µBitx is a bargain-priced, but surprisingly fully featured transceiver. Not surprisingly, the µBitx has both strengths and weaknesses. This article tries to provide a balanced perspective in order to help inform potential purchasers of what they are getting in to.
The µBitx has, since its release in December 2017, been a popular, well regarded and highly affordable all-band QRP transceiver.
Commonly identified strengths of the µBitx include:
- An unbeatable price of around US$129 (for the kit) it is possible to get on the air on all HF bands.
- TheµBitx is near complete, with the main board pre-assembled and tested, so there is less that can go wrong for the home constructor.
- TheµBitx produces 10 watts out on 40m and even more on 80m. Most QRP rigs only put out 5 watts (or less).
- The µBitx operates on both SSB and CW. Most QRP rigs are intended for either phone or CW and not both.
- The µBitx includes an arduino, so it is programmable and the sketch (software) can readily be upgraded to add new features.
- The µBitx SI5351a PLL chip provides three stable oscillators for the mixers that are under arduino control. This increases opportunities for modifying filters, IF frequencies, etc.
- The µBitx comes with a built in electronic keyer. It is quite a cunning design, utilising just a single analogue port on the arduino.
- Most of the µBitx componentry is taken from existing proven designs by Ashhar Farhan. Circuit elements such as the IF stages have been used in a variety of Bitx transceivers over the last 10 years.
- A high quality Coen filter with 8 crystals is used in the µBitx to create an excellent SSB filter.
- The use of a mechanical rotary encoder (without detents, but with a switch) provides a nice tuning dial and doubles as a menu control system. This makes for a very minimal user interface that works well.
- Low pass filters covering all bands from 80m through 10m are intended to ensure that no nasty by-products make it out of the transceiver. This claim, however, is now in doubt.
- The receiver is quiet and is sufficiently sensitive to be typically below the background noise floor on all bands.
- The µBitx can be easily connected to a computer using the USB port on the Raduino for CAT control.
The µBitx v3 and v4 boards had two critical weaknesses that means it should be used with caution, unless you have confirmed that you meet emission requirements in your country using calibrated equipment:
- The LPF bank in the output stage and the associated switching relays have “blow by”. This is the result of the layout configuration, with only one switching relay and a complicated RF path through the relays. However, this can be mostly overcome by replacing the stock relays with Axicom D2n 12v units.
- The 45MHz mixer (on SSB mode only) is easily be over-driven. This causes unwanted spurs on all bands 15m and above. A simple fix is now available that involves replacing two inductors around the 45MHz filter. You should also ensure that the audio level on transmit is as low as practicable to reduce unwanted spurs. Overdriving the audio on SSB will cause spurs, even with the modification installed.
Other weaknesses that you should know about include:
- The transceiver comes with absolutely no warranty. However, there is a very supportive community only too willing to help someone encountering difficulties in getting the rig working. Most people have no problems with getting their rig up and running. A small number of purchasers encounter issues, that are either of their own making (assembly errors), or as a result of shipping damage, particularly with the delicate toroid windings. If your rig doesn’t work, join the BITX20 email group and explain your problems.
- Reverse polarity and board protection is limited to the supplied reverse protection diode. This assumes that the power supply is adequately fused (at a level suitable for the µBitx – around 3 Amps). Incorporating a fast-blow fuse prior to the diode protection is desirable in order to protect your investment.
- The combined volume control and on/off switch is functional (in most cases) but because of the small shaft diameter it can be difficult to source a knob. Many constructors have replaced this potentiometer with a standard 1/4″ or 6mm potentiometer along with a separate switch (which can be located on front or back panels). Suggestions for how to adapt the supplied potentiometer to accept a standard knob can also be found on this site.
- Broadcast interference breakthrough as a result of mixer products has been observed by a number of purchasers, particularly on 80m and 40m. This can be addressed by inclusion of a a BCI filter (see the suggested modification on this website). If you are located near high-power AM broadcast transmitters, you will need to build yourself or purchase a BCI filter.
- The output power of the transmitter section (which uses two low cost IRF510 devices) varies considerably between 80m and 10m. Typically around 14w PEP output is possible on 80m with around 10w on 40m and this declines to around 1w on 10m. This is a function of the IRF510’s that are appropriately biased. Drive level can be adjusted to compensate (at least in part). A scheme for equalising drive across bands has now been developed.
- Low microphone output has been observed by many purchasers. There is a drive level control, but no voice level control in the rig. Amplification of the microphone signal is barely adequate. Modulation using a loud voice is required to drive the output to full power (when compared to key down in CW mode). The solution is to adjust components to increase gain, or build a small mic pre-amplifier either within your microphone enclosure or inside the µBitx enclosure. Several options for how to do this are described on this website.
- A small number of purchasers have found audio tones appearing in their speaker output that are not linked to a specific frequency. These are generally around 6kHz or higher. The solution seems to be to change out the arduino nano or capacitively load the ceramic resonator (‘crystal”) on the Raduino. The v5 board shifts the IF frequency so as to avoid this problem.
- The µBITx has no AGC. Several designs for AGC add ons are already available. The µBITx also has no RF gain control, although this can be easily added later.
- The Raduino in the µBitx comes with only one spare analogue port. All other other Raduino ports are used in switching in LPFs, T/R, encoder control, etc. However, there are workarounds to reclaim ports for other purposes (by multiplexing inputs, changing out the processor, moving to an I2C display or using an I2C port expander).
- The manufacturer’s sketch shipped with the v3 board contained some minor bugs. In particular, setting the CW sidetone didn’t work, and some found the keyer didn’t always key correctly. These were the result of minor software issues and are easily addressable with software upgrades. Within 2 months of the release of the µBITx , several alternative software/ hardware upgrades were available (compare options here)! The v4 board also came with updated firmware that addressed these issues.
- A small number of constructors experienced failure of the audio amplifier (TDA2822) in the v3 version. There were a couple of causes: 1) output shorts created by using a mono plug in the stereo jack, and 2) Chip failure with the WX brand chips (that are probably not intended for 12v operation). The WX chips should be replaced. Several solutions are possible (see fix here). Some owners of v4 main board have found distorted audio (see the fix here) while other owners were disappointed in the level of audio output. The v5 board reintroduced the LM386 from the BITx40 that proceeded the µBITx, and has adequate audio output.