Electronics > RF, Microwave, Ham Radio

What are Radio coil types and how to wind them?

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Hello everybody,
I'm working on making a crystal radio, and i ran into a problem...
 the radio i made was not working, then i salveged one out of an old am radio, it woked fine after that...
Unfortunitly i can not use the coil due to its massive size so i would like to know What are Radio coil types and how to wind them?

It looks like a lot of people have viewed your post but none have answered, perhaps because it is a little too vague. I will make an attempt.

In electronics there is only one type of coil or inductor, a length of wire wound in a spiral and then connected at both ends. Two things make a difference to the inductor, the core material and the 'Q' of the coil.

Coil Material
The simplest core material is fresh air, an air cored inductor, and an example can be seen here: http://www.eham.net/data/classifieds/images/266326.jpg Unless you use a VERY large coil this will be no good for a crystal radio because the value of inductance will be too low. Most coils use ferrite cores and these can either be in the form of a circular ring or toroid or in the form of a slug which is adjusted in and out. There is also the ferrite rod antenna (normally about 15cm long) that can be seen inside most Medium Wave and Long Wave radio receivers, these would be ideal for a crystal set: http://www.petervis.com/Radios/national-panasonic-rf-1103/national-panasonic-rf-1103-ferrite-rod-antenna/national-panasonic-rf-1103-ferrite-rod-antenna.gif Note that a ferrite core will only operate over a range of frequencies and if you use the wrong core your radio will not work.

Coil 'Q'
Coil Q is a measurement of the efficiency of an inductor, the higher the Q the better the coil. If you wind an air cored inductor maximum Q is obtained when a) the diameter of a coil is equal to its length b) the resistance is as low as possible and c) the distance between the windings is equal to the thickness of the wire. This can mean some large coils (see above) but it can be done http://home.earthlink.net/~belundy/xtal-set.html When a ferrite core (especially a ring core) is used and the ferrite is used inside its working frequency then Q is increased by a very large amount which means that a smaller coil can be made. If the ferrite core is outside of its working frequency then the Q of the coil is lowered and you now have an RF choke, something which is still an inductor but is used to suppress RF signals on power lines  https://www.radioshack.com/collections/connectors-connectivity_instore/products/radioshack-100-h-rf-choke?variant=5717358789 Yes, it does look just like a small ferrite rod antenna but the core material is different.

So, in summary, for your crystal set to work:

1) You need either a large air core inductor or a ferrite rod

2) You need the right number of turns on the inductor

3) You need to wind the coil for maximum Q by taking care to space the turns using wire that is not too thin because of resistance losses.

This post turned out longer than I planned but I hope that it helps. I am aware of the Internet restrictions in Saudi Arabia so some care has been taken with the links which are all safe. Hope this helps!

Hi,  It might help if you explain more about what you are trying to do.  The traditional method to make crystal set coils is either pull one from an old am radio (like you did) or wind your own 'air cored' on an old toilet roll tube or similar.  The purpose of the coil is to form a tuned circuit with the capacitor which allows you to (sort of) tune in individual stations.  For this to work as well as possible, the Q of the tuned circuit needs to be as high as possible which usually means large-ish coils as German_EE explained.  You could probably get a small SMD inductor to tune but the performance is likely to be terrible.

For Medium wave AM broadcasts, its 60 turns of enamelled copper wire on a ferrite rod 3 to 5 inches long.
Just wind it by hand, easy to do. Then a dab of hot glue to hold the ends in place and stop it springing out. Keep the turns close together, side by side. :)

Its better to make up a sleeve for the rod to wind the coil on.  Start with the middle of the rod wrapped in clingfilm so you can get the sleeve off again.  Then wrap a strip of cartridge paper round the rod, securing it with superglue.  Build it up a couple of turns, saturating each with superglue. Let it all fully cure, slide it off the rod (impossible without the clingfilm), clean out as much as possible of the clingfilm from the center of the sleeve, and wind the coil using the sleeve as a former.   The start can be anchored with a narrow strip of paper and superglue.  Tack the end into place with stickytape in case you need to add or remove turns.   Once you are satisfied you can get the range you need, lock the end in place as you did the start and 'dope' the coil with superglue.   

If your set design calls for a tapped coil, use magnet wire with solder-through enamel, and for each tap, make a 4" loop, twist up just the base of the loop tightly, and solder the twist, taking care to get a good joint.  Cut one side of the loop at the twist to get an 8" tail.  Be careful NOT to overheat adjacent turns.  It may be advisable to wind the tap turns ONLY over a narrow strip of cartridge paper placed lengthwise along the rod to protect the other turns from the heat of soldering.

The total inductance of the coil and thus the tuning range can be adjusted by sliding the sleeve bearing the coil off-center on the rod.  Adjust it with the tuning cap's padding trimmer(s) initially set to 50% and the main vanes fully engaged (max capacitance) for the desired minimum frequency.   Use the padders to adjust the maximum frequency with the main vanes disengaged (min. capacitance).  You will probably need to readjust the coil (without touching the padders) and then the padders to get the dial limits correct.  When the coil is correctly set, lock it in place with a light dab of hotglue or beeswax either end.

When mounting the rod, its important NOT to have too much metal near it and absolutely critical NOT to have any metal loops round it or plates across the ends.   Cable ties direct to the PCB or plastic P clips are good.  If there is a metal chassis, use plastic standoff pillers under the P clips.


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