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Author Topic: micro 80 circuit  (Read 9565 times)

Posts: 7

« on: December 05, 2001, 03:16:01 PM »

I am building the micro 80 qrp transceiver by oleg borodin RV3GM.

I have all the components, but there is know information on the coil L1 on my circuit diagram.

Please could anyone advise me where to start with the number of turns and where to put the tap on the coil.

any info much appreciated



Posts: 18535

« Reply #1 on: December 11, 2001, 12:31:12 PM »

I believe this rig is similar to the "Pixie" transceivers.  (The latter
use an LM386 for the audio stages.)  You may be able to do a
web site search and find a schematic for it.  If the rig you find is
for 40m, you will have to scale the coil size down to 80m.
(The AL7FS site may have a schematic.)

In this case, there is a 680pf capacitor across the coil, so, by
my calculations (since f = 1/2*pi*sqrt(L * C )  ) about 3 uH.
It would be useful to have some variable element in the circuit
for adjustment, either by making part of the capacitor variable, or
by winding the coil on s slug-tuned former.  (I've successfully
rewound IF transformers, but it takes very keen eyesight to work
with the fine wire, and the resulting inductance is not predictable.
However, you can wind a coil and adjust the number of turns for
the best output.

For the tap, start around the center of the coil.  The position of the
tap will determine the amount of power output you get from the
transmitter.  I'd use a toroid and experiment with tapping the coil
at different turns, using the one which seemed to give the best
performance both on transmit and receive.

Good luck! - Dale WB6BYU

Posts: 7

« Reply #2 on: December 16, 2001, 07:33:47 AM »


Thank you for the info. I was hoping someone may have already built the micro 80 transceiver with the coil data, as i believe it is the simplest transceiver circuit.

I believe a 3uH inductor wound on a T50-2 toroid would be 25 turns with 26 swg.

Would you recomend a tap about every 5 turns?


Posts: 18535

« Reply #3 on: December 18, 2001, 08:13:32 AM »

Actually, I think the "Pixie" can claim a few fewer components
because it uses an IC audio stage, but either can make a simple
and compact rig.  I've talked to at least one "Pixie" on the air over
a distance of 500km on 40m.  (And one of the QRP clubs was
offering a surface mount version, the "smite", which was about the
size of a credit card and would fit in your wallet!)

I don't have my toroid info here, but your numbers look about
correct.  For myself, I would take the pragmatic approach to finding
the right coil values.  An extra 10cm of wire on each end of the
coil won't make much difference on 80m, so you can wind a few
extra turns and unwind the wire one turn at a time for testing.  Just
resolder the end each time, without bothering to snip off the extra
length.  This should get you close enough.  (Actually, I'd probably
use 470pf fixed in parallel with a 365pf variable for the tuning
capacitor, since it is easier to trim the capacitance for resonance
than unwinding turns on the coil.  At some point, anyway, you
should be able to make the circuit resonant at the output.)

To find the tap, you could tap the coil every 2 or 3 turns throughout
the middle third of the coil.  Might be easier to arrange for the
circuit using two separate windings, instead of a single winding
with a tap - this allows you to adjust the turns ratio independently
of the tuning.  If nothing else, put the tap in the middle of the coil
and allow extra wire on each end:  you can then add a couple
turns at one end and unwind them from the other to move the
effective tap point.

But start with the tap in the middle of the coil and see how much
power output you get.

Posts: 18535

« Reply #4 on: January 04, 2002, 11:30:55 AM »

David -

I found another circuit variation in the RSGB Technical Topics
Scrapbook (probably appeared in RadCom around 1996??)
which uses a different input circuit.  I can look up the details
for you if you wish.  (I think one of the coils was still unlabeled,
but should be easy to determine.)

Let me know if you need the reference.  You can contact me
directly at:

- Dale
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