ANTENTOP- 01- 2004, # 005

Construction

We heavily recommend constructing over pieces of un-etched PCBs. They are cheaply available everywhere. See the pictures as a guide to component layout. We recommend the following rules:

►Keep your leads short. Short connections are more important than components that are at right angles to each other. What might look neat to you might look unstable to the RF design.

►Keep the outputs and inputs isolated from each other. We have taken care to keep the high impedance points down to a minimum. But still, maintain design hygiene.

►Make one module at a time, test it completely, then move to the next one. Construct the transceiver in the following steps:

►Make the VFO. Check the RF output using an RF probe. Check the stability on a regular receiver or a frequency counter. With the tuning capacitor fully closed (the plates inside each other), set the trimmer so that the VF0 frequency is exactly 3.9995 MHz (keep 5 KHz margin at the band end)

►Make the BFO. Check the output on the RF probe.

►Calculate the ladder filter values and make the IF strip along with the audio preamplifier.

►Connect the BFO, VFO, IF strip and an external audio amplifier together. When you power on and attach a piece of 2-3 meter long wire to the input of the IF amplifier you should be able to hear the atmospheric noise. Tune the BFO coil by fully screwing the slug in and then slowly tuning it out until the IF noise sounds right (not too shrill and not too muffled).

►Wire up the receiver mixer, connect the VFO. Peak the mixer output and the RF input coils for maximum output. Then tune to a weak signal on the band and tune for the best signal. Be careful to tune for best quality of signal and not for maximum loudness. Take a break, spend a day or two listening to the band with your receiver. Nothing is more enjoyable than using a crisp receiver that you have homebrewed.

►Wire up the modulator. If you have an oscilloscope, you can check the modulation. The modulated output will be too low for you to be able to measure on the RF probe.

►Wire up the linear chain. DON'T solder the IRF510 yet.

►Put the transceiver in transmit mode. Whistle into the microphone and peak the transmit mixer output coils for about 6 volts peak RF voltage on the probe at the 56 ohms resistor where the gate of the IRF510 would be.

►Solder in the IRF510. ATTACH A DUMMY LOAD. We used four 220 ohms two watts resistors paralleled together.

►Keep the bias trimmer totally down towards zero. Attach VOM in series with point X in the power amplifier. Apply power in transmit mode and slow increase the bias until you have 80mA flowing through the IRF510.

►Connect the RF probe across the dummy load.

►As you whistle, You should get about 20-24volts of peak RF on the probe. When you pull out the microphone from the jack, the RF output should drop to complete zero. What if your transmitter is unstable?

• Don't curse your fate. All transmitters start out as unstable beasts. Relax.
• Start disconnecting power from the stages starting from final IRF510 and working backwards. When you have located the unstable stage, there are a number of things you can do to fix it.
• Try increasing the value of the 10 ohms resistor used in the emitter degeneration OR
• Strap a resistor of about 1K across the output transformer of the unstable stage to 'load' it.
• Move the linear amplifier away from the rest of the circuitry.
• Redo the board. This time spread the stages out. We guess that the linear chain should occupy about 6 inches of space, all laid out in one line.

The BF195 transistors can be substituted with any other HF transistor like 2N2222 etc. The 2N3866s are best not substituted. The circuit works with slight increase in the noise figure if BF195 or equivalents are used in place of 2N3866s in the IF stages. The output power on the transmitter absolutely needs the 2N3866s. Substituting them with other switching transistors didn't give good performance.

The IRF510 should not be substituted with any other transistor. The other IRFs, though rated higher, have higher input capacitance which makes them a bad choice for 14MHz operation.

The LM380/LM386 can be substituted with almost any other audio amplifier. Our first amplifier was Cambridge SoundWorks Sound System. If you turn down the bass, they are an excellent system for the shack. We have tried a TBA180, an LM386, an LM380 and even a glow-bug guitar amp. Feel free to experiment.

Final Notes

The first contact we made using this rig was DF6PW. He reported us 57. Within the first evening we had worked four continents. The rig is regularly used at VU2PEP. People are often surprised at how the transmitter quality is 'just like a commercial rig'. Many refused to believe that it is a seven watt rig.

72/73!

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