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Monday 25 January 2010


As variable capacitors are not cheap or easily availabe all the time and the use of a varicap diode does not always appear to be an option then the use of a variable inductor is another option.

The construction requires a bit of thought though.

I had a rummage in the garage to see what I had where a rotary motion could be translated into a push/pull direction easily enough, and the screw in/out action of a window frame fixing seems to fit the criteria and it rigid so I will attempt to recreate one to build a reproducable PTO based on the circuit by Steve Weber (KD1JV) that was modified by KE7HR with a trimmer capacitor to shift the frequency a bit.

I have a pile of straws from MacDonalds, Starbucks etc all with approx 7-8mm diameter that I have meant to use for coil formers to build a PTO with since the GQRP rally a while back.

Construction details.

Circuit build manhatten style on a scrap of pcb. I used J310 instead. Probably obvious but on the diagram orange highlighter blobs are manhatten pads.

On a separate scrap drill an 8mm hole, this is a very tight fit for one of the M5 Nylon nuts I have.

Araldite the nut in place(photo to follow) after making sure the outside is flush with the plane of the PCB.

Solder this board at 90 to the plane so the PTO inductor is above the circuit.

The rest of the construction details and images will follow...

Ideally I want a PTO with 11-11.2MHz, 5-5.2MHz or 2.1-2.3MHz it is all a matter of which IF frequencyI want to use.

First stab... with a scrap of #28 wire.
15T #28 on the clear Starbucks straw is 1.3uH close wound.
Using a 1" brass slug (6mm diameter tube) the inductance reduces to 0.74uH.

This video is of the rough prototype and the frequency shift is from 8.7Mhz to 9.1MHz.
You might need to turn the volume down if you play it!

I'm in the shack and the music in the background is Barclay James Harvest, "Back in the Game" from the album Mockingbird.


Sacrificial Ham said...

What the heck is "araldite"?

M1KTA said...

Araldite is a brand name for a rapid bond epoxy glue.

Check out second picture....

Anonymous said...

Everyone knows "Araldite" is the Greek God of sticky things.

Good work.
Best Regards,

Neil said...

Everyone knows "Araldite" is the Greek God of sticky things.

Good work.
Best Regards,

G7VFY said...

If you are like me, and do not have a fully equipped workshop in your flat then there is a simpler alternative.

If you can get your hands on old AM/MW/LW car radio and a screwdriver, you will find you have the basis of a high quality PTO VFO Receiver. This has the additional benefit of being silly cheap (Bought for about 1ukp on ebay or the local car boot) and, having push button mechanical memory, not to mention, a surprisingly high quality metal chassis.

Stephen Walters