I used to view the inductors of RF circuits in books and magazines as problematic, often shunning any radio circuit having coils which are difficult to find . RF inductors of the correct value, unlike standard components, are not easy to buy, if at all. However, there are some great radio circuits around and it's a pity not to be able to reproduce them just because you normally can't acquire their coils off-the shelf. Many hams still build all their own equipment, everything from antennas to hand-made variable capacitors, which are almost carbon copies of their manufactured counterparts. Hams are by nature an inventive, improvising, lateral thinking bunch - with a little maths, patience and guesswork it's easy to roll your own inductors? Don't be inductor-imponent... make your own!
What you need:
1. Dip Meter. A dip meter has many uses, but we just need it as a means to measure the inductance of our homemade coils.
2. HF Receiver. Since most dip meters don't have very precise frequency panels, use your hf transceiver to find where your dip meter carrier is exactly.
3. Coil Formers. All sorts of coil formers, ferrites and sheilds, can be salvaged from old radios and television sets for rewinding. These small coils were taken from a trashed vhf mobile radio - can you spot the ones which have been re-wound?
4. Simple Jig. A piece of wood with some nails and/or pins is sufficient to hold the coil former in place during winding and glueing. Try using elastic bands or a small weight to keep the coil's wire taught while the glue is drying.
5. Wire. If you can't buy thin enameled wire, old transformers are a good source. Break them open and salvage the wire.
Rough calculation. You can roughly calculate the dimensions for an air spaced coil by following the guidelines, formulas and tables found in much of the popular ham reference literature. Chapter 6 of the ARRL Handbook is a good source. This can give you a good place to start. Or you can experiment with varios number of turns and old coil formers to find a solution that gives the coil specification you require.
Use salvaged coils. I had a few old coil formers in the drawer taken from a trashed vhf mobile radio. Solder a capacitor of known value across the coil. Beware of the tolerance factor which in some capacitor types is quite high, a good option is to use a polystyrene capacitor which has small tolerance values but don't worry if these aren't available, it's not too critical at the experimental stage.
Now we use our dip meter. This device is nothing more than a calibrated oscillator which can be tuned over a wide range of frequencies. A meter measures the amount of feedback to the RF oscillator and will show a dip when its frequency is tuned to the same frequency as the circuit under test. This is because the circuit will absorb some of the energy from the oscillator and thus the amount of RF fed back. Therefore, by placing our unknown coil-known capacitor tank circuit near the dip meter's coil and tuning, we will get a dip in the meter reading when we tune the dip meter to a certain frequency. This is the resonant frequency of the tank circuit under test.
How many Henrys? Since we know the capacitor value and now its resonant frequency we can calculate the coil's inductance using:
Reading frequency. By using a HF receiver to find the dip meter's carrier frequency, we can achieve greater frequency precision. Most dip meter scales are not very accurate so don't rely on these for the frequency reading.
Fine tuning. By adding or removing turns of wire in the coil and repeating the process, we can obtain the exact inductance value we require - remember, the more turns the greater the inductance.
Ferrites. If using coils having ferrite tuning slugs, the inductance value with the ferrite removed, all the way in and at a half way point should be measured. This way we know the ideal number of coils needed and the inductance swing which the ferrite will permit - usually quite large. Don't forget that ferrite slugs must always be adjusted using non metallic instruments otherwise they WILL break! Never compromise on your tools! Ideally, use purpose made plastic adjusting tools, otherwise use a matchstick or a filed to shape plastic knitting needle... don't tell the XYL!
Other points to consider. Coils which are physically short and wide, as opposed to long and thin, will have higher Q characteristics, and if the coil has a shield placed over it this will lower its inductance slightly - easily compensated by adjusting the ferrite slug.
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