Sunday, 27 December 2015

QRP is synonymous with minimalism:

"Experience has taught me that relatively few radio amateurs build complex equipment. Rather more of them are inclined to build small projects, which can be completed in an evening or a weekend, cost very little in parts, and will probably work first time. It is satisfying to go to bed knowing that you have built something with your own hands and it worked. Few of us get the chance to do that these days
Here are two simple projects, both of which could be built in a couple of hours or less. Both of them should work first time if built with care. Hopefully they will provide practical applications for the QRPer."-Rev. George Dobbs G3RJV.
The drawing shows the circuit of the VU Mini Transmitter. T1 is a Colpitts oscillator with C1 and C2 forming the capacitive feedback divider. The oscillator is crystal controlled but VC provides some useful frequency shift. VC can be a small variable capacitor or a trimmer. The maximum capacitance should be in the 50 to 75pF range. Larger values offer more shift but attempting to move the frequency too much will produce instability and eventually the oscillation will cease. The addition of an inductor of about 16uH will offer several kHz of frequency shift. (see VXO Option)
C3 couples the signal to the power amplifier, T2. The biasing resistor R5 controls the output of T2. Usually a value in the range 33K to 100K is suitable. The higher the value - the higher the output of T2. 47K is a useful starting value for experimentation. I suggest that T2 is not run higher than 1 watt of RF output power. It will get warm and a small star heatsink ought to be fitted.
The collector load for T2, RFC1, is a home made RF Choke. Carefully wind 12 turns of 38 swg (about 34 awg) enamelled copper wire (any small enough gauge will do) through a ferrite bead. RFC2 is a small 100uH axial choke which is essential when using T2 as the receiver mixer. It also provides a useful RF load on the input of T2 and increases the drive to T2. The transmitter output is coupled from the collector of T2 via C4 to a low pass filter. The original article appeared in GQRP CLUB Magazine SPRAT and was later included in qrp MAS projects and went viral among QRP enthusiasts. This simple transmitter can easily be converted into a CW transceiver by adding a BC517 audio amplifier to drive a crystal earphone providing a very reasonable sensitivity! In winters of 1992, on first evening of use after its completion it fetched me three UA9 and one EA on 14.035MHz...!!
This is not a sophisticated automatic keyer but it is lot QRP to build and to have fun operating it.
When the paddle is connected to the DOT terminal, C1 starts to charge. When C1's charge reaches sufficiently high value, it causes the transistor BC157B to conduct and pulls the relay and key the transmitter. Paddle connection is then momentarily disconnected through the relay, discharging the capacitor and thus stopping the transistor to conduct. The relay releases and paddle connection is restored to ground. The cycle repeats until the paddle is released.
The dash paddle works in the same fashion. The 10 k preset set the dot - dash ratio. The 47 K potentiometer is mounted on the front panel and acts as speed control. 2K2 preset should be adjusted for soft most operation of the keyer. Can you imagine any simpler QRP Paddle Keyer?
So things should not always be complicated to get go, and there is lot of fun these Simple designs have to offer you. You seldom need to sell your soul to get on the air.............and simplicity and minimalism is the way.

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