Part 10 
by Dick Kelly, W6BKY, w6bky@aol.com

As promised last time, Figure 1 shows a way to generate a 3.58 MHz signal for checking out your 80 meter receiver. 

Q1 can be any bipolar NPN transistor you have salvaged from the IF circuits of a TV set, or you can use a 2N3904, 2N4401, etc. I used a 2N2222A. X1 is a 3.579545 MHz color burst crystal salvaged from a color TV, or JAMECO #14533. Figure 2 shows the parts layout I used.

T1 is wound on a T37-2 core. The primary is 39 turns, and the secondary is 16 turns. I used #30 wire salvaged from a TV set; red for the primary and green for the secondary. The colors make it easier to keep track of which-wire-goes-where when soldering the transformer into the circuit. NOTE: The enamel must, of course, be cleaned from the ends of the wire so there is good electrical connection and the solder can take hold.

Alert readers, such as yourself, may be wondering what in the world R4 is doing in the circuit. Well, the circuit likes to work into a load, so R4 provides a load and keeps everyone happy. The circuit will oscillate without R4, but it may produce a strange looking signal (not a sine wave.

Let’s talk about COILS. Two types of coils are commonly used in HF circuits: solenoids (cylindrical shape) and toroids (donut shape), such as the one shown below. 

Toroidal coils and transformers are very useful because they are compact and, more important, they are self shielding. That is, they do not require external shielding, as do most solenoid coils. Toroids can be either air-core or metal-core. We are most interested in metal-core toroids.

Looking at Figures 1 and 2, you might ask: "Why did you choose a T37-2 core, and how did you know to wind 39 turns?"

"Good questions!", I reply, and here is my answer. First, some nomenclature. The designation "T37-2" means:
 

T - - Toroid

37 - - Size: 0.37 inch Diameter

2 - - Material Type: 2

You need about 6.5 uH in the primary and about 1 uH in the secondary (trust me on this, I don’t want to get into impedance, turns ratios, etc. right now). The number of turns required to get the needed inductance can be derived from information in your ARRL Handbook. There, you will find a coil winding formula for toroidal coils. Have I mentioned that arithmetic gives me a headache? You may have noticed the lack of calculations in this series of articles. Well, there are times when a bit of arithmetic is required, headache and all. So here’s the formula you need to calculate the number of turns, "N", for a given inductance, "L". NOTE: This formula is for powdered iron cores; there is a different formula used for ferrite cores.
 

There is a table in your Handbook that shows values for AL, which is sometimes referred to as L100. Either term refers to inductance per one hundred turns. Look under the column headed "Mix-2" and find a value for the T37-2 core; you will find the value, 42. Cranking the arithmetic yields about 39 turns. 

If you have an older Handbook, you may not find data about toroidal cores. Not to worry! AMADON, and other suppliers, want you to use lots of their cores, so they try to help by sending more information that you ever wanted to know about toroidal cores with each order they ship. You are then faced with the task of interpreting the data. It isn’t as daunting as it may seem; if I can do it, anybody can!

You may wonder why a T37-2 core was chosen rather than, say, a T12-17, or some other core. By examining the data sheet(s), you will find three very good reasons:

1. Physical size. There are some 24 sizes of cores, with diameters ranging from 0.125" to 5.20". What we need is a core that is relatively small, and will allow the number of turns needed. A size 37 (0.37" diameter) is just about right.
2. Frequency. The material used in the cores is mixed for use in a specific range of frequencies. #2 material covers the range of about 3 MHz to about 30 MHz (just what we need). If we were choosing a core for the coil in a VFO, another characteristic needs to be considered …
3. Temperature stability. Material #2 is fine for filters, but it is not a good choice for VFO coils because it is less temperature stable than Mix #6, which is a better choice for VFO coils.

I have found it best to wind coils when I an fresh, and when I am not in a hurry. The absolutely worse time to try to wind a coil is when you have already been at the workbench for quite a while, and you are trying to finish this blankety-blank circuit so you can move on to other things. It takes time to wind toroidal coils, so DO NOT rush the process.

I pre-cut strips of white tape that are a bit wider than the core and a bit longer than the circumference, as shown below.

When the coil is wound, and the windings are still tight, the tape is ready to secure the windings in place. The reason I use white tape is so I can write the inductance and/or number of windings on the coil. When winding a transformer, secure the primary winding with tape, then wind the secondary and use another strip of tape over both windings.

When using small wire (#28, or smaller) the windings will slip and slide as you wind the coil, so you must distribute them uniformly around about ¾ of the core before applying tape. Secondary windings are NOT spread over ¾ of the core; they are, instead, would at the same "pitch" as the primary (same space between turns). When using relatively large wire (#24, or larger) the windings will stay in place by themselves.

The winding of coils and transformers requires practice and patience. Follow the techniques outlined in your Handbook, and you will do just fine. By the time you have finished the 40 meter transceiver, you will be an expert!

‘Til next time, 73, Dick, W6BKY