THE GACW40R
A CW RECEIVER FOR BEGINERS
Guillermo H. Necco LW3DYL
The most common question I hear is: Does the telegraphy still exists? To what I have to answer, yes; and there is not other method as effective to get long distance communications (DX) at which we the hams are so fond of.
The telegraphy (CW) has many technical advantages being some, the simplicity of the equipment designed to this scope and the extraordinary penetration that has under bad propagation conditions. It is easily decodified in situations with high atmospheric noise. By the contrary, a SSB signal with deep fading (QSB) or high atmospheric noise (QRN) is almost uninteligible.It is necessary to take into consideration an essential aspect of the communication: The language. Normally we use English for phone contacts... but have you ever had a chat with a Byelorussian in English? I can assure you that it was a terrible experience for both of us. For him my English was so bad as his for me.
In CW the communication is much easier. We still use English but you send words letter by letter avoiding regional accents and wrong pronunciations. We also use the Q code which also further facilitate the things.
Added to all this is the fact that in Argentina the Novice category has a segment on the 40 mts. band that allows local communications during the day time and DX at night.
This is why we introduce the first component of a series of CW QRP equipment made of: a receiver (GACW40R), a 2 Watts transmitter (GACW40T) and 15 Watts a linear amplifier (GACW40L).
This kind of equipment has a very special characteristic: they trend to be the most simple ones without loosing any effectiveness. Also they have to be made with the most common and cheap components in the market. Because here in Argentina RF toroids are very hard to find , the coils were developed with Guille's (LW4DZC) system, using hypodermic syringes, and the few toroids remaining can be obtained from a scrap PC power supply.
There are also presented two types of circuit boards: in one the traces are cut with an utility knife in the copper side, then drilled and the components inserted as in a normal PCB. This is for those that doesn't like to use etching solutions. I'm also presenting some drawings to guide the beginners in the placing of the components.
The other version is with normal etched PCBs for which I'm providing the drawings and mounting instructions. Originally I was thinking in making a transceiver, but this has some adjusting difficulties that make it unsuitable for beginners so I decided to make three independent units.
The receiver
It is a superheterodyne single conversion. That is, the input signal from the antenna is mixed with that of a local oscillator, passed through a filter and mixed with another oscillator to get an audio tone.
The antenna is connected to a tap of the 14uHy coil, that filtrates all frequencies except those close to 7MHz, that are those that we are interested in. These signals are feed to the gate of a MOSFET that mixes them with the signal from a VFO running at 3MHz. The difference between this two signals is 4MHz and is feed to a computer's crystal filter that allow to pass a very narrow range of frequencies and highly attenuates the rest. This eliminates the adjacent channel interference. After this, the signals passing through the filter are amplified by a transistor BF494 and are sent to a product detector made by three transistors BC548, courtesy of our friend Humi Barreto (LU3AKZ) that used it in his Solvegj equipment, allowing a very simple design with a very good result. This product detector is linked to a 4MHz oscillator known as Beat Oscillator whose mission is to recover the audio tone. It works as follows: By the filter passes the signal that I tuned with the VFO (that is the station I want to hear), it is amplified and fed to the product detector. My ear can't hear 4MHz, so I have to downconvert the tone and I do this by mixing the 4.000.000 Hz signal with let's say 4.000.700 Hz. Making the subtraction I get a difference of 700 Hz which is what we hear as the CW signal.
That is why during the calibration of the receiver we have to adjust the VFO trimmer till we hear a pure signal without distortion at the loudspeaker.
Another fundamental component of the receiver is the VFO. This is a Vakar circuit with very good frequency stability. It operates between 3.002 to 3045 assuring the reception in all our 40 mts. CW segment. As always with all free oscillators one have to wait from 5 to 10 minutes for its complete frequency stabilization. Before this time the drift is irrelevant.
The last component of the receiver is the audio amplifier. This was made around the very common and cheap TDA2002 that presents a high gain an a very simple assembling. It can feed a little loudspeaker or a headset.
The coils were made on 2.5 cm3 syringes that have an outside diameter of 1.25 cm (0.50 inch). The wire is 0.50 mm2 section (AWG#20).
14 uHy = 65
turns, tap at turn #10 from the ground side.
12.5 uHy = 60 turns.
With T37-61
14.2 uHy = 16 turns AWG 24
12.0 uHy = 15 turns AWG24
With T37-41
14.2 = 22 turns AWG24
12 uHy = 20 turns AWG24
English translation made by Juan Ferrari KG4FSN GACW # 599