915 MHz 3DR Radio Tuning With $7 SDR Spectrum Analyzer! Spectacular range boost!?

If you happened to get one of those $7 SDR radio dongles from ebay and need to put it to work... here's one thing I tried...

Tune in with the R820T... (Using HDSDR)

This is both radios running on one channel.  They are out of tune enough that the channels don't even overlap!

Now I switch them into beacon mode and tune to 925.000 MHz...

Now I tune them up by adjusting the EZRADIOPRO_OSC_CAP_VALUE to get this...

Through the magic of special effects, I can overlay them...

Below are the before and after pics of normal transmissions (non-beacon mode)...

Settings used...

Here are the actual results I measured (warming up and final calibration)...

Ground = +7.3 khz @ OV 204 (cold)
Ground = +6.5 khz @ OV 204 (warm)
Ground = +6.1 khz @ OV 204 (hot)
Ground = +0.1 khz @ OV 208 (warm)

Air = -6.5 khz @ OV 204 (cold)
Air = -6.2 khz @ OV 204 (warm)
Air = -7.1 khz @ OV 204 (warmer)
Air = +0.6 khz @ OV 199 (warmerer)

All the pictures in this post are to the same scale.  The FSK signals (wider pictures) are on a different frequency than the beacon signals.

Make any predictions on expected range improvement below...  

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Comment by Rana on January 9, 2016 at 1:16pm

That's really awesome !

Few years back, I purchased a pair of 3DR telemetry radios from the store, infact the store name was not 3DR at that time.

Those radios were 100mw 915MHz and gave good working range upto 13km in air @300mt alt and 13dbi Yagi antenna at ground.

Some other pair of radios did not give, I am able to understand that now that may be by luck my pair of 3DR radios

might have been perfectly tuned.


@Jake, two days back only I got a similar SDR dongle from ebay, which software you are using for the PC ?


Comment by Michael Utz on January 9, 2016 at 2:12pm

thats excellent Jake. Will this software also work with the 433 MHz version. If not, are you willing to make one ? 

Comment by Hugues on January 9, 2016 at 4:18pm

What is SDR ? R820? HDSDR ? This is Chinese for me. And what is this parameter OCS-CAP-VALUE you modify? In which sw is this?

Comment by Thomas J Coyle III on January 9, 2016 at 4:31pm


SDR: Software Defined Radio

HDSDR: Free SDR program for Windows

R820: USB Radio dongle



Comment by Randy on January 9, 2016 at 6:54pm

Great work.  So once this offset is discovered is it pretty stable or does it change with temperature or some other changes in the enviroinment?II wonder if we could create a calibration routine where one radio outputs a signal at a known frequency and the other radio tries to match it.  Perhaps that's the same as the rssi suggestion above.

Comment by Gary Mortimer on January 9, 2016 at 11:28pm

@Hugues don't forget when you have finished tuning your radio you can use the SDR to put air traffic overlays on mission planner. When might we get some LoRa modules? We have been testing them down here for other purposes and very easily get 17km range ground to ground out of 100mW (that is line of sight though not over anything) http://www.instructables.com/id/Introducing-LoRa-/

With one module on a kite acting as a repeater I bet it could cover the entire outback challenge area easily. 

Comment by Hans Müller on January 10, 2016 at 2:25am

Where did you find that $7 SDR Spectrum Analyzer?

Comment by Michael Utz on January 10, 2016 at 2:50am
Comment by Hans Müller on January 10, 2016 at 4:39am


Comment by JB on January 10, 2016 at 6:29am

Nice work Jake. Will be checking the RFD900's now as well to see how good they are.

It's good to see some SDR capabilities trickle through for spectrum analysis.

Is anyone keen on setting the RTLSDR dongle up as a secondary telemetry receiver as well? We could then use the onboard companion computer running linux (Rpi,odroid) as a flying SDR receiver for ADSB or redundant telemetry receiver (either 400-900Mhz) or on the ground PC with some spectrum monitoring to automatically choose and set the best channels to use according to RF conditions.

Even better would be to run a Myriad RF/Hack RF/Blade RF board either end and ditch the telemetry radios all together. ;-)


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