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Tuesday, October 2, 2012

How to Get Ready to Build a BITX-20

Continuing with our workshop inspiration theme, I spotted this in the BITX-20 mailing list this morning.  The response is from Farhan: 


i would suggest a different route. a long and winding one, that will
finally lead to a bitx.

the idea is to learn. you do this by understanding what you build and
building what you understand. by 'understand', i specifically mean, being
able to measure. here is what i suggest, buy yourself a bunch of 2N3904s
from the local radio shack and some resistors and caps. then build this :


this is an oscillator. if you plug a coil between the open ends, it will
become a vfo, if u plug a crystal, it becomes a crystal oscillator. you can
use your frequency oscillator to check the frequency it is oscillating at,

with this, you would have mastered the first of the three blocks that make
up almost every radio circuit. but next, you must make another test
instrument. a power meter. most of us ham start out with a simple RF probe.
while that was fine and dandy for its day, now we can do much much better.
we can make a very accurate power meter that enable you to measure things
as finely as anybody in this business. W7ZOI has designed a super simple
power meter. it is available as a kit.
read about the power meter here :
you can purchase the kit at www.kangaus.com
(I have no business interest with kanga or any other kit manufacturer)

with the power meter in place, you can now measure the power levels coming
out of any circuit with great accuracy.

now, you can build a single stage feedback amplifer (there are six of them
used in the bitx) on a copper clad board. using the test oscillator as an
input, you can measure how much gain the amplifer has (measure the
oscillator output, then connect the oscillator to the amp and ,measure the
amp output. the, amp output - oscillator output = amp gain).

of course, while building both these blocks, you will discover what
voltages to expect at which junction of components in both these blocks.

next, you can build a step attenuator. which is a really simple thing and
of immense value in the home lab. here is a design
or you can now buy it in a kit form from

finally, you can build a simple signal generator like this :
http://www.phonestack.com/farhan/siggen.html  . this will allow you change
frequencies and measure what a circuit does at different frequencies. you
can use this to test how the filters are doing and get them to 'spot' where
you want them to.

so, there it is, a signal generator, a power meter, step attenuator, test
oscillator. four, very simple test instruments that you  can build
yourself. they will give you one helluva education in radio. and within
weeks, you will understand and start building on your own!!

- farhan

> Quoting bfabman :
> Hello Everyone, I have been watching the group for a few months now
> with interest. I have no electronic experience to speak of, but I have
> a burning desire to make one of these, and I am wondering what all of
> you think of someone like myself building one as my first real radio
> project, to be used for qrp mountain topping. I don't have any
> electronic test equipment for the final alignment, other than a
> standard DIG vom meter. (I am willing to buy some equipment if
> necessary) I think that this would be an awesome winter project just
> don't know if it would be over my head. If I got it all built, could I
> actually get it aligned and working properly. I did make a Norcal
> frequency counter project last year and it turned out very well. Thanks
> for your opinions before I spend the money. Blake
> Paul Daulton K5WMS
> beacon WMS 185.302 khz qrss30/slow 24/7
> Jacksonville,Ar 72076
> em34wu

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1 comment:

  1. Hello Paul,

    Farhan gives sage advice; build some simple test gear to use with your DVM. Build the BITX stage-by-stage, testing each one as you progress.

    By following that procedure you avoid the frustration of completely assembling the rig and then wondering why it doesn't work.

    Best of luck with the project and you know where to ask questions you may have along the way.

    73.......Steve Smith WB6TNL
    "Snort Rosin"


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