The 147.120 Tech Net Blog

Well, I’ve been working on linear dials for radios for a long time and just haven’t been able to finalize it until just recently.

I realized that the Tamaya gears I had obtained from Colonial Photo and Hobby would screw right on the front of the variable capacitor in the BiTX20. It didn’t take me long to dig out my Universal Plates and pulley hardware and come up with the nifty item pictured below.

Linear Dial by N4LGH

Linear Dial by N4LGH

Side View showing the gears and pulleys

I’ll make a post to the Wiki in a few weeks with detailed steps on how I go about actually setting the thing up to work with a real radio. Right now this is a prototype and I don’t know exactly how I’m going to go about the final layout. The marvelous thing about gears is they can be physically laid out just about any way necessary as long as the sequence is correct.

The end result will be a 2 or 3 inch cube that can be mounted behind the front panel with the dial pulleys mounted to the panel as well. I have started to lay out a brass tube runner for the dial pointer which will be very small brass rods (almost wire) soldered onto the brass runner.

It is a 72:1 ratio giving 36 turns to cover the 350 kHz wide 20 meter band. This theoretically exceeds my 10kHz per turn requirement but realistically this is not a ‘frequency linear’ capacitor so it will be very wide at one end and very tight at the other. My luck the tight end will be the upper end. Still, with 72:1 ratio it will have a very smooth tune even at the tightest part of the band.

The feel is decent, the action overall smooth for the course gears I used. The only backlash in the system is introduced by the rubber band type belt I put on for the test. I will get some real dial string and a spring to load the pulleys and it should be smooth as silk. With a flywheel on the other end of the knob shaft it should have a nice ’spin’ to it!

The only problem it has right now is the gear ratio is very strong and will twist the screw right out of the capacitor without you even feeling it. Once I have the runner made I’ll put stops at each end and there won’t be any problems with the gears unscrewing from the cap.

I chose these parts because they can be obtained at any hobby store and over the internet, as are the pulleys, rods, and universal plates.

I have purchased a lot of gears over the past months but to estimate I think one would have about $15 – $20 in this. If several were to pool and get the larger bags of the gears and rods and would be about $10 – $15 each to make as a group buy.

I have a large gear set up to take the 1/4″ shaft of a regular tuning capacitor. With the strength this thing has, there’s no doubt it will turn it as well.

Dig in!
Tracy N4LGH