ΜBITX

Notes on µBITX assembly and use.

Assembly

Parts that took some time to decide on approach to:

• Case: used a cookie tin from Goodwill for $3 as a case.
• Mic assembly: TODO

Antenna setup

20m dipole

Inverted V dipole made with Amateur Radio Supplies 20m dipole antenna wire (two 18' strands, trimmed, wrapped around insulators, strung up in attic) and 450Ω window line.

• Originally was sent two 16ft strands, which is really too short. Customer service was great and sent me the 18ft strands and a prepaid return envelope for the 16ft strands.

QRPGuys Multi Z Tuner

• See my notes on assembly for the Multi Z Tuner

QRPGuys 12W dummy load

• Still no luck getting power measurements working.
• dummy load works when diode is removed (??)

Items from QRPGuys took about 2 weeks from date of order to arrive.

7-band OCF dipole

Ordered 7-band OCF dipole from gniklat1 on eBay.

Mods perfomed

Fuse before diode

Applied this fix during initial assembly.

The diagram there appears to be labelled incorrectly, though.

Have blown the fuse twice (first time forget, second time transmitting on 40m with untuned antenna) quite a few times, so it seems to work.

Mitigating heat on LM7805

I started by adding a small 5V switching power supply before LM7805. This created a ton of noise on RX, so I took it out.

Instead, I relocated LM7805 to case (up against case and screwed to it). The first time I got the leads switched, though, and smoked the nano, so had to replace that.

This arrangement seems to dissipate heat nicely.

I also have a heatsink in some old electronics that I've been meaning to try out too.

Dim the LCD

As documented here. At first it had no effect, but then found I hadn't cut the trace between the two leads first, which I did 2018-09-06.

CEC firmware

After getting some extra nanos, I flashed one with CEC firmware. Works great!

TX LED

Soldered up red LED per BITX20 message 46853 on 2018-09-06. Works great!

Leveling out power

Replaced some of the transistors (etc.) per this solution:

• Replace the 2N3904 transistors in driver stages (Q911, Q912, Q92, Q93, Q96, Q97) with 2n2222 transistors (in TO18 metal can),
• Parallel up R941, R911, R96, and R942 to get 11 ohms each by placing a 22 ohm resistor across each existing resistor (stacked 1206),
• Replace Q90 with a BFR106 transistor,
• Increase R81 to 2.7K,
• Change C81 to 470 pF.

Reducing spurs

• Replaced L5 and L7

Reducing harmonics

• Replaced relays with Axicom D2n 12v relays (model V23105-A5403-A201)

Power measurements

• Before and after the 3 mods above (levelling out power, reducing spurs, reducing harmonics)

Mods planned

• SWR meter (rx)
• Red-on-black or amber-on-black display (for night-time operation)
  • And/or a 6-8 digit LED display
• More buttons - I like PH2LB's approach (part 2)!
  • band up, band down, left/right digit selection for tuning, ...?
• Foot-pedal controlled TX
  • Have obtained an old (working) foot pedal switch for this purpose (edit: now have two somehow?)
• Some sort of audio pre-processing?
• Fix filters??
• line out
  • Again, PH2LB's approach is great.

Other things on wish list

• external SWR + power meter
  • YL got me a DAIWA CN-501H for my birthday, 2021-03
• Antenna analyser / antuino