Inside Geoff's Workshop

By Geoff Roberts, G8DHI


Winding A Radio Coil

A radio coil wound on a lathe

The coil was wound on a 4 inch Diameter Paxolin former/tube. Paxolin is a material no longer manufactured. It was used extensively in the Radio industry in the early years from 1920's up to the late sixties. Paxolin is dry to RF and does not have a dampening effect on the inductance like some modern plastics. I found this piece at a local ham radio rally a few years back and snapped it knowing its rareness.

I have standardised on copper wire sizes for my projects ie 0.25mm 0.5mm and 1.00 mm this simplifies winding and calculation of number of turns as each is a fraction of 1mm. This coil was made with 1mm wire which is firm enough and thick enough to withstand the slider running over many times without breaking the windings. It was also easy to work out by practical means the pulley ratios for my winding machine with the 1 mm diameter wire.

A lathe ready to accept a radio coil former

It is very hard to wind a coil by hand and the time factor if you do so is very slow and frustrating, so I set about designing a simple coil winder using an old Drummond Vintage round bed lathe. The lathe was never very accurate for brass turning in my workshop so I put it to better use as coil winder. The lathe has a lead screw and a traversing tool saddle which with the right combination of pulley wheels could wind accurately every time a very tight coil. It works a treat and I have made dozens of coils over the years.

The Coil former is supported between two nylon cones and can be tightened by moving the tail stock. The tube does not slip because I put a blade in the cone which just cuts into the former material to secure it on the cone. The wire is passed over the top nylon pulley and tensioned by the bottom pulley on the saddle traverse.

Old lathe showing the three pulleys

There are three pulleys on the back gear shaft which can be altered to give the lead screw the correct speed when hand turning the shaft. I had to make Nylon pulleys as there were no back gears available for this very old lathe when I bought it. With a little calculation of circumference it was easy to work out the ratios for my standard wire sizes. In bygon days simple machines like this were made by well know companies like AVO Douglas. It was a great project and works very well speeding up winding times and getting a very tight coil every time.

On a slider it is nessassary to secure the ends of the coil so that the winding does not spead apart with the friction of the slider. I did this by turning down a screw head for each end of the coil winding. See above photo. The winding length diameter and number of turns determines the inductance. I knew from previous loose coupler windings that I made that the coil would need to be 200mm to give me the 1000 µH for a Medium wave coil.

Here some of my handmade radio coils.

Bye for now Geoff.


Geoff's Perikon Detector

G W Pickard of the USA was a pioneer radio engineer in the early 1900's who invented the Perikon Detector and the Catwhisker Detector. For a detailed history of these please read the Wikipedia article.

I have made several of both of this type of devices but cannot praise more highly the virtues of the Perikon Detector compared to the more commonly known Catwhisker Detector(Galena and Gold wire) the Perikon Detector (Zincite/Bornite) is in my experience is much better and more stable.

It is easier to set up in that it is a pressure contact device as opposed to a point contact device so not subject to vibration or not being fiddly to use like the Catswhisker. However I have had very interesting and unusual results by making the Perikon both pressure and point contact by selecting a Zincite crystal with a sharp point. Searching for a spot on the Bornite crystal with the tip of the Zincite crystal in a similar way to the Catswhisker yields some very much higher sensitivity 'sweet spots' and because it is spring loaded it is also stable to vibration. No mention of this has been made in any articles that I have found on the web. Diagrams of Perikon Detectors I have seen on the web are just pressure contact devices with two crystals of about equal size in an adjustable spring loaded device.

My detector has both spring loading and point contact . I use a pointed Zincite crystal of reddish orange and fix it in a tube with a grub screw.I also bind a small piece of pewter which is very soft( Tin alloy) to cushion the crystal against the sides of the tube. The Bornite (or Peacock ore)is similarly held in place in a slightly larger tube or cup. I have experimented with the Woods metal method of securing the crystals but found it to be too messy with molten metal ending up all over the place on the brass turned parts.

Here is a breakdown of my detector into its component parts. There are over 60 simple parts to this device although when assembled looks much less. G W Pickard experimented with many thousands of combinations of crystal. This is a few of the more well known combinations of crystal.

I can custom build you a Perikon detector. Have a look at my detectors page.


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