Radio Projects
First Posted March 8, 2008
Last Modified March 13, 2014

    I have been fascinated with building radios since I was 13 (and that was in 1966) (See Below).  One of the funnest projects I had to do when I was a senior at UCSB was to build a super heterodyne radio.  Our goal then was to make the best radio we could.  And mine turned out very well.  But I remember back to the days when I was in Junior High and High school when I would build simple radios, generally involving one or two transistors.  And I also had fun building AM transmitters.  Several years ago I started getting the bug again.  My goal is to employ the experience I have had over the last 30 years to help me build some fascinating radios with a minimum of parts.

    The projects I have in mind are a simple TRF receiver using just two transistors, and also a simple superhet receiver that will be pretty much completely scratch built.  Right down to the IF transformers.  Although, that is not really so much by choice.  Getting IF transformers is not easy.  And it doesn't look all that hard to build my own.  And that will enhance the challenge of the project.

TRF Reflex Receiver

The above of March 8, 2008, is very preliminary...and untested to say the least.  This is a double tuned circuit.  Both the input and output is tuned to increase selectivity, although, this will make it more difficult to align.  The two RF transformers are going to be custom built.  When the time comes, I will provide details on how to do that.  Of course...those who think they can build it, I say, go ahead.  There are a few things I am sure that will have to be tweaked.

Simple Reflex SuperHeterodyne

And this is a first cut at a two transistor superhet receiver.  This one is going to be interesting.  First off, all of the rf transformers are going to be built from scratch.  The last time I built a superhet receiver was when I was in college.  That one was a real doosie.  Here I am going for minimalist.

Very Simple Broadcast Band Transmitter

Common Base Oscillator with AM modulator

The above is a very basic AM transmitter circuit.  Now, this one I find it interesting because this is the circuit that I used to make my first transmitter.  I got it out of a book I checked out from my high school library.  It was always iffy if I could get this thing to work.  However, with better components, it is not quite so hard.  This circuit is a Common Base Oscillator.  A common base amplifier, in general, has a lot of power gain, and also is non-inverting.  I would love to put a name to this oscillator.  Searching on the web, I have heard it referred to as a Colpits, Hartly, and Miesner oscillator.  These are all wrong.  Well, maybe someday I will learn what the name of this oscillator type is.

Now, how does this circuit work?

Starting with the oscillator, Q1 is the active device and is configured as a common base amplifier.  Components C1, C2 and L1 make up the tank circuit that determines the oscillation frequency.  This may look a little different than what you normally see.  C2 is a 365pF tuning capacitor that is used to set the oscillation frequency.  It goes from the hot side of the coil (L1) to ground.  C1 connects from the cold side of the coil to ground.  C1 is there really only to insure that the cold side has a good path to ground at high frequencies.

C4 provides positive feedback around Q1.  It comes off a tap on the tank coil so that the signal being fed back to the input has a nice low impudence.  It then feeds into the input of the amplifier, which is the emitter of Q1.

Q2 serves two purposes.  It provides the bias current for Q1, and is also the amplitude modulator  for the signal that Q1 generates.  The AM modulation is achieved basically by having Q2 change the bias current depending on the input signal fed into Q2.

Now, please be advised, I have not actually tested any of these circuits as of this time.  I don't have time :-(.

One thing I have noted over the years of building radios is that they oscillate.  You can never get an oscillator to oscillate.  But it is no problems getting a receiver to do so.  Radio receivers must have a lot of gain in order to receive the weak signals that are propagating through the ether.  When you combine this with all the coils and rf transformers, well, you have a recipe for disaster.  All I can say is good grounding techniques, careful wiring, and shielding are your friends.

I hope to do so real soon.  And I hope to have some real fun projects that other may find amusing and educational.  I have a very soft spot in my heart for radio circuits, because this is where the hobby started for me.

Here are some links for finding parts:

August 23, 2011

    Today, I finally ordered some chassis boxes and some trimmer caps to start building and testing these projects.  I plan on building the super heterodyne receiver and the transmitter.  I still need to build another IF transformer yet, and get some more cans for coil shields.  But I am gradually gathering the materials that I will need.

Winding Coils
First Posted March 15, 2008

Winding coils has never been one of my favorite things to do.  They almost always turn out looking very crude.  But then I remembered I have some pretty good tools.  I used a piece of 2 inch PVC to make the coil form you see below.  I cut off a piece about 8 inches long so that I could chuck it in my lathe.  #22 wire can be wrapped at about 33 turns per inch.  The closest thread I can cut with my lathe is 32 threads per inch, so that is what I set it to.  I made two passes so that I could make fairly light cuts to keep the plastic from melting.  Then using a cut off tool, I trimmed off what I needed.

I then used a #60 drill to make the holes that the wires go through.

Then I got out my spool of wire and began winding the coil.  With the thread pattern cut in the form, winding was a lot easier.  It wasn't too much problem keeping it looking fairly neat.  Although, it was the little tickler coil that was the hardest to wind, for some reason.  I wasn't able to keep a hold of the wire and had to start over twice.  The big coil I got the very first try.

I intend on using this coil in an RF generator.

August 19, 2011

Well, you know, to use radio coils in anything that has some gain, the coils will need to be shielded. But, how can this be done easily.  Got the answer at the local Michaels Craft Store.  They sell these cans for putting homemade Christmas goodies into.  What I saw was the perfect thing to put an RF coil into.

Coil Shield

I will probably have to sand the paint off where the cans join for good electrical conductivity.  What I plan on doing is bolting the lid down to the chassis.  Then I will mount the coil on top of that.  Then, just press the can over the lid.  Yeah, I know it may look just a little too cute...sure beats having to fabricate something like this.

Building IF Transformers
First Posted July 7, 2011

    Hey, it is been a while since I have updated this page.  Well, last time I posted, I could still see.  Things are a lot more tricky for me now.

But, I am trying to forge ahead with my radio projects.  And I need IF transformers.  There are several ways to skin this cat.

The above are two other links I have located on building IF transformers.  Either of these methods will work.  I am going to outline another way to accomplish the same thing.  I cannot say my way is any better or not, but what the heck.

The IF can is going to be made of a 3 inch long piece of 1.75" square aluminum extrusion.  The stuff I purchased has .125 inch thick walls, so the inside is 1.50" square.  The coil for is going to be made from 3/8" evergreen polystyrene with3/4" by .060" thick disks to keep the coils in place.  The top of the can will be a 1.75" square of .125" aluminum.

This is going to be kept together by using threaded spaces.  The spacers will hold two .062 fiberglass perf board apart.  The perf board will have a 3/8" hold in the center.  The coil form will pass through these two holds.  On the top perf board I will mount one or two trimmer caps, depending on the type of transformer, to tune it.  The whole mess will be screwed together hand hopefully look pretty clean.  I have ordered the materials for this, so I am hoping I may actually get a chance to build a few samples soon.

    Hey, these drawings don't look too bad.  On screen, I can magnify them.  But, I see at least one omission.  There should be a capacitor across the primary winding (the one with the tap).  I was planing on using a fixed cap in parallel with an adjustable trimmer.  The cap will go across the the entire winding...i.e. will not be connected to the tap.  My intention is to mount the "interior part of the transformer down to the chassis.  The 1.75" sq extrusion will slip over this, and then the lid will be screwed on top, into the top of the interior part of the transformer, and this will hold the whole thing together, I hope.

August 12, 2011

    Ok, today, I actually started building the IF transformers.  Today, I made the IF Can sides.  It is just an aluminum extrusion cut to size.

IF Can Picture

    These are the first three I cut out.  I just cut them with a hacksaw, and then cleaned them up with a milling machine.  And then a little filing with a single cut file to make things smooth.  This was, I think, the HARD paart.

August 13, 2011

Perf Boards

Here are the perfboards, before they are drilled.This was a real quicky tonight.  The boards are 1.5 inches (about) square.  I will clean them up with a file later to get them more exactly to the correct size.

August 16, 2011

IF transformer Core Parts

    Above, you can see the perf boards after they have been drilled.  Most people out there probably could have drilled them quite easily.  Not me.  Because of my very low vision (20-200 uncorrectable), I had a very hard time drilling the holes in the correct place.  I solved this by drilling a pilot hole that was about .08 inches.  I could see this much better.  But I also had to use a brad tip drill that was 0.140 inches in the drill press.  I would use the brad to locate the pilot hole, then I would turn on the motor in the drill press.  I took me about 3 hours to make to two little boards.  The tube was easy.  I just cut the 3/8 inch dia tubing in my miter saw.

Coil form Washers

    The washers were easy to make.  I clamped a sheet of 0.060 inch thinck styrene on the X-Y table of my milling machine.  I put a drill chuck in the quill and started with a 3/8 inch brad tip drill.  I drilled a hole every 1.500 inches.  I thin put a hole saw in the chuck and went over the same places again.  The guide drill in the hole saw is smaller than 3/8 inches so really, I was only using the saw part.  I drilled slowly, trying to keep the styrene from melting.  This was pretty tough to do with the hole saw.  So, after I got the rough cut washers, I put them all on a 3/8 inch bolt, and held them together with a nut.  I then chucked them back up and used 100 grit sand paper to smooth them down.  Use a very light touch.  Things can get hot very fast.

Coil Form Assembly

    Here is the perf board and coil form all put  together.  I still need to get some spacers to hold the whole thing together.  As of tonight, the only real peice I need to fabricate is the lid that goes on top of the can.  This is another job that is NOT going to be easy for me.

August 17, 2011

Coil Form Ready for Can

    So, close, I can taste it.  These are the main mechanical components for the inside of the IF transformer all put together and ready to put inside of the can.  The standoffs on the bottom will be bolted to the chassis, and the standoffs on the top will hold down the lid.  Now, I just need to finish the lid.  I am not looking forward to that task.  It is going to be a very tough thing for me  to make.\

Morris Coilmaster
July 9, 2011

    Did a lot of research today for a book on building radios.  I was looking into coil winders.  It turns out, that somewhere in my collection of stuff I do have a Morris Coilmaster universal coil winder.  Didn't even really know what it was.  No, I need to find it.  I sure hope it is not lost.  I ordered a booklet from Lindsay Publication on building a clone of this device.  I am interested in that partly because I need to make sure that this is something that everybody would have access to one way or the other.  It seems pretty evident that nothing like this is made today.  It also has gotten me thinking that an NC version could be built with a couple of stepper motors.  With stepper motors, one would not have to mess around with gears and cams.  I am pretty sure that some sort of coil winder would sure make my IF transformers a lot more cool.  And a universal coil winder will help to make the Q of the coils that much better.

July 10, 2011

    Hey. I found my Morris Universal Coil Winder.  It is missing at least one spring.  Hopefully I can replace that.

August 7, 2011

    Well, today, I managed to whip the Coilmaster back into shape, more or less.  It is going to take a bit more work.  I replaced the missing spring and very lightly lubed the bearing points.  I also mounted it on a wood base, and added a bar to put the wire spools on.  The big problem is the shaft coupler for the screw that holds the coil form.  I can't seem to get it  to hold the scdrew on in any way shape or form that is straight.  I also did not have anything to put on the wire to make it "sticky".  Hopefully, I can pick up some bee's wax.  The coils I managed to wind today were, well, not impressive.  My reaserch has suggested that getting good coils will take a bit of practice.  Anyway, here is a picture of what it looks like:

Coil master mounted on wood base

I am still doing research into building a NC coil winder, but it is rather obvious that I need to learn my chops on this hand powered unit so that I can know the best ways that coils can be constructed.  I have to say that the video I saw on You tube using a similar machine to the coilmaster to wind a coil was quite impressive.  And there is obviously something I am missing here.

Building Your Own Variable Capacitors
First Posted July 18, 2011

    Now, here is a very high goal to achieve.  I have been doing a lot of research on this subject and have found quite a number of poeple that are already doing this.  Some of the results are quite spectacular.  Some of the results are more crude.  However, all pretty much seem to be functional.  I am trying to come up with a way to make an effective variable capacitor that does not require fancy tools.  I don't know if this is possible.I would rather not make the plates out of soft aluminum.  But, working with harder alloys is a lot more difficult.  Plus, I have very limited vision, so what ever I do must be something that can be done blind folded.  If I can solve this problem, then just about all of the major components in the radio can be scratch built.  A very exciting thing for me at least.

July 28, 2011

I have been doing some more thinking on this problem.  I am going to try and come up with some sort of a jig that will make cutting out capacitor plates "easy".  I still have no idea how I am going to do this.  By far, the best way is to build or gert an NC mill, but this is not very simple or inexpensive.  I will keep working on this problem.  There has to be some solution..

One Transistor Super Het Receiver
First Posted July 18, 2011

    A one transistor Super Het?  Ha!  Not Possible.  Or, at least that is what I would have said up until a few days ago.  In doing my research, I ran across a circuit for just that.  It consisted of an Antenna Coil that feeds into a transistor that is also connected  to an oscillator coil.  Pretty much like the normal converter stage used in a typical transistor radio.  But, this is where it ends.  The converter stage outputs intio an IF transformer.  There is no IF amplifier, however.  The transformer goes directly into a detector.  The output of the detector then goes back into the converter transistor.  So it is a reflex receiver, but I have to say, it is a bit strange.

    So, now I see the challenge.  I am now going to have to see if I can come up with something similar.  I thought I was veing very clever by doing a two transistor reflex superhet (see above).

Radio Projects from my Past
First Posted August 23, 2011

    I did not think that anything from my electronics past existed any more, but I was wrong.  My older sister arrived yesterday from Seattle with a box of stuff that my mother kept from when I went to school.  In that stuff was the Journal we were supposed to keep in my English class.  So, I wrote about things I enjoyed.  The first thing was about the transmitter I was working on at the time.  For a time frame, this is when I was in the tenth grade.  Most likely, this was the spring of 1969.  I am pretty sure that the journal was one of the last things we did in that class.  Anyway, here are those pages for your amusement.
High School Paper Page 1

High School Paper page 2

    This project was partially built in the electronics class I was taking at Dos Pueblos at the time....


    After reading the text, I have to say I am not really too sure exactly what I was trying to say.  The circuits, however, are fairly plain.  I can sort of see why I would have a hard time getting this thing to work at times.  The transistor biasing is, well, just not very good.  The bias current would depend very much on the characteristics of the part used.  Still, you can see the basic circuit.  It is very similar to the one I show above.

LC Circuit Calculator
First Posted September 11, 2011
Updated March 13, 2014

Circuit Calculator Program

This is a handy dandy program I created to make it easier for me to do the mundane calculations for the radio projects I am doing.  There are four calulators in the program.  There is resonant frequency, capacitive reactance, inductive reactance,  and a coil calculator.  The coil calculator figures out the length, number of turns for a given coil value.  These are single layer solenoids only.  And even then, this is only an approximation.  Source code is included.  This program was compiled with Microsoft Visual C++ 6.0.

March 13, 2014

I added some calculations for determining the inductor size and the pad capacitor size when given a tuning range and a maximum value for the tuning capacitor.

    Circuit Calculator

    This program is in the public domain and is open source.

Electronics Books that I enjoyed and Influenced me, and some contemporary Books
First Posted October 2, 2011

    When I was younger, there were a lot of books I enjoyed.  Most of them, however, I don't remember the names of.  A few I have been able to locate.  Some, I am sure, I will never locate.  Anyway, here is my list.

Using Electronics by Harry Zarchy

    This book is one of the ones that I considered the most fun.  It mostly deals with radio receiver circuits.  All of the circuits are pretty basic.  It should be noted that several of the circuits probably will not work with modern transistors and will require modification.  Many of the circuits have no bias for the base of the transistors.  Back in the 50's, you could get away with that because the transistor junctions leaked so bad the base would bias itself.  And in some cases, in order to work, some of the circuits require a germanium transistor.
Now, it just so happens that the only copy I have is electronic.  So here it is (see above).

    A Boys First Book of Radio Electronics by Alfred Morgan.

        I am still trying to get a copy of this as well as the three books that followed it.

Contact Information: patchell AT cox DOT net