Hi Carl!
One more quick question:
What kind of modulation transformer did you
use on that successful test you had?
This is great stuff!
Bruce, MICRO1690/1700
Hi MICRO1700 and Hello to your Wife and everybody you see on your upcoming trip.
The transformer I have in the circuit is the same as the one in the Pixie2 Illustration, and is Radio Shack Audio Output Transformer Part # 273-1380 ($2.49), 1,000 ohm primary/8 ohm secondary. It is confusing that the Pixie2 Data Sheet advises against using a miniature transformer, which this is. But since it works, who's complaining.
In my first hunt I couldn't find any larger transformers in the catalogs with the correct ratings.
Thank's Carl for your info.
I intend to have this thing built as soon as
I can. Maybe next week.
I have to be careful. Because of my vision
situation it is really easy for me to mess things
up. I have to use LOTS of light and magnification
and go real slow. I was able to build an SS-Tran
AMT-3000 a few years ago, which works fine, but
it took several months to do that.
My intention is to put the 13 MHz transmitter in
the attic for starters, with a dipole and attenuator
which will be built to get down to the
proper Part 15.225 field strength. I have a
wonderful Sony ICF-2010 shortwave portable
that I bought in 1991 that still works very well.
I guess the first tests will be just with me
walking around the neighborhood. Hmmm.
Maybe I'll use a smaller receiver while I'm
walking my dog. I don't want to attract too
much attention...
Again, I don't want to sound like a broken
record, but I have learned a lot from you
and Neil and have enjoyed it.
Thanks again!
Bruce, MICRO1690/1700
P.S. Say Hi to everybody out your way, too,
from me!
Hi Carl, Neil, and everybody!
I just got a modulation transformer. It's
1200 ohms/3.2 ohms. It will either work
or it won't.
I will finish the transmitter as fast as I can.
Best Wishes!
Bruce, MICRO1690/1700
Your transformer is in the ball park, MICRO1700, and I expect it to do the job. What are you using as an audio (modulation) amplifier?
Hey, last night I had an active brain and borrowed a few ideas from schematics that I found, including a manual linked on this website titled: CARRIER CURRENT TECHNIQUES by Ernest G. Wilson, which has a very interesting section on building Part 15 AM transmitters, and the modulation methods are interesting.
I ended up drawing about 5 alternate modulation possibilities, including several that are transformerless! I will test some of them and if they work I'll post the details.
Hi Carl!
You know, it's funny, but I don't know
what I'm going to use to drive the
modulation transformer. Not yet, anyway.
I think it's great you're looking at those
other modulation schemes. Any information
you find will be very useful.
I found your website. It's great!
I'll be back with an update as soon as I can.
Bruce, MICRO1690/1700
Inspiration came with the dawn and I tried direct (transformerless) modulation of the Pixie2. The results are extremely pleasing and I prefer it over the previous transformer modulation which also worked.
MICRO1700, you might want to consider the audio amp I described earlier in this thread, which seems ideal with this transmitter. It's a Velleman Kit available from Jameco and other suppliers.
I will describe the changes which accomplish this
Replace R3 with 10k resistor;
Replace L1 with 10k resistor;
Replace R4 with 330 ohm resistor;
Add a capacitor across R4 .2 uF (I had a .1 which is working fine);
Connect the ground from the audio amp to the ground of the Pixie2;
Connect the positive audio line from the Velleman Amp directly to the emitter of Q2;
If you use another amplifier you may need to feed it through a 100uF/50v capacitor with the + (positive) side at the emitter of Q2.
(The Velleman already drives audio by way of an electrolytic, there's no point in having two of them)
This modulation idea comes from a circuit on Page 14 of James R. Cunningham's LOW POWER BROADCASTING, which describes an interesting circuit for a 100 mW Part 15 AM transmitter.
If you search "Cunningham" on this website you'll come up with a lot of stuff.
Further comment: with only a 50 ohm load on the output of Pixie2 I'm hearing the signal everywhere in the house. For some reason the radiation of the transformer version didn't seem that strong.
I just walked uphill outdoors about 150 feet and the signal was there the whole time!! It's actually working better with no antenna than the medium wave transmitters which have antennas!
Analyzing further, running the Pixie2 with only a 50 ohm load, there tends to be a buzz on the signal within the building, but outdoors the buzz is gone and only the modulation is heard. Moving around with the portable radio inside the building there are buzz free spots. Who has a theory on what's happening?
The 2nd harmonic of 13.56 mHz is 27.12 mHz, directly between Citizens Band channels 13 & 14, which occupy 27.115 and 27.125 mHz, respectively. Using the Sangean receiver which goes way up to 29.999 in 5 kHz increments, the 2nd harmonic from Pixie2 seems about the same as the main signal, but I will later test this at a greater distance.
Then I need to hook up the CB transceiver to see if this 2nd harmonic bleeds onto channels 13 & 14.
At question is the filter circuit and whether it needs to be changed for better harmonic suppression.
The buzz I referred to does appear on the outdoor reception after all, but both inside and outside there are spots where the buzz vanishes but the modulation is clear.
The 27.120 mHz 2nd harmonic is as strong as the main signal as received 150 feet away on only a dummy load, therefore the lowpass filter isn't sufficient. It consists of three inductors in series with the RF (1 uH, 1.5 uH, 1 uH) with two 270 pF caps to ground from the two junctions within that string. I'll guess that the inductors don't need to be changed, only the caps.
Using the Sangean tuned to CB channels 13 and 14 it is found that at least in the near field the signal does splash onto those channels.
There don't seem to be any spurious emissions outside of the harmonics. The 3rd harmonic will be up at 54.24 mHz, which I haven't checked yet. That's in VHF TV range I think.
The buzz was cured simply by grounding the power transformer.
Now I'm running on an antenna, simply a straight wire antenna 1/2 wavelength strung from room to room indoors. In house it comes in like a professional shortwave station and the same outdoors, where is covers the yard much better than our AM or FM transmitters.
I'm worried about that strong harmonic on 27.120 mHz, but will start trying capacitors on the filter until I attenuate it.
Then all the parts need to be mounted in their case and installed with a good outdoor antenna and we'll start checking the distance reception.
Hi Carl!
WOW!!! Very encouraging! I will probably
reread your posts a bunch of times and
then redraw the schematic and put in
the audio modifications.
It will take longer than I originally thought
because of other things that are going on
here. I also will have to build the attenuator
circuit to bring the RF output down to Part 15.225
specs.
Your Velleman amp has a four ohm output, is
that right? With the transmitter in it's
modified state, how are you feeding the
Velleman into the transmitter at Q2? Do you
have any resistors across the Velleman's output?
(I hope I'm asking the right question here.)
This is great stuff, Carl!
Best Regards!
Bruce, MICRO1690/1700
I'll describe the audio hookup that is working.
My audio feed comes from the output jack of a C.Crane Radio Plus tuned to KDX-FM at 101.9 mHz. The next stage is a 10k variable resistor which feeds a step-up transformer 600/50,000 ohms, to the input of Velleman amp.
The Velleman output is designed to drive a speaker as low as 3.2 ohms, but is in fact a very low-impedance output which can drive any higher impedance. The connection from the Velleman to the base of Q2 is a straight wire with no other parts. The amp does not need to be "loaded" with 3.2 ohms.
I've been running it all day and using it as the monitor for our streaming station and it sounds very good.
By the way, I have two power supplies, both 1-amp, and plan to get a single 2.5 amp power supply for the job.
I suppose the lil' transmitter is designed for a 50 to 75 ohm antenna system.
You might want to try cutting that 1/2 wave antenna in half and feeding it with coax, 1/4 wave to the center conductor and 1/4 wave to the shield.
That would present a better match for the transmitter. Even better if you use a balun at the feed point. Although my simple dipole wire antenna always worked well without a balun.
With only 1/4 watt on the 40 meter (7 mHz) ham band I worked a Florida station from here in northern Ohio. The antenna was horizontal and only about 25 feet high.
Thank you MRAM for the ideas, and I will use your pattern when I build an outdoor antenna.
Upon second measure my indoor antenna is 1/4 wavelength, since there's not all that much room in here, but the local coverage is great. I haven't walked any distance to see where it fades out.
I'll start without a balun and if there's any desire later on to experiment I'll try a balun.
Having expressed concern about the 2nd harmonic at 27.12 mHz between CB channels 13 & 14, I just skimmed over some of the previous postings in this thread, and noticed reference to legal Part 15 operation between 26.96 & 27.28 mHz right in the CB band, although with lower power allowance than the 13.56 mHz channel. Well, what if the filter padded the 27.12 just enough to meet the Rules, then the one transmitter would become two transmitters!