This question is about the output network
of Carl Blare's 13.560 MHz Big Talker.
However, it is specifically about the RF
output network coils, so I'm making
a different thread.
This question is about the output network
of Carl Blare's 13.560 MHz Big Talker.
However, it is specifically about the RF
output network coils, so I'm making
a different thread.
The output network that comes after
the final transistor DC blocking cap
is made up of three coils. I did not
have these coils - but they were made
available to me by a kind and generous friend.
They are in the 1.0 to 1.5 uH range and
are actually referred to as RF chokes.
They are small - about the size of a
quarter watt or half watt resistor.
And (this is a long story) - these coils are
sitting 15 miles away at my brother's house.
I will be getting them back soon.
However, just for the sake of experimentation,
I thought I would wind some temporary coils myself, to
use in the transmitter output network in the
meantime. And then, when I get the coils
my friend had sent, I will put them in the
circuit instead.
So I went on the net and found an "open air coil
calculator." Since this is just an experiment, i
ended up winding some #24 wire around a
permanent marker and when the calculator told
me that the values were correct, I put glue on the
coils so the turns would stay together. When the glue
dried, I pulled the coils off of the marker. (I actually
put a piece of paper in between the marker and the
inside of the coils so the coils would not accidentally
get glued to the permanent marker. I used
"Super Glue," actually.)
So these homemade temporary coils are now sitting
on the "Big Talker" final stage circuit board. The
transmitter is almost done.
The coils are about .75 inches in diameter. They are
close wound. (No spaces in between the wire turns.)
They are about .5 inches in length. One is a little
longer, because it is a slightly higher value. The smaller
coils are about 7 or 8 turns. The single larger one
is about 10 or 11 turns. (I don't have them with me,
so I can't remember the exact specs.)
Making the coils was fun. That was probably worth it
just by itself.
However, after putting them onto the "Big Talker" RF
final stage board, I am beginning to question whether
or not these coils will work correctly in the circuit.
First of all, they are much larger physically than the
original coils that I was supposed to use. This means
a higher Q. The RF output filter might not function
the same way because of this. The design calls for
the little RF chokes - which are a much lower Q, and maybe this
was done for a reason.
Also, because they are much bigger, the fields that
are generated by them might interact.
Lastly, they might have some capacitive characteristics,
which might result in VHF parasitics (?)
I don't know. But to me, it's all good fun. And when I
get the correct coils back from my brother's house,
I can just put them in instead.
However, for these experimental coils, which could be
running in an operating transmitter very soon, well...
I guess we'll see what happens.
What do you think?
Any input?
Thanks you guys!
Bruce, DRS2
I am glad you brought up this particular question. I have also wondered the same thing.
The simulations which contributed to the design of Big Talker were done by Neil Radio8Z, which specified the inductance values but did not describe what the inductors should look like physically.
I ordered the inductors by their value from Jameco Electronics, and the only such inductors they had in their catalog were the small ones you have.
Of interest, the same type small inductors are used in the AMT3000 transmitter output.
But the LPB transmitter has big donut toroids with inductors wound on them. Quite a difference in appearance.
I do not know how these two design approaches differ in performance.
"I do not know how these two design approaches differ in performance."
This was answered some weeks ago when you (Carl) asked about toroid cores.
Toroid coils have very little field leakage, meaning the injected signal stays on the inner side of the toroid, with very little of it if any at all escaping. This means reduced losses in the coil compared to the in-line coils and inductors which do not have this unique trait as the toroid cores have.
RFB
If you hadn't built the first one,
and helped me out so much. I would
never be doing this.
Things are very busy here, so it's
a little bit at a time.
But I don't have much soldering left
to do.
The osc., buffer, and final are all on
different perf boards. If I had the
materials, I would have done "dead bug"
construction. Are you familiar with that?
You get a piece of copper board and
put the transistors upside down and glue
them to the board, hence the "dead bug"
name. Then you connect the other parts.
They will all be up in the air, but you
use 1 meg resistors to support them.
The 1 meg resistors are kind of like
posts, and they hold the parts up in
in the air over the copper board.
The great thing about it is that the
ground bus is everywhere underneath all
of the parts, because it's the copper
board, so it's really easy to ground
what ever you have to ground.
I really want to try "dead bug" construction
next time I build another project.
So it's only taken 2 or 3 years, but
I'll be firing this thing up hopefully
in a couple of weeks. So, there's just
a little more soldering and then I
have to connect the boards together.
The oscillator board will run on just
about any voltage. The buffer board
works best with 7 volts. I plan to
run the final amp at very low power to
begin with, because the modulation amp
is a small one. So it may be running
at 4 volts, or something like that.
I will probably use a 12 volt supply
and make a voltage divider to feed
the stages.
And as for the coils, they will either
work or they won't. I had a lot of
fun making them, that opened up another
little "radio door."
Thanks for everything, Carl. Maybe we'll
get a few more opinions on these coils,
but I might have already come up with all
the different things that could go wrong
anyway.
Bruce, DRS2
Thanks RFB for reminding that we discussed to toroid based inductors in the past. I remember that the subject came up, but I don't recall what was said.
Would everybody slow up in your postings so I can go into the past and review the tons of previous info?
I do recall that PhilB allowed that the tiny inductors in the AMT3000 are lossy, and reminded that building a serious large-size inductor coil to bypass the tiny inductors was the real way to gain control over output efficiency.
Therefore it would be worthwhile to design improved inductor coils for the Big Talker, exactly what DRS2 is now doing.
"I do recall that PhilB allowed that the tiny inductors in the AMT3000 are lossy, and reminded that building a serious large-size inductor coil to bypass the tiny inductors was the real way to gain control over output efficiency."
Exactly. That large toroid in the 5K aids greatly in the unit's efficiency spec and would be quite different if that unit used the other in-line inductors.
Keep in mind however the differences in the use of these inductors. In the 5K transmitter, it is used to maximize RF output transfer by taking advantage of the toroid core's ability to contain all the magnetic fields within the core, meaning less loss.
In the LPB's the toroid core inductors are used in a low pass filter, again taking advantage of the toroid core's ability to minimize losses and free radical fields that would be subject to outside inductive influences, creating even more of a mess in design and layout etc.
RFB
Many hams, including me, built
a very popular 40 meter 300 to
400 mW output Morse code transmitter
called the Tuna Tin 2.
When you buy the kit, the circuit
board is round, and it fits on the top
of a tuna can.
I love mine. There are hundreds of
them out there. Mine is very stable
and has perfect keying characteristics.
But the point is, it uses torroids to
couple the osc. to the power amp, and
the power amp to the ant. out jack.
Both stages are 2N2222s.
Well, anyway, I guess this really is just
trivia, but it's a good little QRP (lower
power) transmitter, and the design is
good, and it uses torroids.
Nuff said.
Bruce, DRS2
The subject of coil loss has been mentioned a couple of times and the toroid's containing the "field" was given as a reason for low loss compared to "inline" coils. If the inline coils are small they probably have a ferrite or powdered iron core and as said before they do not completely contain the magnetic flux. The fact that the toroid contains the magnetic flux is a plus because this means the flux is less affected and affects other things less but the price is loss due to the core material dissipating energy. Toroids are probably equal to or better than inline coils with a core in terms of loss.
Air coils have no core loss but air coils have to be used carefully since the flux is present in and around the coil. The toroid may have an advantage on I squared R loss in the winding because fewer turns are needed for a given L compared to air coils but this may be offset by the core loss.
If you are using air coils, keep them as far away from other components as possible and align them at right angles to one another to minimize mutual inductance.
Toroid cores are available for the Big Talker frequencies and the core material must be chosen for the intended frequency of operation for best results. A 1 uH inductor wound on a toroid would only need a few turns and would be easy to wind and change if needed but the downer is that the last time I ordered cores there was a $20 minimum (Amidon) for a $1 part.
Neil
I'll be ordering toroids for the MRAM End-Fed 13.560 outdoor antenna, so I'll add a few to that order for the Big Talker because I think the toroid look on a transmitter output makes it look "transmitterish."
I built the Tuna Tin 2 when first published in QST all those years ago using standard RF chokes, it worked.
I have seen the updated version with toroids and someday I'd like to try it.