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Offline Token

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Re: Neophyte to SDR- - question
« Reply #10 on: April 14, 2014, 2109 UTC »
Will post a screenshot when I get home.


Also, what could the meandering frequency sweep be?  Ionosonde?  But over such a limited range (I can't even tell what the range is on this waterfall)

That sweeping signal does not look like an ionosonde of any kind.  It looks like voice, and why will be explained below.

I am going to assume the voice mode used for these signals is USB, as that is what the waterfall looks like.  I am further going to make another assumption, and this could well be wrong but without knowing which sat you are monitoring it gets tough.  My assumption is the sat being monitored is using a Mode B configuration.  Mode B simply means you transmit to the sat on 70 cm SSB/CW, and the sat then translates your signal (cross bands or retransmits) to 2 meters SSB/CW.  Mode JA would be the other way around, you would transmit up to the sat on 2 meters SSB/CW and listen on 70cm SSB/CW.  There are several other modes, with different band and transmit mode (FM/SSB/CW/digital etc) combinations.

Let’s take a virtual tour of communicating using the VO-52 satellite in its Mode B inverting operation.

As I said, Mode B is 70 cm SSB/CW uplink, and 2 meter SSB/CW downlink.  So you transmit up to the sat on 70 cm, and it retransmits your signal on 2 meters.  In this case the sat uses an inverting translator, so you transmit up on 70 cm in LSB and your signal is retransmitted down on 2 meters in USB.  The uplink window of operations is 435.220 MHz to 435.280 MHz, or 60 kHz of bandwidth centered on 435.250 MHz.  The downlink window is 145.870 MHz to 145.930 MHz, or 60 kHz of bandwidth centered on 145.900 MHz, but inverted.  

Think of that bottom waterfall as representing that downlink range, 60 kHz.  0 kHz, or the center of the waterfall, is 145.900 MHz (remember I am just using this as an example, I have no idea what sat is really represented there).  So the + numbers are on the high side of that freq, and the – numbers on the low side.  +25 kHz would be 145.925 MHz, and -25 kHz would be 145.875 MHz.

VO-52 uses an inverting transponder.  This means if I send up to the sat on 435.260 MHz LSB, 10 kHz above or + 10 kHz of the center frequency, I will show up on the downlink 10 kHz low, or – 10 kHz, of the center freq, at 145.890 kHz, and in USB.

Of course you want to monitor your downlink audio while transmitting, so that you can hear anyone who returns if for no other reason.  So, the translation issue can make finding your exact downlink frequency a matter of quick mental jumping jacks (or there are software programs to do it automatically).  To top it all off Doppler shift can move frequencies around on you.  The satellite is moving fast enough to shift the received and transmitted frequencies noticeably, and as the radial velocity of the sat changes this Doppler shift changes.  Sometimes, rather than calculating an ever changing target to hit or if using older equipment, it is just easier to select an empty receive frequency, key the mic, watch the downlink waterfall for your signal to pop up, and then tune your transmit frequency to yield the desired receive frequency.

Ergo…your meandering signal on the waterfall.

T!


(edit)  At home now, and I can run the web page (work does not allow it to run).  The center of the bottom waterfall appears to be on 144.425 MHz, and I do not know which Hamsat that would be the downlink of.
« Last Edit: April 15, 2014, 0041 UTC by Token »
T!
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Offline tybrad

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Re: Neophyte to SDR- - question
« Reply #11 on: April 18, 2014, 0040 UTC »
Wow.  Most everything that you wrote is new to me.  Hamsats?  I did not know, and will investigate it further.

Thanks!
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