Home > Ham Radio > Building Bitx part-1

Building Bitx part-1

May 30th, 2008

   Bitx 20 is a cute QRP transceiver for 20m. The original Bitx 20 was developed by Indian engineer Ashar Frahan. The details of the circuit and other details  are available at his website. There is an active yahoo group for bitx builders and  there are several mods and add-ons  developed by bitx enthusiast all over the world.

  I am already having a Bitx20 and I have made contacts  with several European and Russian  amateurs on 20m.  I have been  thinking about  building a  modified version  of  bitx for 40m  for quite some time now.  Now that I am on vacation, I have started working on this project last night.   I am following the version 3  layout as  prepared by VU2WJM OM Rahul for this.   The diagram is here.

I will describe my progress  along as i build here.

   For converting the Bitx 20 to 40m two modifications are to be done. The band pass filter must be redesigned for 40m and the VFO must oscillate around 17MHz.  I am planning to get the transceiver working for 20m and then replace the VFO and band pass filter.

I am following ugly construction method for this project.  In ugly construction a plain copper clad sheet is used as a ground plane. All components having connection to ground are soldered on to the copper clad sheet. All other connection are between component leads and not supported on the board.

The  following  photographs show my  progress so far

  Bitx_microphone_amplifier

  The  microphone amplifier.

This stage was  built first and tested  by injecting a 50mv 1Khz signal from a signal generator. The stage was giving a gain of nearly 10.

bitx_Beat_frequency_oscillator

Bitx beat frequency oscillator

I built the above stage next  and  powered up. The waveform was observed on a CRO and frequency measured with a frequency counter. I was able to vary the oscillator frequency about 20khz by turning the trimmer.

The next step was to wind the trifilar transformer for the mixer.  The details are shown below.

  

 bitx_trifilar_transformer_winding_1

Take approximately 105cm of 29 SWG enameled copper wire. Fold them into three as shown above.

bitx_trifilar_transformer_winding_2

Twist  the wires together.

bitx_trifilar_transformer

Wind 13 turns into a toroid. You can use a TV balun . In that case choose  smaller wire  diameter( something like 32SWG).  Cut the ends of the  wires and isolate the three windings. Identify the ends of the windings with a multimeter and mark the windings with marker pen. (You need at least 2 colors). Now the transformer can be installed. Be careful about the polarity of the windings.

  Bitx_mixer_installed

The bitx mixer installed on the board.

  I will describe further progress later

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  1. N4DDP don
    June 2nd, 2010 at 18:01 | #1

    Why not use a 2.7 – 3 mhz VFO (what I am trying with my bitx?)
    I redid the board (up to the mixer) to use almost all SMT components.
    I just got my VFO working well.

    I am stuck on the bandpass and after and am looking for ideas.

  2. vu2swx
    June 3rd, 2010 at 09:15 | #2

    I built a second bitx with 9mhz if and vfo from 1.9 to 2 Mhz . Working perfectly on 40m.
    Used 4.7 micro henry moulded inductor in parallel with 100pf and 22pf trimmer for band pass filter. If there are sufficient interest I will post schematic and pictures.

  3. October 15th, 2010 at 20:00 | #3

    Dear friend,
    I made two bitx ,one for 20 mt and one for 40 m,but I need the dada for the coils for the filter and for tha lowpass filter on power amplifier.
    Could you help me in this?I can’t find any data on the web.
    Thanks a lot.
    Ciro IK6AIZ

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