[[  Most recent Update:  27 October 2012.  ]]


After you have built a few electronic projects, you may wonder why they are called electronic instead of mechanical.

By mechanical, I mean such things as:

[] front & rear panel layout
[] placement of parts on the circuit boards
[] arranging circuit boards on the chassis
[] a box to put it in
[] cables and connectors
[] etc.

I have built some items that look pretty good, but most of my home brew projects are built by simply wiring the circuits without much thought or effort toward appearance. I even use ugly (point-to-point) wiring technique as opposed to printed circuit (PC) boards. If I were in the business of building stuff in great numbers to sell, I would, of course, use PC boards and assembly line techniques. Yes, I have made some PC boards, but for me they are more trouble than they are worth for one-of-a-kind projects.

Packaging is important, particularly if you are planning to use your home-brew equipment for many months or years, and it’s none too early for you to start thinking about what you want your 40 meter transceiver to look like when you have finished it. Whenever possible and/or practical, I will present alternative approaches for packaging.

Three packaging methods I have used successfully are briefly outlined, below.

[1] Build on a single, relatively large circuit board, then put it in a suitably sized box.

[2] Build on two boards, one for the receiver section and one for the transmitter section, then put them into a suitably sized box or into separate boxes.

[3] Build and test each of the many module separately, then hook them together so they fit into a box or boxes, then connect the boxes together.

Building technique number [3] is the method that will be presented here, using three boxes:
[] Control Head, contains the Control Panel, plus VFO and VFO buffer.
[] Receiver, contains all receiver circuitry, except the VFO.
[] Transmitter, contains all transmitter circuitry, except the VFO. The transmitter box also houses the antenna switching circuitry.

The builder (YOU) are, of course, free to use any method you choose. That’s one advantage of building your own equipment – you can, within certain limits, package it any way you want.

Before we start building, perhaps it would be a good idea to take a quick look at what all this is leading to.

The simplified functional diagram below shows the contents of the RECEIVER section of the WT40 transceiver .

Some things to notice in the diagram:

[] Only the units pertaining to the Receiver are shown.

[] The Audio Amplifier module is contained within the dotted-line enclosure.

[] The VFO is used to determine the frequency of both the Receiver section and the Transmitter section of the Transceiver.

[] the VFO is tuned with a Potentiometer, as opposed to the traditional variable capacitor.

[] The transmit / receive (T / R) switching is done by relay. Yes, there are more elegant and clever ways to do T / R switching, but they are usually employed to implement break-in keying and/or to achieve minimum physical size for the unit. Neither minimum size nor break-in keying were high priority for this transceiver.


First, we will build the Receiver section, including the VFO because the receiver can not function without a VFO. When all the modules in the receiver have been built and tested individually, the receiver modules, along with the VFO and VFO buffer, will be assembled and tested.

Next, the Transmitter modules will be built, tested and assembled.

Finally, the whole transceiver will be assembled and tested to verify it is working properly.

There is much work to be done, but first . . .


We are bathed in electromagnetic radiation (radio signals) 24 hours each day. There is no escape. Some of this radiation is useful, most is not. That which is not useful is, by definition, “noise”.

A radio receiver has two primary functions:

[1] The radio receiver must have enough Sensitivity to be able to detect the electromagnetic signal of interest to the user.

[2] The radio receiver must have enough Selectivity to be able to eliminate most, if not all noise, and present only the signal of interest to the user.

For the WT40, the signal of interest is in the Ham radio 40 meter band.

There are many auxiliary functions performed inside a radio receiver, but unless the receiver has good sensitivity and selectivity, the other functions are of little or no use.

SELECTIVITY is accomplished by good circuit design in general, and good filtering in particular.

SENSITIVITY is accomplished with amplification circuits of various kinds.

NOTE: The Antenna contributes to both sensitivity and selectivity, and will be addressed after the WT40 is assembled and tested.

In order for a receiver to be useful for Ham Radio communications, it should be sensitive enough to detect very weak signals. Most communications receivers, including receivers built for the Ham bands, are capable of detecting signals of less than one micro-volt (uV). That’s 0.000001 volt, and that’s a pretty small amount of voltage. It is so small that it can’t be detected with ordinary test equipment such as your DMM. Having said that, I should point out that there are Ham Radio signals “on the air” that will present 5 to 50 microvolts (uV), or more, at the antenna input on your receiver.

SELECTIVITY is accomplished with with good circuit design in general, and good filtering in particular. The receiver section of the WT40 has two types of filters:

[1] A radio frequency filter that attenuates all signals except the 40 meter signals, such as the one pictured below.

[2] An (optional) audio frequency filter is designed specifically for receiving CW (Morse Code) signals.

In order for a receiver to be useful for Ham radio communications, it should be selective enough to eliminate most of the “noise” that comes into the receiver. This is particularly important in receivers used for communications because the signals are usually much weaker than those of commercial broadcasts, and are usually packed much closer together.

It is good to have as much filtering as possible at the “front end” of the receiver – the first circuits encountered by the signal of interest when it enters the receiver. This will eliminate much of the “noise” before it gets into the following circuits and causes trouble. The receiver 40 meter filter shown above does this for the WT40.

The signal-to-noise relationship is more complex than the simple explanation presented above would indicate. For example, if the signal of interest is 10 uV and the noise is 20 uV, the signal will be obliterated by the noise. Not only that, but there is a certain amount of noise generated within your radio by the components simply doing their job. All those electrons rushing hither and yon can create quite an uproar!

Be that as it may, sensitivity and selectivity work hand-in-hand to grab the signal of interest from “thin air” and process it in such a way as to make it clear enough and strong enough for you to hear.


The Schematic Diagram, below, shows virtually all the circuitry for the WT40 Receiver section.

OMITTED circuitry includes Antenna Switching, Voltage Regulation & Distribution, and the Variable Frequency Oscillator (VFO).
The schematic diagram shown above may appear a bit overwhelming to the inexperienced builder. Notice, along the bottom of the schematic, that the circuit is divided into SIX modules:

[] Receiver Filter
[] Radio Frequency (RF) Amplifier & Mixer
[] Audio Buffer
[] Audio Preamplifier
[] Audio Amplifier

Notice, also, the TWO OPTIONAL units: 1) An Audio Filter and 2) A Speaker. More about options after we have completed the non-optional modules.

Each module will be built and tested, then the modules will be assembled and tested as a complete receiver.

For the benefit of inexperienced builders, much detail will be presented for building the Audio Amplifier Module because this is the first module in to be built in this series. It is not my intention to insult anyone’s intelligence by dwelling on boring details, and I think it is equally important to present enough information so the inexperienced builder does not get lost in the process.

Subsequent modules will omit some of the detail.

For the subject at hand, which is building the Audio Preamplifier & Amplifier modules, a Schematic for the Audio Amplifier Module, which includes both Preamplifier and Amplifier circuits, is shown below.

Incoming signals will be carried by a twisted pair (or by miniature coaxial cable such as RG174 or equivalent, if you prefer). Notice on the Schematic Drawing that signal entry points, such as the one for Audio In, require TWO Tie Points: one for the signal (TP28) and a companion Ground connection (TP29).

The Layout Drawing shown below is one of many that work equally well. I think this one is well suited for the inexperienced builder.

A photo of a finished Audio Amplifier module is shown below.

Why build the Audio Module first? (you might want to know).

Because that is where “the rubber meets the road”, so to speak, or more exactly, the Audio Amplifier module creates the electrical signal that drives the earphones or speaker. Your ear is, after all, the final arbiter of the quality of the radio receiver.

Also, and more importantly, the Audio Module contains many of the types of components that are used throughout the transceiver. After building and testing the Audio Module, the inexperienced builder will be ready for learning about more advanced building techniques, such as coil winding.

Coil winding?!

Yes, you will be winding a few coils for use on some of the modules. No, this is not simply to make life difficult for you – some of the inductors (coils) that are required for the transceiver are not off-the-shelf components. Not to worry – after you have wound a couple of coils, the task will be a “piece of cake”; I promise!

FYI:  You can see some of the coils you will be winding in the photo of the Receiver Filter shown earlier in this post.

Next time, in WT40: AUDIO MODULE, AN INTRODUCTION, we’ll take a Parts List along with some information about preparing and populating the circuit board.




About w6bky

Retired 29 May 1987. Now do hobbies: blogging, ham radio, gardening, etc.
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