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Author Topic: SDRPlay and L band Immarsat  (Read 241 times)

KB5SLF

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SDRPlay and L band Immarsat
« on: September 30, 2022, 07:24:47 AM »

I am planning on using my SDRPlay RSPdx for Immarsat decoding. I'm thinking of purchasing a l-band active antenna as154. I do have 2 problems.
1) Is it possible to install this in the attic if I'm pointing out through a window with direct line of sight ?
2) Is it possible to use a sma splitter on my SDR? My other ports except for my C port are taken
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WA3SKN

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Re: SDRPlay and L band Immarsat
« Reply #1 on: September 30, 2022, 03:55:11 PM »

At 1.5 GHz things start getting "picky".
They recommend an OUTDOOR installation for that antenna.  Wood, glass, etc can cause problems.
That is a "active" antenna and uses a Bias-T to supply power (3-5v).  You will need to plan around that if using a splitter.

-Mike.
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KA4WJA

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Re: SDRPlay and L band Immarsat
« Reply #2 on: October 01, 2022, 11:52:30 PM »

John,
I'm not certain if you're experienced in sat comm?  But, I'm thinking probably not? 
(please correct me if I'm wrong, and sorry about that...)
But, I'm gonna' give you the straight skinny...and adding some further info based on my above assumption... :)

{BTW, I was not aware that you could decode the INMARSAT signals and listen to them, using SDRPlay/RSPdx?}

First the very short answers...
I am planning on using my SDRPlay RSPdx for Immarsat decoding. I'm thinking of purchasing a l-band active antenna as154. I do have 2 problems.
1) Is it possible to install this in the attic if I'm pointing out through a window with direct line of sight?
Not really...it might work, but probably not....see below for the details

2) Is it possible to use a sma splitter on my SDR? My other ports except for my C port are taken
I'm not clear on what the "C port" is?  But, why in the world do you desire to "split" your L-Band signal input? (I'm assuming you wish to "combine" the INMARSAT L-Band signals together with whatever other antenna you're using into your SDR?  So, in that case, yes [assuming you power any active antenna, on the "antenna-side" of your "splitter/combiner"]....but, I have little understanding of what you're attempting to do?  So, just use a small outdoor antenna, and be done with it...)

A)  While my more recent use and experience is with Iridium, currently with my Iridium 9555 satphone, both on land and at sea, as well as other sat comm systems.....(btw, I started as beta-user/tester of Iridium, in the late 90's)....I have used INMARSAT in the past, and have recently had clients use it (BGAN) as well....heck, I even go way, way back to the old days of the old analog MARISAT/COMSAT system when I was a teenager...(and the "modern" INMARSAT-A terminals of the 80's...where the radomes were now smaller/lighter, at ~ 5' in dia and weighing in at only ~ 250 lbs!, plus all the below deck equipment!)  Now, 40 years on, we have 10 times the capability that fits in our pocket, costs 10 times less, and runs on a small battery for days (in standby)!....and, since all modern INMARSAT traffic is digital, and most of it is just data (whether internet IP traffic, text, email, etc.) I'm not sure you'll find much of interest to listen to?  {there just isn't much telephone voice traffic anymore on INMARSAT....the Geo-Stationary round-trip-path latency has always been an issue for telephone voice traffic on INMARSAT, but now add in the A/D and D/A conversions and IP protocols, and you find most business / routine maritime sat comm telephone traffic handled by Iridium these days.  Especially now, since Iridium Certus systems are GMDSS cert...}

And, you are aware that this is a duplex system....meaning the mobile terminal / phone / etc. has a discrete L-band downlink freq that you can receive, but a separate discrete uplink freq that you will not be receiving---- so, even if you do demod / decode some voice telephony, you're only gonna' be hearing one-half of the conversation.


B)  But if all you want to do is experiment / and play-around....go for it.


C)  And, understand that from central Texas, you'll only have line-of-sight to INMARSAT I-5 Amer (AOR-W) and I-4 Amer (I-"americas")....

While it is possible to shoot thru glass on L-band, you're much better off shooting thru plastics like acrylic ("plexiglass") or poly-carbonate (LEXAN), or polyethylene ....or even use polystyrene ("styrafoam")...

Now, while I don't know your attic, nor your antenna, I suspect you'll have issues shooting up (at ~ 32* elevation and ~ 118* Mag, for AOR-W....or ~ 53* el  and 180* Mag az, for I-4 Amer...) without picking up significant thermal noise from the surrounding structure?  Seriously effecting your rec S/N?

So, why not just use a small unobtrusive antenna, outdoors?  Heck, even an Isatphone antenna is about the size of a cigar....an entire BGAN terminal and antenna are smaller than a briefcase.....is there some reason you wish to place a sat comm antenna INSIDE? :( 

A small unobtrusive antenna in the clear is likely to outperform any antenna in an attic!  :)

Just saying, why complicate the matter?


John, I answered your question, but as I wrote, there might not be much to listen to, and your proposed antenna set-up is a rather poor idea.

So, I do hope this helped?

73,
John,  KA4WJA

P.S.   No worries here, just an fyi....
It's INMARSAT (with an NM not a MM), and yes it is in all caps....it's a British company whose name is an acronym for INational MARitime SATellite corp....  :)
And, BTW, they are really super guys...."brilliant" in both our American definition and the UK definition!  :)
« Last Edit: October 01, 2022, 11:57:45 PM by KA4WJA »
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KB5SLF

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Re: SDRPlay and L band Immarsat
« Reply #3 on: October 11, 2022, 04:35:44 AM »

Thank you, John. Yes, I am new to satcomms. I did get my antenna outside and am getting plenty of ACARS and NAVTEC signals, but the software for it is a problem to download, which will be my next question. Yes, SDRPlay devices do receive Inmarset transmissions. They have put out videos on YouTube detailing the procedures to do this. It's pretty awesome. When I'm not playing CW, FT8 or phone, I'm playing with the RSPdx. I highly recommend it. You have MUCH more experience than me and I really appreciate your advice and input. 73
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