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 Hugues on January 10, 2016 at 12:38pm

Thx Thomas & Gary for the info : I'll get learning this right away !

I have a RF explorer for monitoring frequencies, I guess I could use that instead of a SDR dongle.

Issue is I saw on github for the SIk Firmware it can only be compiled on a MAC/Apple ?; Did I get that right ? Compilation is required to change the OCS-CAP-VALUE, right ?

Comment by Jake Stew on January 10, 2016 at 2:05pm

Darn, lost a post...

Anyhow...  SiK firmware compiles in linux.  I use the Kali linux VM with VMware.  There is also a Kali VM for virtualbox.

I use the 32-bit Kali VM and run it in my normal Win7 x64 environment.

Basically you just have to... download and run the Kali linux VM.  Then I run the following commands...

sudo apt-get update           # Fetches the list of available updates
sudo apt-get upgrade         # Strictly upgrades the current packages
sudo apt-get dist-upgrade   # Installs updates (new ones)
apt-get autoremove            # Removes junk

sudo apt-get install sdcc    # Installs SDCC and dependencies

cd SiK                              # Enter SiK directory

cd Firmware                      # Enter Firmware directory

make install~radio~hm_trp  # Compiles bootloader and firmware for HM-TRB based radios.

Compilation IS required to change the OSC value, BUT I  have already compiled a bunch of them for people.  I uploaded them to my fork of the project.  I did versions for values from 196-212.  This is -8 to +8 relative to the default value.

This should probably cover any value needed.  If not drop me a line and I'll make one for you.  You can find them here...


Comment by Patrick Poirier on January 10, 2016 at 2:18pm

Thanks Jake for this very interesting topic, that made a nice sunday project :-)

@JB, next time that OutBack Joe goes outdoor , make sure He get his keys so we can go Fox Hunting !!

Raspberry Pi2 running Osmocom Spectrum Browser on 315 Mhz with RTL2838  :Green Peaks are Key Pressed

(source: http://www.rtl-sdr.com/testing-gnu-radio-raspberry-pi-2/)

Comment by Hugues on January 10, 2016 at 2:21pm

Thx Jake. I am using 433Mhz radios. I have a few pairs I will test with RF explorer to determine how much they are detuned. 

Comment by Jake Stew on January 10, 2016 at 2:28pm

Point about 915 vs 433...  My test firmware "Jake_Tune_Adjuster.ihx" is written for 915 radios only.

It *should* work on 433 radios, BUT it will still set them to 925 MHz.  The 3 lines are these...

MyFreq = 925000000UL;

Should just have to change those to a lower frequency.  There is no problem setting the 433 radios to 925, but the PA/antenna matching/filter circuitry will probably filter out 99% of the output.  Might still be usable for tuning though, we don't need much signal to reach a few feet.

I'll try to make a 433 version today, but I don't have any 433 radios to test it on currently.

Comment by Hugues on January 10, 2016 at 2:35pm

@Jake, I can test your future 433 compiled firmware for you if it can help.

Comment by Jake Stew on January 10, 2016 at 6:55pm

Done! Files here...


So the basic procedure is to download Jake_Tune_Adjuster_(925 or 434).ihx, and flash it onto your radio using the standard 3dr radio config program.

Then open a terminal window to your rado's com port.  I highly suggest using putty for windows users.

Enter: +++

You should get back: OK

Type ATJ and you should get back some info.

Now enter ATK and the radio will switch to a 600 hz test signal on 925 or 434 MHz for 30 sec..

After 30 sec. the radio should emit a pulsed tone on it's frequency.  Now you can use ATU and ATY to tune the OSC value up and down.

ATK takes ~30 sec. to complete.  You cannot enter any commands during this period.  ATU and ATK should take ~3 sec. to complete.

Shift the frequency up or down as needed, on one or both radios.  Note the correct OSCval, then load the corresponding numbered firmware from:


Now you have tuned radios running the latest firmware.

Note that the 434 version will say some wrong settings in ATJ and ATK.  The calculations were for the 915 version.  Stuff like this is why I had it output multiple ways of figuring things.  Since this is a utility firmware and I don't have 433 modems to check it with I don't plan to fix it.

Next task is a beacon mode firmware.  Essentially the firmware already does this perfectly, just want to clean things up.  The idea is for the scenario where you have a radio link on a lost vehicle, but aren't getting GPS data.  You would then enter "ATB" or some other AT command to switch to beacon mode, which would be easier to track with handheld radios and foxhunting gear.

Comment by Gianluca Galeno on January 10, 2016 at 7:23pm
sorry guys, I present are Gianluca Galen. for a month I got hold of a CX20 open source. I have already done my flights. about 3 days ago, I installed the 433 MHz telemetry. all good works. But after I made a mistake. I updated the drone, with telemetry attacked. result? telemetry flashes me quietly. but it does not connect to the PC. I noticed that first lit up the green light and the red light a few, but now I will always turn on the verse, but see little of the red. anyone can help me. I do not know what to do. thank you all. I do not know if it is the right section. excuse me
Comment by JB on January 10, 2016 at 9:58pm

Nice one Patrick! It's a shame Joe locked his keys in the car and that's why he's lost in the bush! ;-)

Have you tested the range yet on the key fobs? Might actually be useful to find people in collapsed building etc, if the people know that it can be used to locate them!


A modified 3DR/HopeRF radio firmware in an energy saving beacon mode might be a good kickstarter though for cheap EPIRB. EPIRBs run at 406mhz, so a 433Mhz version should easily work in that band from memory. RF penetration through solids is always an issue but better than nothing...but it's ideal for using it outdoors. Having a $30-40 EPIRB would be pretty good value for money, instead of $200-300. The tracking and locating hardware used by emergency responders is setup for it already so no extra costs there. 

Could be used for asset tracking in fire prone areas like we've been having here in WA as well. 300km fire front and 700sqkm in 6 days...not fun at all when it comes within 7km with 60kmh winds and you have to evacuate! Fire plasma wreaks havoc with RF though... :-( A flying comms platform would help though, seeing that if you already have fire over your head, means you're already in big doodoo. Time to put the sprinklers on again.

Comment by Art on January 11, 2016 at 8:31am

I confess I didn't read the whole thread but in what I read I didn't find any data about the "spectacular range boost"  Seems like an entertaining albeit rather pointless frequency analyzer exercise but I don't think it affects you radio performance a bit. I'm not that familiar with SiK code so I don't know if the AFC (Automatic Frequency Control) is enabled but I would venture a guess that it is.  If not, it should be.  So with AFC enabled it will pull in a frequency with a 13kHz offset with virtually 0dB loss in receive sensitivity.  So no, there is going to be no  spectacular range boost.


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