SDRplay as satellite downlink receiver?

Discussion in 'Satellite and Space Communications' started by WD9EWK, Aug 24, 2015.

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  1. WD9EWK

    WD9EWK Ham Member QRZ Page

    The recording is a function of the software you use with your dongle. Make sure to point the recording to a drive with lots of space. You can easily chew through 200-300 MB to record 96 kHz of bandwidth for a few minutes. But you get everything in that bandwidth, so you can play around with it later. HDSDR gives you the option to chop the recordings once they reach a certain size, so you don't exceed any limits on your hard drive's file system.

    To make use of those SDR recordings with other software, you may need some virtual audio cable software so you can have the audio output from your SDR software go into another program. Some Windows 7 systems have a "Stereo Mix" recording device that can do the same thing, without having to install additional software. Receive bandwidths vary with these devices, of course - from 192 kHz from the FUNcube Dongle Pro+, to around 3 MHz for the RTL dongles, 8 MHz with SDRplay, and up to 20 MHz with the HackRF devices.

    These SDR devices are not that expensive. Of course, the "not that expensive" assessment is relative. I can buy 7 or 8 RTL-type dongles, or the SDRplay, for the price of a certain 2m/70cm FM HT discussed ad nauseam in this forum. The HackRF Blue was just over $200 through a crowdfunding drive, and the HackRF One retails around $300 from a few different sellers. Since I'm not purchasing a new IC-9100 or a used radio like an IC-910H or IC-821 for a home satellite station at this time, it's not hard to look for other items to try out for satellite work - or just for general receiving. They all have their places - the RTL dongles are OK for VHF/UHF receiving, as they don't typically receive below 24-25 MHz without modifications. I have one of those dongles at my office mainly as an FM radio, but sometimes I will punch up ham repeaters, weather radio, or other stuff. The FUNcube Dongle Pro+ could be used as a receive-only station copying telemetry from AO-73, if it's not with me in the field or back yard. I'll probably keep it with me when I travel, since it is so small. The SDRplay might end up being my first choice for an SDR receiver when working satellites. And the HackRF devices, as transceivers, could do a lot more than just receive. Being able to see the entire 88-108 MHz broadcast FM band on my screen using a HackRF device (either one) is impressive! (Only my Win7 laptop, of the computers I have at home, is capable of seeing the full 20 MHz broadcast FM band - my tablets can't handle that much bandwidth)

    Last edited: Aug 30, 2015
  2. W7CJD

    W7CJD Ham Member QRZ Page

    What antenna?
  3. WD9EWK

    WD9EWK Ham Member QRZ Page

    For my satellite work, I use an Elk Antennas handheld 2m/70cm log periodic. Since log periodics have only one coax feedpoint, I have to connect my FT-817ND and receiver (either another FT-817ND, some other radio like a Kenwood TH-F6A with an all-mode receiver, or one of the SDR devices I've been writing about) through a diplexer to use this antenna. Diplexers also act as bandpass filters, which can be helpful if I'm transmitting on 2m and the diplexer can block harmonics from going out to the antenna (or over to the radio or SDR device I'm using to receive 70cm downlinks).
  4. WD9EWK

    WD9EWK Ham Member QRZ Page


    Going back to the screenshot I posted earlier from HDSDR playing back the AO-73 pass I worked last night... you can see a diagonal line near the top of the shaded area from the AO-73 transponder downlink. This would be an example of a CW signal that was drifting down the passband. Had I wound the recording back a little further, you'd see that CW signal zigging and zagging through the passband.

    Also, the reason the passband is a lighter color than the rest of the passband HDSDR recorded from the SDRplay has nothing to do with either the software or SDR device. Transponders are retransmitting what they hear across their passband. There is some signal being retransmitted by the satellite across all 20 kHz in the screenshot above - even if it is just noise. There is one relatively strong SSB signal in the middle of the transponder passband, and a second signal near the high end (right side) of the transponder in the screenshot.

    Once I figure out the SDRplay with the linear transponders, I will also try it with SO-50. HDSDR has an AFC function, which I have used in the past with my FUNcube Dongle Pro+ to track the SO-50 downlink. AFC helps to cut down the number of adjustments I have to do for the downlink, as long as there aren't long gaps between signals from that satellite. Whether in real time or playing back a recording from an SO-50 pass, you can clearly see the Doppler effect on the downlink - moving the downlink signal lower in frequency through a pass.

  5. W7CJD

    W7CJD Ham Member QRZ Page

    Thank you.

    What diplexer do you recommend?
  6. WD9EWK

    WD9EWK Ham Member QRZ Page

    I use an MFJ-916B 2m/70cm diplexer, but any of them from Comet, Diamond, etc. for these 2 bands should work fine. I use this MFJ diplexer since it is about half the price of the equivalent diplexers from Comet or Diamond. Also, the MFJ diplexers are made in Taiwan for MFJ, not in Mississippi - if there is any concern about MFJ quality control.

  7. WD9EWK

    WD9EWK Ham Member QRZ Page


    I was successful in using the SDRplay as my downlink receiver, during an AO-73 pass this morning around 1745 UTC. I set up my station in the back yard - one FT-817ND as my transmitter, HP Stream 8 tablet with HDSDR connected to the SDRplay for my receiver, Elk antenna with diplexer connecting to the 817 and SDRplay. I used an audio splitter so I could hear the downlink in my Heil Traveler headset/mic and record audio on my Sony recorder. I made an RF recording in HDSDR, a 8:35 recording that took up almost 450 MB on my tablet. I figured out the settings I needed to change, so I could hear myself and do a better job hearing the AO-73 downlink, compared to my unsuccessful attempt last Friday evening. There was a little delay between my voice and when I heard it from HDSDR - probably related to the limited memory and CPU in the tablet. I've gotten used to that, and will be ready if/when we have access to higher-orbit satellites AMSAT has been discussing in the recent past where this would be more common than with our current low-orbit satellites.

    When using an SDRplay with HDSDR, you have access to a controller pop-up for the ExtIO driver. Click on the ExtIO box to the right of the Tune frequency, and you get this pop-up:


    On here, I changed the IF Bandwidth to 200 kHz, unchecked the Enable Tuner AGC box, and changed the Gain Reduction (db) value. The 200 kHz is the lowest IF bandwidth supported by the SDRplay, and is the closest to the maximum input/IF bandwidth of the FUNcube Dongle Pro+ (192 kHz). Enable Tuner AGC is checked by default, so I had to uncheck it to change the Gain Reduction value. I probably could have set it to zero - something to try next time. This was better than the value I saw when I first unchecked that box (51 dB). For satellite work, dealing with weaker signals, the last think I'd want is something reducing the amount of gain in my receiver. More is better. For other uses, there may be valid reasons to reduce the amount of gain in the receiver. Click on the Exit button to close this pop-up. I did not make any changes in the Advanced menu from this pop-up.

    Next, I wanted to remove the center spike that appears in the waterfall (or, at least, reduce its presence). This is done by pressing F7 (or the Options button on the left side of the HDSDR window), then clicking on Input Channel Calibration for RX. Another pop-up appears:


    The only change here is near the top of this pop-up. Where it says RX DC Removal Mode, click on the button to the right of that until it shows Auto. Then click on OK to close this pop-up.

    In the main HDSDR program, I set the mode to USB. I also wanted to adjust the bandwidth I'm trying to hear from the program. Press on F6, or click on the Bandwidth button on the left side of the HDSDR window, to get to this. Normally, the input bandwidth is set here (it is for other receivers, but not the SDRplay).

    On the output side of what appears in the lower-left corner of the HDSDR window, make sure you have a smaller value. I left this at 12000, but for SSB you could use 4000 or 8000 here.

    With this setup, I heard the 145.935 MHz telemetry once the satellite was up a few degrees from the horizon, and then could hear activity from the transponder. You can see from this screenshot of my mobile phone's AmsatDroid Free tracking app how this pass went over my location:


    Starting due north, ending to my southwest. Nice elevation, too. During this pass, I was able to work Leo W7JPI in southern Arizona and Brad KG7NXH a few miles southeast of me here in the Phoenix area. There are times my signals are weak, and they popped up as I twisted my antenna.

    If you want to take my HDSDR RF recording from this pass and play it back on your computer, you can get it from my Dropbox:

    Look for the filename starting with HDSDR_20150830 for today's AO-73 RF recording I made. The other HDSDR WAV recordings were done with my FUNcube Dongle Pro+. Most of those are smaller, since I can use bandwidths smaller than 200 kHz with that device, which cuts down on the file sizes. I also uploaded the images posted in this message to my Dropbox, along with 3 photos I took Friday evening showing my station. Other than those photos being taken after dark, I used the same setup in my yard this morning - with the settings adjusted as I described in this post.

    Last edited: Aug 30, 2015
    W7CJD likes this.
  8. W5SAT

    W5SAT Ham Member QRZ Page

    Patrick, loving the HDSDR recording - probably ran through it about 3 or 4 times now. Very handy to see the whole band and different QSOs going on and signal level of each.
    KD8ATF likes this.
  9. WD9EWK

    WD9EWK Ham Member QRZ Page

    That's the idea behind making the RF recordings. :)

    I have some RF recordings of SO-50 passes, even though those are just one FM channel, to illustrate the Doppler effect on the downlink signal. You can clearly see it move downward in frequency throughout a pass, and with AFC you don't have to make many adjustments in HDSDR. It does a decent job of tracking the SO-50 downlink, as long as there aren't many quiet points - or fades - in the pass where HDSDR's AFC loses track of the downlink. Depending on how others are operating, listening to FO-29 with an SDR receiver can be nice, seeing stations spread out across much of the 100 kHz transponder. You can also see times where (too) many stations try to jam around the center of a transponder, and refuse to spread out.

    I saw that K0MDJ posted on the web site that he heard AO-73 during that pass. I heard him above where I was on the transponder, for the first few minutes of the pass (before he went out of the footprint). W7JPI was a little below me initially, then moved up to where I was for a QSO before you and I made our QSO. That's a perfect example of hearing a station I did not work, but was available to work, on that pass. I was more concerned with hearing myself and then working anyone who tuned across my CQ calls, so I didn't go searching for stations up and down the transponder.

    SDR Sharp has a recording function, either in the program or as an additional plug-in, that can do what I'm doing in HDSDR. Other SDR software will also write RF recordings as well. I've preferred using HDSDR for my satellite operating since it has everything in one place, and has worked better for me on the tablets than SDR Sharp. If I used a laptop, I could give consideration to more software, but for portable work I'm always looking at what works with the less-powerful (but still useful) Windows tablets.

    KD8ATF likes this.
  10. KD8ATF

    KD8ATF Ham Member QRZ Page

    I used HDSDR to listen to that recording in I like it! Looks like using the mouse wheel will make it eaiser to tune around. Will try to AO-7 tomorrow with it.
    WD9EWK likes this.

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