VUHF Radio

A dual band VHF/UHF module based around two AX5043 radios. Built to work with HAM Radio communications using AFSK and G3RUH modulations.

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A plug-and-play radio module that I developed to communicate with my University CubeSat that is scheduled to launch next year. The module uses two AX5043 transceiver for dual band, VHF and UHF, communication using AFSK-1200 and FSK-9600 (G3RUH) modulations and AX.25 framing. It is also capable of receiving the Morse Beacon of the CubeSat in VHF.

For now, using the current hardware and software, the module can:
- Receive and transmit AFSK-1200 using AX.25 in both UHF and VHF
- Receive and transmit MSK/G3RUH-9600 using AX.25 in both UHF and VHF
- Receive and decode Morse in VHF
- Output receiving signal tracking info, like RSSI and Frequency Offset
- Basic configuration changes over an AT-like interface

Functionalities I am still experimenting with and like to add latter are:
- BPSK-9600 support
- Try the “Analog” FM communication, this is for fun, like a walky-talky type of thing
- Using the Raw bit input/output of the transceiver


During the development of the TT&C for my University CubeSat I started to think about an easy and cheap way to receive communications from the CubeSat once it is in orbit, projected to be next year in 2021. The CubeSats TT&C transmits in VHF, and receives in UHF, using AFSK-1200 with AX.25 as the basic communication modulation. It also has a Morse beacon in the same frequency band. The idea of the developed module is a plug-and-play receiver that anyone can use to receive the satellites communication without needing any additional hardware or software, besides an antenna.

The module is based around the AX5043 Sub-GHz transceiver, it is as far as I can tell the only transceiver of its kind that supports FM-AFSK. It is an amazing little transceiver, full of functionalities and highly configurable. It even supports analog interfaces (1 DAC and 1 ADC channel) and a raw bit interface (DATA & DCLK)! The module uses two of these transceivers so that it can both receive communications from the CubeSat in VHF and transmit to it in UHF, useful for me during development and testing of the CubSats TT&C. The control and data transfer of the transceiver to/from the PC is done over USB using the Virtual COM Port library of the STM32F103C8 MCU. The module also features current consumption monitoring capability of the module and each transceiver. It also has analog signal conditioning to use the ADC and DAC of the AX5043 for the possible "analog" FM and other analog outputs like RSSI. The module was designed to fit inside a cheap aluminium enclosure (80x50x20mm in size) available on eBay.


The localization/layout of the different modules can be seen in the image below, each LDO has output current monitoring using a INA138. The analog input/output for each transceiver is done through a 4-pole 3.5mm audio connector. A few "mandatory" LEDs for status indication are also present.

There are some mistakes in the hardware design, visible as wire bodges, diagonal components and bypass/shunt resistors. I'm working on a revised hardware design (Version 3) to fix those and also exchange the XO for a TCXO for much better frequency accuracy and stability.


So far I have successfully tested AFSK-1200 reception and transmission as well as FSK-9600 (G3RUH). The transmission spectrum for those modulations can be seen below. The emitted power is around 15 dBm just as expected from the datasheet.

The power consumption during transmission is around 140mW for the transceiver alone which gives a efficiency of about 20%.

Characterization of the reception is still ongoing, so far only AFSK-1200 in UHF is characterized with a BER of >10-5 for input powers over -105 dBm and 10-4 at -110 dBm with similar resutls for GMSK-9600 in UHF. The VHF receiver has significantly worse performance with a BER of >10-5 only at input powers over -95dBm. This requeires further testing.

The current hardware schematic and Gerber files can be downloaded and the current firmware is available on my GitHub page. Fill free to use them however you want. Also more in-depth performance analysis are available on my webpage as well as an explanation on how to set up the AX5043 transceiver which is a bit complex with all the possible configurations.

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  • 2 × AX5043 Ultra-Low Power Narrow-Band Sub GHz (27 - 1050 MHz) RF Transceiver
  • 1 × STM32F103C8 Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers

  • Audio Filter Improvements

    NotBlackMagic10/06/2020 at 11:57 0 comments

    As seen in the last project logs/updates, the module is capable of demodulating and outputting audio FM and AM signals. The audio quality is OK, not very good due to the way the DAC works and the not very good filtering of it. Over the last few days I researched different filter typologies to improve the audio quality and I tested a new one that uses the same circuitry as the previous filter, that is already present in the Module.

    The previous filter used was a double RC Low Pass Filter, as can be seen in the figure bellow on the left. The new one tested is a double LC Low Pass Filter where the resistors of the RC filters are exchanged with inductors, as can be seen in the figure bellow on the right.

    The double RC filter is a second order low pass and the new tested LC filter is a fourth order low pass, increasing the DAC filtering substantially. Bellow is a figure comparing the frequency response of both filters with ruffly the same cut-off frequencies:

    Tests showed that the new LC filter improved the Audio quality as can be seen on the figure bellow where the Audio Output of the Module is shown, when in FM Demodulation mode. This can be compared with the previous project log, "FM Audio Demodulation", figure. The test conditions were the same.

    The improved Audio quality is also audible, a sample is available on the website, but it has now a lower pitch/tone.

    On the website a more in-depth comparison between the filters used, and two others tested before, was also added.

    Besides this I'm working on a better command interface for the Module, inspired by the CAT command interface used by some HAM Radios like the ones from YAESU and Kenwood. It will not be compatible as it is a very different kind of device but the syntax is the same, eg. Setting the VUHF Radio Frequency command is: "FA145895000;"

  • FM Audio Demodulation

    NotBlackMagic09/02/2020 at 20:04 0 comments

    Analog FM demodulation, and modulation, is a unique and officially supported special feature of this transceiver. ON Semiconductor even provides an application note on how to set up and use the transceiver as an analog FM transceiver, "AX5043 Use as Analog FM Transceiver". Both the application note and the configuration used are for Wideband FM signals, the type used in FM Radio Broadcast. FM Radio Broadcasts were actually used to test the FM demodulation with good success!

    As with the AM Demodulation, the basis of the FM Demodulation is the internal DAC. For FM Demodulation the DAC is setup to output the Frequency Tracking value. With the transceiver setup properly, the FM signal is first down-converted into the IF stage where the frequency change over time is tracked and output by the DAC. This tracking information is also accessible over the SPI interface of the transceiver.

    To show the FM modulation in the IF stage, the DAC was configured to output the SAMPLE_ROT_I (or SAMPLE_ROT_Q) and a RF signal generator connected to the module is used to generate the FM signal, using as the modulating source a pure 500 Hz tone. In the figure bellow the FM signal in the IF stage is shown in blue and the pure 500 Hz tone is shown in yellow.

    With the DAC setup to output the frequency tracking value, the FM demodulation, the signal observed is shown in the figure bellow. In blue is the FM demodulated signal and in yellow the original audio signal used as the modulation source.

    As with the AM demodulation, a lot of noise is visible on the demodulated signal. This noise is also very clearly audible in the audio output of the module, through the audio jack. The main source of this noise is from the not well filtered DAC output.

    Samples of the output audio can be listened to on my website, as well as all the configurations used to achieve the FM demodulation.

    A little teaser on what I'm testing and working on for this project can be seen on my twitter, very early stages still.

  • Audio AM Demodulation

    NotBlackMagic07/26/2020 at 20:15 0 comments

    This very unconventional use of the AX5043 transceiver was triggered by a question posed to me by a fellow Hackaday member, if the transceiver could be, somehow, used as an Audio AM receiver. Over the past week I explored this idea and it turns out that it actually can!

    The basis for this is the very versatile DAC output of the transceiver which, among others, can output the amplitude tracking information of the signal at the entrance of the demodulator block. With the DAC set up to do this, the transceiver has to be "tricked"/setup to keeping the amplitude modulated signal at the IF stage, where this tracking information is calculated. The signal at the IF stage is visible in the figure bellow in blue (DAC set to output either SAMPLE_ROT_I or SAMPLE_ROT_Q), in yellow is the original audio wave:

    With all properly setup, the DAC outputs the demodulated AM signal, the desired Audio signal, as can be seen in the figure bellow. In blue is the demodulated AM signal as output by the DAC and in yellow the original audio wave:

    It can be seen that the demodulated signal is very noise. The main source of this noise is the not well filtered DAC output, even after a second order low pass filter. The transceiver uses a Sigma-Delta DAC. Even with this noise the performance is much better then expected, and by plugging in a headphone to the audio jack of the Module one can listen to AM audio transmissions!

    More information on the set up used to achieved this as well as an audio sample file output by the Module are available on my website.

  • Hardware Revison 3

    NotBlackMagic07/15/2020 at 19:31 0 comments

    The newest hardware revision, revision 3, arrived and I assembled and tested it over the last weeks. This revision fixes all the hardware errors and problems discovered with the revision 2 like the AX5043 Oscillator Input, analog output amplifier/filtering and in the power supply/distribution circuits.

    The RF circuit, matching networks, were kept the same and this is reflected by the RF performance and input matching (S11) which are the same as with revision 2.

    Besides that, the PCB color was changed to blue which in combination with no bodges looks much better and clean then the previous revision:

    With this, I now have two assembled and working modules. This, with the antennas that arrived and where tested a few weeks ago, enables now communication testing between modules as well as range testing/comparison with different antennas and parameters.

    Files for the new revision are available on my website.

  • Antennas

    NotBlackMagic06/30/2020 at 21:16 0 comments

    Some months ago, I ordered a bunch of dual-band antennas to use in this project and they finally arrived! The antennas I bought are shown in the figure bellow, from left to right they are a Nagoya NA-771 Clone, a Nagoya NA-771 Original, a UV-106UT Clone and a UV-108UT Clone.

    All four antennas where tested with a R&S VNA to get there S11 parameters and in the future, I will perform field tests with the VUHFRadio to compare their ranges and “gain”.

    The figure bellow shows the comparative result of the two NA-771 antennas in the Sub-GHZ band:

    And the results for the same band for the UV-106UT and UV-108UT antennas are shown in the figure below:

    Overall the NA-771 antennas have better S11 characteristics for the desired bands of this project. In general a S11 of bellow -10dBm is desirable (VSWR better then 2:1) as this means that over 90% of the power is delivered to the antenna, less than 10% is reflected.

    A bit more in-depth results are available on my website as well as all S11 results files for download. I will also, overtime, add more results for other antennas to that page.

  • BER Tests

    NotBlackMagic06/21/2020 at 19:42 0 comments

    During this week I performed some BER testing on the transceiver for both AFSK-1200 and GMSK-9600 in UHF and VHF. In UHF the results were great with both modulations having a BER over 10-5 with input power above -105 dBm. For VHF, the results were not so good, with a BER over 10-5 only achieved with input power over -95 dBm, for both modulations. I am not sure why this is, and it requires further testing.

    The effect of increasing the receiver bandwidth on the BER was also tested and it is very noticeable, as is expected. The BER decreases about 10-1 for each doubling of the receiver bandwidth, this with input power of -105 dBm.

    Full results are available on my website at the bottom of the page.

View all 6 project logs

Enjoy this project?



justin.h725 wrote 09/23/2020 at 01:14 point

I am very interested in VUHF Radio. When will it start selling and what is the price

  Are you sure? yes | no

NotBlackMagic wrote 09/29/2020 at 08:12 point

Thanks for the interest! I have no plans for manufacturing/selling it yet. If the interest is big enough I will for sure consider it.

  Are you sure? yes | no

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