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Version:

June 12, 2010:
Revised: v2.0

ZL2PD TuneAid

A little device to help tuning up those transmitters and antenna tuners

Introduction


After I built my KN-Q7 kitset 40m QRP single sideband (SSB) transceiver, described elsewhere on this site, I went on to build a matching low power "Z-match" antenna tuner and a sensitive SWR meter. For some reason, I found I needed three hands to tune this apparently simple arrangement – One hand to hold the PTT button down, and then two more hands to simultaneously adjust the controls on my antenna tuner, while simultaneously whistling into the microphone, after delivering the essential voice station identification, of course!

For those new to ham radio, an audio tone or a reasonably continuous level of speech is required into the microphone so an SSB transmitter will generate some RF power. That RF power allows the SWR meter to measure the match between transmitter and antenna, and determine when the antenna tuner is correctly adjusted.

My problem didn’t stop with the lack of three hands. A really bad cold had left me with a decided lack of "whistle-power". Those who suffer from asthma can probably appreciate this latter problem all too well.

Adding a tone oscillator into the KN-Q7 QRP transceiver would have solved the problem, but space inside that radio is very limited. This external TuneAid oscillator was my solution.

The Design


The TuneAid is a simple combination of a timer and audio tone oscillator. It's a 100% discrete parts design, built with common small signal audio PNP and NPN transistors. The circuit diagram is shown in Figure 1.

Pressing pushbutton SW1 turns on Q1. This then turns on the oscillator (Q3 and Q4).  With C1 initially discharged, Q2 is turned hard on, keeping Q1 turned on after pushbutton SW1 is released. So, only a brief press of the power-on switch is required to start the tone generator.

C1 slowly charges through R4, steadily reducing the voltage on the base of Q2. After about 10 seconds, Q2 turns off, turning Q1 off, and of course, this then turns off the oscillator. Diode D1 ensures C1 is rapidly discharged for the next time around. Battery drain in this standby state is minimal, ensuring excellent battery life.

The Q3/Q4 low component count audio oscillator is an arrangement often used for morse code practice oscillators. A small 8 ohm speaker generates a surprisingly loud tone level, and this is held close to the transceiver microphone during the tuning process.


Figure 1: TuneAid Circuit Diagram

Building the TuneAid


The PCB is shown in Figure 2 and the component layout in Figure 3. It measures 36 x 20 mm (1.4” x 0.8”)
 
The prototype was built into a small plastic box just large enough to hold the PCB, speaker and 9V battery. I used one similar to Jaycar’s part number HB-6015. I milled out the speaker slots on the box cover to match the speaker. The speaker was a flat 25mm diameter type recycled from an old cordless phone, but any 8 ohm speaker, large or small, will work fine.




Figure 2: TuneAid PCB




All other components are readily available from the usual sources, the transistors being any small signal types such as a BC548 (NPN) or BC559 (PNP). C2 is a mylar capacitor, but a disc ceramic type will work perfectly well in this circuit.

My pushbutton (Jaycar part number SP-0622) has a bright blue LED built in, and it looks great while the TuneAid is operating. However, this is a relatively expensive part to use for such a simple switch. Feel free to substitute any cheap pushbutton you can find.

 


Figure 3: TuneAid PCB Component Layout




Using the TuneAid


While I still need three hands to manage the tuning process, I don’t have the whistle problem any more. A quick press of the TuneAid button will give me a 10 second tone burst, easily long enough to manage the tuner, and the PTT key.

The battery has lasted for almost 12 months of intermittent use, and it is still going strong.



 

Figure 4 : TuneAid Front Panel Graphics









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