’29 MOPA Progress

'29 MOPA Plan
I have completed manufacture of the three RF chokes required for my new MOPA project. Two of the chokes (the ones in the amplifier) are broken into three sections, presumably to reduce stray capacitance or unwanted resonances while the choke for the Hartley oscillator section is one single winding.

The choke forms were first turned on the wood lathe, using Mahogany hardwood scrap, to a diameter of 3/4", and then stepped down slightly, for the space occupied by the windings. The windings, using single silk enamel #31, were then done, still on the lathe, while rotating the spindle manually.

I've always been surprised at how little inductance is really needed for effective choking on HF and it would seem that the 'standard' 2.5mH is much more than is actually required. The single-ended version is just 190uH, while the three-section chokes measure just 240uH, about 10% of what normally might be specified today.
Steve McDonald, VE7SL, is a regular contributor to AmateurRadio.com and writes from British Columbia, Canada. Contact him at [email protected].

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