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Author Topic: My efforts to use a DVB-T stick as a " cheap" satellite RX...a question...  (Read 6293 times)
W4HIJ
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Posts: 367




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« on: June 05, 2013, 06:24:52 PM »

Since I'm on a tight budget, I've been experimenting with using a DVB-T stick as a cheapie SDR satellite RX. I have it connected to an omnidirectional "scanner" antenna that I had lying around and using HDSDR coupled with SatPC-32 to track the doppler, I'm getting OK results. I don't have any sort of RX preamp built yet and I imagine I will get  better performance once I have that in line. A directional antenna would help some too and both it and the preamp are on my to do list. My main concern is when I get to the point of TX to the satellite.  I'm concerned over desense because of the wide front end of the DVB-T stick. Experiments with a 2 meter HT while trying to receive on 70 cm have indicated it will be an issue even with some spacing between the RX and TX antennas. I'm wanting to use one dedicated dual band antenna for the RX  and another dedicated dual band antenna for the TX. In this configuration I won't be able to use the old trick of putting a diplexer up at the antenna as a filter because I would be blocking one of the bands (2 meters) I'm trying to receive. I was thinking of putting a couple of switchable filters in front of the DVB-T RX at the shack end. Something like a cavity filter is way to elaborate and beyond the low cost spirit of this experiment. In the May/June 2007 issue of the AMSAT journal in WA5VJB's article on his excellent "cheap yagi" antennas he shows some circuits for simple 250 MHz high pass/low pass filters designed to work as band splitters for yagis for the two frequencies built on the same boom. I'm wondering if I could adapt a filter circuit such as this down at the shack end, being able to switch between the 2 meter filter and the 70 cm filter to be able to combat desense. Would an arrangement like that work or am I all wet?  I know I could just use a couple of HT's to work SO-50 and the upcoming birds but I wanted to try this experiment to see if it has feasibility for using the DVB-T stick as an all mode RX. I would like to have something like a Funcube Pro dongle  but that's just not in the cards right now.  I would love to hear some  opinions on my experiment. Hopefully good ones..... Grin
Tnx and 73,
Michael, W4HIJ
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W4HIJ
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Posts: 367




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« Reply #1 on: June 06, 2013, 10:26:50 AM »

Hmnnn...118 reads and no one has any thoughts on this? What happened to the experimental nature of ham radio?  Guess I'll just try my ideas and see. It's only a handful of parts. I'm convinced there is merit in the idea of simple inexpensive SDR RX's for satellite use, I just don't have the know how to design my own or the finances to buy a Funcube Pro right now. I don't understand why everyone seems to think that the only valid way to work an FM sat anymore is standing outside with an HT in one hand and an Arrow antenna or a clone of it in the other. That's always seemed cumbersome to me. I prefer the comfort of my shack!
73,
Michael, W4HIJ
« Last Edit: June 06, 2013, 10:49:55 AM by W4HIJ » Logged
W5PFG
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Posts: 80




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« Reply #2 on: June 06, 2013, 11:02:31 AM »

Sure, with the right experience and knowledge you can build the right filter for your Chinese TV SDR.  However, it might be better on the budget to buy a second-hand FunCube dongle (not Pro.)  The time and money invested in experimenting would not yield the performance of even the original FunCube dongle.  If you're experimenting for the sake of doing so, and not economics, that's another story.
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W4HIJ
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« Reply #3 on: June 06, 2013, 11:34:46 AM »

Sure, with the right experience and knowledge you can build the right filter for your Chinese TV SDR.  However, it might be better on the budget to buy a second-hand FunCube dongle (not Pro.)  The time and money invested in experimenting would not yield the performance of even the original FunCube dongle.  If you're experimenting for the sake of doing so, and not economics, that's another story.
I don't mind spending the time, I just don't have a lot of money to spend. If the filter looks like it will become a high dollar affair, then I agree with you. However, if I can get buy with something cobbled together from a few bucks worth of parts then I think it's worth a shot. That's why I was looking at the simple high pass/low pass filter circuits. The DVB-T actually performs surprisingly well for what it is but I know it's not a Funcube and I don't expect it to be. I really don't understand what's wrong with experimenting for the sake of economy. If I could afford something better right now, all this would be a moot point.
73,
Michael, W4HIJ
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N1EN
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« Reply #4 on: June 07, 2013, 08:43:16 AM »

Just out of curiosity, what are the cool kids using for satellite receive and doppler-adjustments on FCD's (FCDPro+, in particular) these days?

In addition to the annual futile attempts to get through on an FM satellite at a local club's Field Day, I've been thinking of seeing if we could "do something" with an IC-7000 for TX and a FCDPro+ for RX, if time permits researching how to make it happen.
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W4HIJ
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Posts: 367




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« Reply #5 on: June 07, 2013, 10:30:30 AM »

Just out of curiosity, what are the cool kids using for satellite receive and doppler-adjustments on FCD's (FCDPro+, in particular) these days?

In addition to the annual futile attempts to get through on an FM satellite at a local club's Field Day, I've been thinking of seeing if we could "do something" with an IC-7000 for TX and a FCDPro+ for RX, if time permits researching how to make it happen.
I'm not sure about other options but HDSDR will work with the Funcube Pro if you download the proper ExtIO DLL interface.  You can then track doppler via DDE using HRD, SatPC32, Orbitron or WISP. It has all those options listed anyway, I've never tried anything but SatPC32, I know it works. Go to http://www.hdsdr.de/ and click on hardware to get the link for the ExtIO DLL.
Hope this helps,
73,
Michael, W4HIJ
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KA4NMA
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Posts: 336




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« Reply #6 on: June 07, 2013, 08:57:04 PM »

Do you have a dual band radio? One that transmit channels stored on one vfo and the receive on the second vfo?

Randy ka4nma
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N1EN
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« Reply #7 on: June 07, 2013, 09:46:23 PM »

It looks like we'll have 4 IC7000's and 1 TS590 at our Field Day.

I'm open to other suggestions, but considering the limitations of the 7000's, I was thinking that if we're going to attempt a contact on one of the linear birds, we might have more success listening on a separate receiver, and I'd prefer not to pull any of the other rigs off of HF duty.

Of the gear we'll have available, transmitting on a 7000 and receiving on a dongle seem like the combination "most likely to succeed" on a linear satellite pass.

I had also thought of programming a bunch of frequency pairs into memories, and adjust for doppler in a stepwise manner, but that feels a little herky-jerky, and the implications of making a math error with the inversion through the transponder and the doppler adjustments have me shying away from that (especially since the 7000 won't do full duplex operation).  If I'm wrong about that, and if someone would like to share some suggested memories for the major linear birds, I'd be happy to be corrected.

We will use the pre-programmed memories when joining the FM horde on SO-50, but I'd be shocked to get a successful contact out of that scrum.
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W9IQ
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Posts: 102




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« Reply #8 on: June 08, 2013, 07:49:38 AM »

I would think your original concept of a 2m low pass or a 70 cm high pass filter (depending on the satellite mode) inline with your dongle would be worth the experiment. The ARRL handbook has a filter design tool in the companion CD that should get you some nice designs with homemade air wound inductors and a handful of capacitors.

- Glenn DJ0IQ and W9IQ
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KC5CQW
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Posts: 98




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« Reply #9 on: June 13, 2013, 10:33:58 PM »

I also have a DVB-T; lots of fun!
A notch filter to block the TX band may work very well for you.
I saw one once made from a 1/4Δ coax stub.
All you would need is a "T" fitting and coax terminations (BNC, PL-259 etc.).
 
A good antenna for RX would be something like the Texas Potato Masher II or other loop types that can switch polarizations.
Most scanner types are vertical mostly so you'll miss out on higher passes.

Good luck and have fun!
73, Damon
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N6JSX
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Posts: 217




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« Reply #10 on: July 31, 2013, 05:53:50 AM »

Have you looked into 'repeater' builder's type sites?
Hamtronics or DownEast have some good tunable band-pass pre-amps.
Check Digi-Key for small PCB tunable cavities too.

You could look into making a PCB three-strip band-pass notch filter with variable caps to allow tuning the notch, I did that for Magellan GPS  - cheap but requires some very high end test equipment to nail the design. I had to make one for PC-Electronics ATV RX down-converter as near cell phones use to mess up the 1.2G ATV (back in the 80's).   

Got to think outside the box and dig around, stuff is out there just dig around on the web. I'm constantly finding new web sites commercial & personal blogs that have a wealth of designs & cheap products. 
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