Several people have sent me E-Mail regarding the coil formers I am using for my projects. Well, I was fortunate enough to get my hands on some scrap radio boards, each of which contains about 15 coils that I re-use. The formers will fit snugly in a 6mm hole in the PCB. If you know of a source of these formers then PLEASE contact me so I can buy a few hundred more.
These formers are 5mm dia. and fitted with an adjustable ferrite core. If I do not require a ferrite slug then I do not waste the former, I then use a bit of wooden dowel the same diameter. These formers agree with my usual formula for turns / capacitance for a given frequency:-
For example, if I wanted a tuned circuit for 10MHz then the wavelength of 10MHz is 30 meters, so with 30 turns and 30pf the coil resonates at about 10 MHz. In practice, it will resonate between 10MHz and 14MHz as the ferrite is moved from top to bottom so the formula is a little high, but it will get you in the right area.
VHF coils do not follow this rule but for 145 MHz a 6.8pf capacitor will resonate with 4.5 turns of tinned copper wire. Wind the coils on a 5mm drill bit so that they can then be slipped over the former when required. I usually form the leads before fitting to the former in order to reduce the possibility of damaging my precious formers.
The completed former and VHF coil can then be fitted to the PCB. Note that I usually use double sided PCBs for VHF work and countersink the upper "gound" plane when a component lead is not to be soldered to earth.
These coils I use on my VFH FM Transmitter (BUG) (not the PCB version), Simple FM transmitter, General Coverage valve (tube) receiver, in fact all my projects where the coil formers are not specified. I hope that this clears up a lot of the confusion and will enable many of you to follow my circuits a little more easily. If you use any other former then you will have to adjust the number of turns to suit. The size of wire is of little importance and even the turns spacing has only a minor effect on frequency. Have fun, de HARRY, Upplands Vasby, Sweden.