Well That's Always A Good Reason 🙂
Indoor Antennas will usually never work as good as they should. I'm sure once you get that antenna outside it will work much better.
Given the in-inefficiency of the antenna, I would not sweat over 3 milliwatts at 13 MHz. For example, a single circularly polarized antenna with 100 feet of coax, for FM broadcast is negative gain. A local LPFM has to run 277 watts into the antenna to get an E.R.P of 100 watts out.
So running 8 milliwatts as opposed to 5, is less than 3dB. Unless the FCC monitoring station is farther than 500 to 1000 miles under normal conditions, they won't even hear you. And if they did, I serously doubt they would pad over to your house to make a measurement. Now if you were running 100 watts, that would be a different story.
Hi Guys:
I just about finished my version of the
13 MHz transmitter. Carl, I am about
where you were 2 or 3 weeks ago.
Q2 doesn't seem to be doing much,
but my output network might be wrong.
(I'm using different parts so it's not
quite the same.)
Also, Carl, I'm not quite sure about your
schematic. About the modulation
transformer- does the low impedance
side go in the B plus part of the
transmitter and the high impedance
side connect to the audio source? Or do
I have it backwards?
Thanks everybody!
Bruce, MICRO1690/1700
Welcome another day MICRO1700 and everyone
The transformer is wired exactly like the original Pixie2, low impedance (4-ohm) is the audio input, high impedance (1k ohm) is the B+ side.
It may be unclear for you to observe, but the drawing of the transformer has more loops on the high impedance side.
If you don't mind taking the time, describe the parts you are using on Q2, especially at the base and emitter.
Hi Carl and Everybody:
Carl, the transmitter is exactly like yours until
the output network, right after C5. And thank
you for the transformer info.
From another website, I found the component
values for a Pi Net for 20 meters (right next
door to 13.560.) I don't know if they are
correct, but I wanted to try to fashion a Pi Net
from what I could get here. It is just a Pi
section and is not as elaborate as your five pole
output network. Right now, the output
is soldered to a 1/2 watt 50 ohm resister.
The literature specified that the caps be 270 pF
and the coil be .57 uH.
All Radio Shack had was 2 bags of 100 pF caps
2 in each bag. So I made 2 200 pF caps out
of what was there by paralleling 2 100 pF caps
twice. I hoped that a larger coil would make it
work, so I wound a coil with #22 wire on a pencil,
about 8 turns, which seems to be in the
ballpark, at about .7 uH. I can add more turns.
But, somehow all of this doesn't feel right. I
am also concerned about
the external field that coil puts out. Will it make
the transmitter unstable?
The transmitter has barely been built, so there
may be other problems. If it doesn't work, I'll
just keep trying other things. I did rush the
assembly job, with a big light and lots of
magnification to compensate for my lack of vision.
We'll see what happens.
Thanks again,
Bruce, MICRO1690/1700
Good Going MICRO 1700
If you could parallel a couple of 70 pF caps to the 200 pF set that you have, that would take care of that. Do you have the receivers needed to monitor the harmonics?
My circuit is on the air for yet another day, so we know it works, so yours should work too, I don't believe the slightly different output should matter very much.
My belief is that inductor coils do not radiate but are self-canceling because the relative phase of the signal is constantly opposing itself as it goes through the circle. Somebody will correct me if I am wrong.
See if you can read AC voltage on the output at C5 and also at the 50-ohm resistor.
Hi Carl and everybody:
Carl, I do have receivers. Hearing the
harmonics is no problem.
I think you said you are using a meter
to take your voltage readings. I have
seen the formulas on how to calculate
RF wattage. Does your meter give you
peak-to-peak voltage, like a scope? That
is one thing I did not understand. Do
you divide the reading by two, multiply
it by .707 to get RMS, and then plug it
into the formula? If you am REALLY far
off on this, don't worry about commenting,
I'll just keep reading what has been posted
here until I get it.
I am learning how much there is to know
about this stuff and I really like it.
Bruce, MICRO1690/1700
Dear Bruce MICRO1700
My AC voltmeter is a little $10 B&K Meter. It's probably not p-to-p
My better meter is broken.
My scope still hasn't been drafted into service.
I need to upscale the lab standards here.
For me, at least some voltage is a good sign.
A standard VOM isn't up to reading 13 Mhz voltage. You could build an rf probe that uses a diode right at the measuring point and then measure the rectified DC. Dave
Take the collector current, mutiply it times the collector voltage, thats the input power. Multiply the input power times 0.65, and that will get you into the ballpark.
I've inquired about something like this before, but this handy suggestion from wdcx makes it seem clear: could not a permanent milli-amp meter and volt meter be installed at the final collector of Part 15 transmitters for a constant view of key operation? This might prove very useful during an inspection.
By the way, during an inspection how does the inspector determine the input power to the final?
Hi Carl and everybody!
I'm getting closer! I got modulation!
But, it didn't sound very good. Then I
realized that the 9 volt battery I was
using to power the transmitter was
running down. (MAYBE that was the problem.)
I hooked up the variable supply. Since
my eyesight isn't good I misread the
voltmeter as I was ramping up the
voltage and blew up Q2. I think it
destroyed itself at about 11 volts.
So I'll replace Q2.
Here's an added experiment. With Q2
blown, I wondered if I could modulate
Q1 just for fun, because it was still
running. (Half a transmitter is better
than no transmitter at all!)
So I ran audio right to the Q1 emitter
(I think) and ground without a transformer.
I got great modulation. That was fun.
Then I tried to put an antenna on the
oscillator to make a one transistor AM
shortwave transmitter. I disconnected
some components after the Q1 output
coupling cap. Every time I tried to connect
an antenna wire the oscillator stopped
running. That was interesting. The
other interesting thing was that, at 9 volts,
the oscillator was drawing around 90 milliamps
from my supply. That's a lot of power for
a little oscillator that doesn't go anywhere.
Still, the oscillator was radiating a little bit.
I could get it around my house. (I don't have
a very big house.)
I feel like I'm 15 years old again because that's
pretty much the last time I did anything like this.
This is fun!
Bruce,MICRO1690/1700
MICRO1700
I love your flamboyant experimental approach! By the way, how are you measuring how many milliamps are being drawn?
Thanks Carl!
On my variable supply there is an
ammeter, which reads up to one amp,
and a voltmeter, which goes up to
35 volts. It's an old supply, but I
figure the meters are still pretty much
on the mark.
Experimenting with this transmitter is
getting me to read about all kinds of RF
literature.
Thanks for the kind words!
Bruce, MICRO1690/1700
MICRO1700 I've been thinking about the results you've reported, including the possibility that Q2 blew out simply by applying 11 volts. My original Q2 also blew out, and I was using the type specified in the Pixie2 diagram, 2N2222A (metal can type). There must be some reason that transistor is sensitive in this application. My replacement was simply an old spare part, an old now discontinued GE transistor I mentioned somewhere in this thread, but right now I have misplaced the exact description. I think ultimately we should try all the types mentioned in the original diagram, 2N3866 and the FET 2N7000.
Also, do you now have a Velleman audio amp, or are you using something else. Is your amp capable of driving the low 4 to 8 ohm transformer?