What is it about amateur radio ops that they feel they can dump all over other constructor’s efforts or ideas? I am sure it happens in your local amateur radio club, in your national association and in every email reflector and facebook group. The IO Groups BITX20 list seems to have one or two trolls at present.
Email groups are a perfect opportunity for trolling activity. It’s best not to respond. Complain to the moderators instead. If somebody is putting down their competitor’s product offering, what could be worse? Don’t support their product.
uBITx.net is not affiliated with the moderators of the BITX20 email list or HF Signals. The views on this website are those of the site owner.
Jim AB7VF suggests that much of the harmonic cruft in the µBITx is from DC current flowing through the L7 and L8 ferrites that effectively lowers the inductance as current increases and allows RF to go everywhere.
Jim replaced the electrolytics as can be seen from the picture ..
He suggests that the transformer is effectively a centre tap. He wanted tighter coupling between the two windings …. The electromagnetic field set up by the DC passing through the coil will bias the ferrite “magnetic domains” causing a shift in the B-H field resulting in loss of inductance and the generation of spurs .
When feeding the center tap – current flows up toward the “dot” or start of the top winding “left hand rule” will give you the polarity of the magnetic field around the top coil … current also flows through the bottom coil away from the dot or start of the bottom winding creating a magnetic field opposing the one created by the top coil. The net result is no magnetic field to bias the little bitty magnets in the ferrite allowing the inductance to remain the same as without the current flow.
The following photo shows the 80 meter output of Jim’s unit after doing the L7,L8 mod and the output transformer mod.
Jim suggests putting a proper inductor on the IRF510’s that is NOT affected by DC current flowing through it and you will get legal output on 80m CW.
uBITx.net would be interested in whether this approach works for others in cleaning up the harmonics, because it will be a lot cheaper and easier than sorting the LPFs.
Howard WB2VXW previously mentioned that he would try to reduce the harmonics by changing the output to 25 ohms and adding a step up transformer to go back to 50 ohms for the output. This would allow tripling the values of the capacitors at the input and output of the filters, reducing the effect of the stray relay and layout capacitance.
In the end after more simulation, Howard decided just to change the filter characteristics and retain the 50 ohms termination impedance.
Howard was able to come up with a filter that doubles the capacitance. At least on 80 meters, with this new filter design, the harmonics don’t exceed -45 dB in his tests. Not as much margin as he would like, but legal.
Howard changed the 3 inductors to 1.6 uH by adding 2 turns on each. He added an extra 1000 pF cap in parallel with the input and output caps, bringing the total to 2000 pF. He also added a 620 pF cap in parallel with the one of the two paralleled 1000 pF caps in the middle two sections for a total of 2620 pF. (750 pF would have been better, but he didn’t have one).
Howard is asking others to give this ago. After validation of the 80m LPF redesign he plans to try a similar solution for the 40 meter band.
George UR4CRG/RX3ARG suggests some mods to improve RX sensitivity:
- Add a parallel capacitor to R12. Try C=470 pF. George found in his signal analyser that this gave a 3 dB rise in gain at 30 MHz. He placed an 0805 SMD (or your could use a 1208 SMD part) over R12.
- Add two 100 Ohm resistors to the collectors of Q11 and Q12. You will need to cut the PCB tracks to add in the resistors. The emitter-followers do not like HF signals and a small collector resistor gives light feed back
After these modifications, reception was noticeably more sensitive/
W0EB’s uBITX is built with a 5″ RA8875 type Color TFT display connected to a BITeensio Card. This gives an “uncluttered” display.
This version of the BITeensio TSW firmware is almost ready for release. It is just awaiting completion of the accompanying instruction manuals and the arrival of a small adapter board for connection to the display. The adapter boards will be supplied in the BITeensio kits upon request.
This display, like the previous 2.8″ Colour touch panel display and the 4 Line I2C display (2004) are capable of utilising an optional external USB Keyboard for rig control and CW. The W2CTX RCP (Remote Control Program) works with this as well.
For information and updates see the TSW website.
George Ellis has a fully transparent µBITx. Nice!
This is Kees K5BCQ 6x LPF/BPF board showing one LPF plug-in. The LPF parts were removed from the uBITX and moved to the plug-in board. This is easy with a hot air rework station like the ones all over eBay for about US$50. The relay removal is also “relatively ” easy with no pulled pads (using the large square tip which heats 4 pins at a time).
The plug-in footprint is the same as QRP Labs uses on their LPFs/BPFs. Kees mounted the pins on the other side of the board because it reduces the overall height. You can solder the coax to the SMA connector footprint or use SMA connectors. Many options.
The photo below shows the four transplanted filters from the main µBITx board and the main board LPF corner that is now stripped bare.
John VK2ETA some time ago provided information about his experiments with the SSM2167 module available on eBay.com and Aliexpress.com for a few dollars. The following shows the wire up diagram:
Picture of 4 pin molex connector added to the V3 Raduino for pickup of T/R (D7), +5V, I2C SDA, I2C SLD.
Note that the Arduino was replaced and put on headers as John uses a remote LCD display.
Andy RW9RN has developed a wee circuit for insertion in the audio line to provide variable bandwidth control for CW and SSB.
He has a video about it here.