A few thoughts and observations re. BITX-40v3
|
The BITX20 discussion group is now growing at a speed that equals what it
did in the early days of BITX20 design and construction.? I am seeing 5 to
20 new members added almost every day.? Most of this growth is due to
availability of the new BITX-40v3 from .
From some of the questions being posted it seems that several members may
be trying to build their own BITX-40 from the schematic provided by Farhan at
HFSigs.com.? This is commendable and a worthwhile project.? To provide some
information and answers for those builders... - All the inductors on the BITX-40v3 are wound on T37-2 or T37-6 cores.
Inductance for these is clearly labeled on the schematic.? Using the
stated inductance you can use the calculator at to find
how many turns and length of the wire to wind your own equivalent
inductors.
- All the transformers in the BITX-40v3 are wound on FT37-43 cores.?
These are broadband transmission-line transformers.? Windings are
trifilar using 10 turns of twisted together wire.? These do not have any
inductance shown on the schematic because they are effectively RFCs
(Radio Frequency Chokes) with close to unity coupling factor over most
of the HF range.? This applies to mixers and transformers in the RF PA
section.? If you really MUST have an inductance value for these transformers
you can go to and enter 10 turns for an FT37-43 core and
see that the inductance is 35 uh.? At 7 MHz this is an impedance of 1539?
ohms or way above the approximately 50 ohms for circuits that they are
coupling together.? The calculator is also useful because it
tells you that your 10-turn winding will take approximately 7 inches or 17.8
CM for each of the three wires you use to make up the trifilar winding.
- The BITX-40v3 is rather unique in that it has no tunable stages, other
than the VFO.? The two BPF (Band Pass Filters) are broad enough to
cover the full 7.0 to 7.3 MHz band.? The mixer transformers and those
in the RF PA section are broad banded enough to cover most of the
HF range.
- As with all BITX designs, it is imperative that you use an antenna that
presents an impedance close to 50 ohms at the transceiver antenna
connection.? Too low an impedance and the RF PA transistor may draw
excessive current and overheat.? Too high an impedance and the RF PA
transistor may go into oscillation and destroy itself.
- Some of the problems thus far discussed regarding commercially built BITX-40v3
boards seem to result from tinned leads on the FT47-43 transformers becoming
shorting together.? The leads are close together and the tinned part is sometimes
longer than necessary.? If the transformers have been bent over in shipping
or due to the board being dropped, it may be worthwhile to take a close
look at the transformer leads to insure that they are not shorted together.
- Transformers and inductors on the BITX-40v3 boards are not glued to the
PCB.? This provides easy means to modify the circuit to meet your particular
desires.? This is particularly useful if you need to change the VFO frequency
range.? After you are sure you have everything working the way you want you
can secure the VFO inductor, and other inductors and transformers, to the
PCB with melted wax, hot-melt glue, or other suitable adhesive.? Securing these
components to the board is not mandatory, but many may want to do it.
- It is possible to build SMT based circuits on cheap breadboards.? This
involves bridging gaps between pads with the SMT component and
connecting pads together with small diameter wire.
??? ? Cheap and frustrating to use
??? ? Better due to plated-thru holes
??? ? Wire for connecting components
SMT ICs and larger components can be mounted upside-down with
glue and connecting leads soldered to the exposed pins.
Others probably use different methods, but this seems to work for me.
- The original BITX-40v3 from uses a 270-degree potentiometer
to adjust VFO frequency.? There are a number of schemes to improve
frequency resolution.? Simplest is changing this potentiometer from the
single-turn unit to a 10-turn unit.? This potentiometer can be any value
between 10K and 100K.? It acts as a voltage divider to bias the varactor
(Voltage Variable Capacitor).? This potentiometer gets its voltage from U2
which is a 9 volt regulator.
Building your built-own from-scratch BITX is a fun and rewarding endeavor. Arv ?K7HKL _._
|
