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Author Topic: Pixie Mods  (Read 5156 times)
KI4JGT
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Posts: 114




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« on: November 08, 2010, 03:51:48 AM »

Hey guys, I was wondering if it were possible to modify this.

http://www.indianapolis.net/QRPp-I/KA8MAV_Pixie2.html

To include:

An internal Antenna
A solar panel for power
A button on the top for a keying device?

Or if your guys have heard of a CW transceiver of this size with the entire package. Thanks 73
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KE3WD
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Posts: 5694




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« Reply #1 on: November 08, 2010, 05:01:40 AM »

Internal antenna on the HF bands? 

Not likely for signal to get out of the box...


73
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KI4JGT
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Posts: 114




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« Reply #2 on: November 08, 2010, 06:05:02 AM »

What about a size efficient external one? Like about the size of the box itself or a little bigger?
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KI4JGT
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Posts: 114




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« Reply #3 on: November 08, 2010, 07:30:36 AM »

Can we not twist and glue a wire antenna around the box? I don't want range as much as communication. 10 or so miles is fine.
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W5FYI
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Posts: 1044




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« Reply #4 on: November 08, 2010, 07:43:12 AM »

A 50-ohm 1-watt resistor makes a good internal antenna; SWR will be perfect, but expect limited range. Solar power can be useful alone, but it would be better to use a solar panel to recharge an internal battery. Typical 9-volt rechargables are really 7.2-volt batteries, so power output will be reduced somewhat from the typical 12-volt supply or 13.8-volt battery.

A small microswitch will work for a key. The kind with a lever is even better, and you may want to play with various models to find one that feels and sounds right.

Pixies are fun rigs. GL
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W3LK
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« Reply #5 on: November 08, 2010, 09:14:52 AM »

Can we not twist and glue a wire antenna around the box? I don't want range as much as communication. 10 or so miles is fine.

If you can find a way to rewrite the laws of physics ... maybe.

The range of a QRP rig like the Pixie with an internal antenna (if you could make one fit) or wrapped around the box will be measured in yards, not miles.
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
WB6BYU
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Posts: 13113




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« Reply #6 on: November 08, 2010, 10:07:37 AM »

If you just want a few miles I'd look at building a 6m version instead, since the antenna would be smaller.

You could use a solar panel, but it has to provide enough current to power the radio.  That's why
the suggestion was made to use a smaller solar cell to keep a battery charged - the battery
provides the peak current when needed.

A button for keying is no problem - there are a number of types that have a good action and aren't
too difficult to use.  You could even mount two buttons on the case and include a keyer chip.

Built-in antenna is a bit of a problem - a half wave dipole for 80m is about 125' long, so a lot of
shortening is required.  You could fit a RECEIVE antenna inside, or at least use a short whip
with an internal amplifier, but it won't work well on transmit.  If you're not in a position to drape
a wire over a tree branch, you might consider a loop antenna of some sort.  For example, start
with a couple of 1x2s nailed in a cross shape and wind a few turns of heavy wire in a spiral shape
towards the outside of the frame.  Stick a variable capacitor across the ends of the winding to
tune it to resonance.  Add one more turn (likely smaller than the others) that connects to the
antenna.  You'll have to experiment to see how much power you can get to radiate, but it might
work.  Higher frequencies require smaller antennas, but make it harder to cover distances as short
as 10 miles until you get close to VHF.

Actually you can build the transceiver even smaller - there was a version called the Smite that
used surface mount parts and was about the size and shape of a credit card, and it could be
shrunk even further.  But the practical limits are the size of the battery required to generate
enough output power, and the size of the antenna.


Also remember that there are two propagation modes that work on 80m:  ground wave and
sky wave.  Ground wave requires vertical polarization and depends on output power (due to
the losses as the wave travels over the ground.)  Maximum coverage is perhaps 40 to 50
miles running 1kW, and about 20 miles at 100W.  This might work for a couple of miles with
the Pixie under perfect conditions and with an efficient vertical antenna, but not much more.
Sky wave involves reflection off the ionosphere, so the signal has to travel much more than
10 miles to get to the other station.  In fact there is little difference in total distance that
the wave travels between 10 miles and 100 miles - it is still basically up to the ionosphere and
back down again.  So until you are close enough to hear the ground wave signal, you might
as well be 100 miles away as far as propagation and signal strength are concerned.

[edited to add the last paragraph]
« Last Edit: November 08, 2010, 10:14:46 AM by Dale Hunt » Logged
KB4QAA
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Posts: 2324




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« Reply #7 on: November 10, 2010, 09:54:08 PM »

Keep in mind about power, that the Pixie II is quite sensitive to lowered voltage.  You need to keep a firm 9volts or else the LM386 will start motorboating and keying action will become unrealiable.
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KI4JGT
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Posts: 114




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« Reply #8 on: November 12, 2010, 06:33:37 PM »

I was wanting a pocket transceiver for CW. That's why I was wanting an internal antenna. One which I can pull out of my pocket somewhere and sit on the table as I use the key on the side of the box.
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WB6BYU
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Posts: 13113




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« Reply #9 on: November 12, 2010, 07:05:37 PM »

It's no problem building a pocket-sized transceiver - I've seen commercial multi-band HF SSB HTs.
With some work you can even fit a battery inside.  But the problem is the antenna efficiency:  the
smaller you make the antenna, the higher power you need to run for the other person to hear you,
so the larger battery you need. 

If you want to talk from one room to another, perhaps over a few hundred yards, you probably
can manage with a small loop antenna (perhaps even a ferrite loopstick to save space, again
at the cost of efficiency.)  If you need better range with a small antenna, move up to the VHF
range where full-sized antennas are shorter:  a 54" telescoping whip on the back of a radio
works far better at 6m than it does for 80m (though there may not be many stations to work.)
If you have a friend who you want to work, you can make a pair of sets and talk to each other.

When I was a teenager I built a single transistor transmitter that probably ran as much power as
the Pixie and mounted it on my bicycle along with a CB whip antenna - I think I was heard 1/4 mile
away.

But the best thing to do is to build a Pixie (or some other small transmitter/transceiver) and try it
out to see how much range you can get.  My suggestion would be to use a simple transmitter like
this one (which is one of my favorites) and try it out while listening on a standard receiver.  Load it
into various sorts of small antennas and see what sort of range you get.  Try various loop antennas
(you can connect them directly to the collector of the output stage if they are sufficiently selective
and have a DC path through them.  You may be able to rewind the loading coil on a mag-mount CB
antenna, then replace the top stinger with a telescoping whip.  Measure the relative signal levels
with a wire laying on the floor vs. wrapped around the box.  You may find a combination that works
for your needs, or you may come up with some other creative solution.  Once you've found an antenna
that works there are various transceiver circuits that you can build to go with it - consider it a modular
 project, where you can keep improving different pieces.  Perhaps you can design a rig that operates
on 440 MHz that is linked to a home HF station instead, which would allow you to key 1000 watts on
40m with a full-sized antenna from a tiny box in your pocket.

But the first step is to build something simple and start experimenting.  You never know where it will
take you from there.

Good luck!

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K4FH
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Posts: 52




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« Reply #10 on: November 15, 2010, 05:25:06 PM »

I need to debug my 40M Pixie II and hopefully someone can give me pointers on where to use the RF probe.

My Pixie II motorboats.  There is no noise.  I can't even tell if it is oscillating or not.  Behavior is the same with or without a crystal.  If I key it, it does go into TX mode.

Chris
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W5FYI
Member

Posts: 1044




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« Reply #11 on: November 15, 2010, 06:31:43 PM »

Motorboating is generally caused by low voltage. Try a beefier battery, or boost the voltage up a volt or three and see what happens. You might also want to experiment with the value of the resistor feeding the audio amp. GL
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WB6BYU
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Posts: 13113




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« Reply #12 on: November 16, 2010, 08:39:32 AM »

I've had a lot of problems with the LM386 motorboating in other circuits as well.  I found that I need a
good stiff power supply for it - the problem is worse with carbon-zinc batteries than with Alkaline ones
due to the higher internal resistance.
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WB6THE
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Posts: 128




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« Reply #13 on: November 16, 2010, 06:42:24 PM »

I think the PIXIE kit provides an LM386-N integrated circuit.
I have a couple of PIXIEs lying around, I built one and it was
horrible. Only wanted to motorboat.

Go find an LM386-L version of the LM386.

I've done some tinkering and I just can't make an LN386-N
work properly but using exactly the same circuitry the
LM386-L works perfectly.

The little Radio Shack Mini Amplified speaker uses a dash L
version of the LM386 and is perfectly stable.

So try an LM386-L version in your PIXIE vs the -N. I've not tried
it but I think it might work better.

I got my dash L  versions on Ebay.

Alan
WB6THE


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KB4QAA
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Posts: 2324




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« Reply #14 on: November 17, 2010, 06:58:51 PM »

There are a number of discussions on the web that provide solutions for improving (i.e. fixing) the Pixie II.   Besides ensuring you have steady 9v, and/or swapping IC's another recommended fix is to add some bypass capacitors to the the LM386....nothing magic, just good engineering design 101 that got overlooked when the circuit was slapped together.  If you can't find it on the web, then take a look at the spec sheet for proper installation.

Also, triple check your Inductor specifications and that they are appropriate for your intended band.   There are at least 3 schematics floating around, some with hand changes, and some have been noted to be incorrect.   The sheet I got from (HEB?) in Mountain View has one thing on the schematic and then a list to the side with at least one variance.  fun.  Smiley
« Last Edit: November 17, 2010, 07:02:36 PM by BILL MERRITT » Logged
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