Setting the ouput power of Pixie2 to meet Part 15 requirements, two methods have been discussed including a designed attenuator or a circuit power control setting.
May I propose a 3rd option, simply shortening the antenna?
Hi Carl!
It's midnight here in Connecticut, and I just
thought I'd check to see if you had posted
anything. I see you have put down a lot
of info and I will reread it a bunch of times
tomorrow. Shortening the antenna sure would
be nice. It would be more convenient. I guess
as long as Q2 is happy it wouldn't matter.
We would have to determine the correct
field strength somehow, but I'm sure we could
figure it out.
Thank you for posting all the information on
what you have done. When my brain is
working tomorrow, I will look it over.
Best Wishes
Bruce, MICRO1690/1700
After much experimentation I have reached a working version of an AM Pixie2. I've posted a drawing and parts list here http://kdxradio.com/pixie2.html
This version is on the air, the audio is full bodied and undistorted, the signal covers the yard and it's been on the air for two whole days.
Please study it and give me your ideas about improving it.
There's still a lot of work to do. Is it within FCC specs? Don't know.
Thank you Carl:
I printed out your version 1.0 circuit of
the Part 15 13.560 MHz AM transmitter.
I intend to get a working version running
this week. My output filter will be different
from yours because of the parts I have
here, but it will be something (for now.)
I understand some of the changes you
made.
I will report my progress as soon as I can.
You have done a lot for the Part 15 group.
To have a practical working transmitter for
22 meters is very useful.
Thanks again!
Bruce, MICRO1690/1700
P.S. If I come up with any useful info regarding
design changes I will let you know.
Also, I'm sure we can figure out the Part 15.225
aspect of this. People have done work on that in
the past and we can look at that work.
Changes or updates to the present version of AM Pixie2 Version 1.0 will be added down at the bottom of the schematic diagram at "Addenda." The first addition is:
DC Volts = 9 VDC
Hi Carl:
I found Pi Network info for the Pixie
for all ham bands from 160 through 6
meters. These networks don't have
as many components as yours does,
so they might not be as good, but
it's something anyway. I read on
your web site the mention of putting
one of these transmitters on 27.000 MHz,
so when the time comes, if you want
the network values for ten meters, I
can give you those. They will almost be
the same as the values for 27.000.
As I'm sure you know, the field strength
level for that band is lower than the 13.560 MHz
value, but it might still be fun to try it there.
Best wishes
Bruce, MICRO1690/1700
The 2nd Harmonic from 13.56 mHz falls at 27.12 mHz, so if it were attenuated enough to meet the lower radiation limit at that frequency it could be viewed as "a 2nd transmitter" which repeated everything from 13.56. It is between CB channels 13 & 14.
You are right Carl.
I think the original Sputnik satellite
transmitted on 20 and 40 MHz
in the same way- there was just one
transmitter.
Best Regards
Bruce, MICRO1690/1700
Here I thought that power in milliWatts could be calculated simply by knowing voltage and impedance. I think I may be wrong. Here's what we have:
The transmitter being tested has .66 volt at the RF output into a 50-ohm load.
Elsewhere on this thread WILCOM LABS offered the tip that .5 mW into a 50-ohm load is within 50% of the allowed field strength at 13.56 mHz.
What's the next step in solving this puzzle?
0.008712 watts
The relationship between transmitter output power and the field strength at 30 meters depends upon the antenna gain. The actual gain of a particular antenna is usually not known. Here are a some examples: 0.5 mW radiating from an isotropic radiator (one that radiates equally in all directions) produces a field strength of 4 mV/m at 30 m. A half-wave dipole in free space has a power gain of 1.64 (2.15 dB) with respect to isotropic. A quarter-wave monopole above ground has twice the power gain of a dipole in free space, and is 5.16 dB over isotropic. An antenna gain of 6 dB over isotropic would give a field strength of 8 mV/m at 30 m. This is about half of the field strength that is allowed by Section 15.225. So, it is unlikely that 0.5 mW of radiated power would cause the field strength limit to be exceeded if a simple dipole or monopole is used.
Using the formula P = (E^2)/R, 0.66 Vrms across 50 ohms gives a power output of 8.7 mW. 0.5 mW corresponds to 0.158 mVrms across a 50 ohm resistor. I use an oscilloscope with a 10X probe to measure the RF p-p voltage across a load resistor. 0.5 mW corresponds to 0.447 Vp-p across a 50 ohm resistor.
[wdcx replied when I was typing. We agree about the power applied to a 50 ohm resistor with 0.66 Vrms applied to it.]
Thank you gentlemen for the guidance in the garden of mathematics.
Starting with WILCOMs proposal, that .5 mW / 50-ohms will produce about 50% of allowed FCC limit per Part 15.225 of 10,000 microvolts per meter at 30 meters (leaving lee-way, I guess, for, among other things, the variability of antenna performance), can we therefore assume that 1.0 mW would put us somewhere near the FCC limit?
If that's roughly true then the 8.712 mW that you've calculated for my situation, again not considering antenna performance, puts our output over-power by almost 9-times.
Is it getting simple enough for me?
Let me change to another observation about this transmitter. Today for the first time, with an indoor 1/4-wave horizontal antenna, I walked down the block with the Grundig handheld and copied good signal until about 250-feet when it dropped away.
Still in guess-land, I'm hoping the brick walls of the house are attenuating enough to avoid an immediate problem during testing.
Now, instead of adding a fixed attenuator, I'd like to make the output variable. Can this be done by adding a variac to the emitter of Q2?
Why use A Horizontal Antenna? Usually on MW (Or Shortwave)
That doesnt work well at all. Try Vertical you should get better results.
This is great stuff!
I'll have more to report when I get my
version going. I just got the parts to
modify the circuit.
Thank you everybody for all the info!
Bruce, MICRO1690/1700
Hi mighty 1650 oldiez 99.9
Since this testing is an indoor project I don't have vertical space for anything more than about 9 feet and the antenna is two-rooms long! Later on, out doors, vertical becomes an option.