easy40
A 14-components high-power transmitter to get you quickly on 
the air
please ask for KIT availability!

Introduction

QRP is all about doing more with less. This is more than true, with the construction of this cheap, small-size, keep-it-simple transmitter presented here. It is just remarkable how 14 components can lead in so much output power, that lets you commnicate with a big part of the globe, when propagation conditions are right. From all my other constructions, this one stands out in terms of simplicity in balance to the performance.

Following my instructions, the transmitter can be reproduced easily, within hours. The result is always success, this is one of the circuits that are not critical at all and a successfully working transmitter can be reproduced every time. I have built this transmitter several times, using similar components and it always worked. However the components and the way of construction I present here, ensures stability and low cost and guarantees that the transmitter will always meet the next specifications:

Main carrier output power (excluding harmonics): 2.5mW-8W (adjustable) at 50R
Band of operation: 40m
Output power adjustment: Vcc=2.5V for 2.5mW out, Vcc=29V for 8W out

Output impedance: 50R
Modulation: CW (with external key), Feld-Hell (with external switching circuit) and AM (with external series modulation transformer, or electronic series modulator)
Key operation: key breaks full transmitter DC power




A little history

For years, I wanted to design a transmitter that would be very easy and cheap for almost anyone to build, yet quite powerful. I have seen hundrends of circuits on the net and on old magazines and very few come as close as this one, when compared to simplicity and power output. Yet not extremely stable especially when operating at high power, however many contacts can be achieved with this tiny transmitter, more than anything else of the same cost and components count I have seen.

I got the idea for this tiny transmitter, after reading an article written by AE6C on QST (1, 2, 3), so I do not claim authenticity.  However, even his idea, comes from pre-WWW2 valve circuits, where components were big, bulky and expensive, so saving some of them was worth it back then.
The article describes in detail the operation of such a circuit. However, the maximum power output of the circuit in the article, was 2W. Where I claim authenticity, are the modifications I made to the circuit to boost the quality of the output signal, the stability and the output power, as well as a completely different mechanical approach. As radio amateurs, we ought to improve circuits to our benefit!

The transmitter construction

The schematic of the transmitter is shown below.

Vcc

KEY


T

150pF

T3

470pF

T1


L

47pF

Out

470pF

470pF

40m

7-25pF

T2
 

10k

2sc
2166

10nF

1n4148

FB43-
101
The inductor at the base of the transistor, is just an FB43-101 ferrite bead, passed through the transistor base lead.

The output filter inductor L, is about 850nH. It is wound with 13 turns of 1mm diameter enameled copper wire, evenly spaced, on an Amidon T68-7 toroidal core.

The transformer T, has three windings (T1, T2, T3) and it is made the following way:
It is important to wind all three windings of the transformer in the correct phase. That is, the beginnings and endings of the wires must be wound exactly like shown in the picture below. For example, for T1 winding, the wire must start from the KEY position and pass in front of the core (not hidden below it), then inside the core. The other end of the T1 winding, should then go to the variable capacitor, leaving the core from it's back side (hidden below it). Each time the wire passes from inside the core, this counts for one turn.



The picture below, shows the finished transformer and the phasing (direction of winding) of the wires can be clearly seen.




The enclosure of the transmitter is purely homebrew. Not only it looks better than the commercial enclorures, but it is also very cheap, easy to build, lightweight, in the right dimentions and plays an important role on both the mechanical and the electronic construction of the circuit. It has been made using a wooden base (breadboard), which holds the components using nails and metal walls that are made out of a single thin sheet of aluminium, cut and bent to the right shape. The aluminium sheet is thin enough, so that it can be easily cut using a pair of scissors and bent using finger pressure and a ruller.

The aluminium walls, host the connectors for the transmitter, but they are also used as a heatsink for the transistor, eliminating the need for a separate heavy, bulky and usually expensive heatsink. The tests have shown that even this thin sheet of aluminium, can easily take the heat apart from the transistor, even at high power levels and prolonged transmitting times. Note that in this transmitter, it is best to electrically insulate the aluminium heatsink from the components, so a piece of mika thermoconductive insulator was used in conjunction with plastic-ring screw insulator, to attach the transistor to the heat sink. Thermoconductive paste, is absolutely not necessary to be used, althought it is not a bad idea to use it if you have it on hand.




Since one of the goals was to make a cheap transmitter for the field, expensive connectors are not used. For the power and the antenna connections, ordinary cheap (but high durability) stainless steel screws, nuts and bolts were used, to create the connectors. The screws are larger in length, to allow both crocodile clips and bare wires (using locking nuts and bolts) to be easily attached. This solution is highly durable (large number of connection cycles) but more importantly it is cheap and very convenient in the field, where suitable coaxial connectors are not always available and there is a high risk to forget taking compnents with you.

However if using it with coaxial cable from inside the shack, you may want to make yourself (or buy) a wire-to-coaxial adaptor, to avoid RF leakage in the shack. The output of the transmitter is 50 ohms, so you would need no matching to the coaxial cable. Most possibly, your antenna would need matching to 50 ohms at HF, not your transmitter.

Just as in the transistor case, the screws of the connectors (even the ground) must be isolated from the aluminium heatsink, to minimize hand effects and ensure stable operation. For insulators, a few pieces of transistor insulating plastic rings were used.








The circuit is made in breadboard-style onto a piece of wooden base, cut in the right dimensions, which also serves as an electrical and thermal insulation between components. Nine solderable nails, nailed onto the wooden base, serve for the mechanical stability of the components and help solder the components leads together.

The nails positions are not arbitrary. They are placed like this, to achieve minimum connection lengths between components and avoid the use of extra wiring. To make it easier for you to place the nails, I have created a template. Simply put the template onto the wooden base and nail the nails onto the wood, at the predefined positions.



The next pictures, show all the components of the transmitter, soldered onto the wooden base nails. This construction may look ugly, but from the RF point of view it is optimal, since the connections between components are minimized.





Since the output power of the transmitter can be varied by varying Vcc, the transmitter can be easily AM modulated, by replacing the key with a series connected transformer, as shown in the schematic below.

Vcc

Audio
in

TR


T

150pF

T3

470pF

T1


L

47pF

Out

470pF

470pF

40m

7-25pF

T2
 

10k

2sc
2166

10nF

1n4148

FB43-
101

I have tested this configuration with a small series transformer, taken out of an old TV speaker, which has a primary of 4 ohms (audio amplifier side) and a secondary of 120 ohms (transmitter side).

To drive this transformer, a TDA7360 integrated amplifier, working in bridged mode and driving the 4 ohms primary directly, thought to be a simple way. Indeed this class-AB amplifier combined with the transformer modulator, form an efficient AM modulator. If efficiency is the goal, this is the way to go. However, this approach requires three additional components for the modulation. A suitable audio transformer (which is not always available and easy to find), an amplifier IC and a separate PSU for this amplifier (TDA7360 works on 18V max).

If efficiency can be sacrificed though, AM modulation can be achieved with a series BJT or FET in place of the transformer. Similarly, an LM317 can be used and this can double it's use as an AM modulator as well as a variable PSU for the transmitter (to vary the output power of the unmodulated carrier). Since the goal of this transmitter is simplicity, this was the chosen way.

To be continued...

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