— Posted by Hergor on 6:30 am on Jan. 23, 2002
i got a problem building fm-bugs.
i got nice circuit-plan for a 2-transistor fm-bug that is powered by a 9V battery but it does not really explain what the air coils are supposed to be like. unfortunately those seem to be the important/critical parts in that circuit.
can anyone explain to me what their function is?
— Posted by ActionTimedDeath on 7:12 am on Jan. 26, 2002
They’re inductors and they’re for resonant parts of the FM oscillator, filters, chokes and so on. Their function depends on which part of the circuit they’re in.
If you know the wire thickness and the diameter of the coils you can easily make them around a pencil (you knew that anyway) If you are told just the number of turns that will do – being air-cored inductors the wire will be pretty thick anyway.
If you are told the inductance (in mH) then you can calculate the number of turns. Can’t remember this off the top of my head since I’ve just had a Speech Recognition and Communications exam and my head is fried. (must be fried anyway to write like this..)
But if that’s the case, then just say.
— Posted by Hergor on 4:41 pm on Jan. 27, 2002
yeah, thank you ActionTimedDeath ! 🙂
i went to the radioshack and asked them till they turned nuts and found that out, too.
btw, the formula is:
whereas: L= inductance in microhenries, a= diameter of the coil in inches, b= length of the coil in inches, c= radial depth of the winding in inches, n= total turns of wire
— Posted by drybilgeltd on 5:26 pm on Mar. 9, 2002
You done good. I looked in several of my Electronic books and only came up with a nomograph that has five vertical colomns of various tables of values that you use a straight edge across. From left to right the column labels are (1)number of turns, (2) plot axis no values just a pivit point, (3) inductance in microhenries, (4) ratio diameter/length, (5) diameter.
Just like more than combination of coins will add up to a dollar, more than one combination of length – diameter – number of turns wil equal so many microhenrys. This chart is for AIR core coils. Ones with Ferrite or Iron in them is different and will depend on the nature of the core material and a bunch of other variable stuff.
I’m going to write your formula in my note book. I think it may be in another book that I own but do not have it with me called ” Radio Operators Q&A Manual by Milton Caufman”. I studied that book for many years to get my FCC 1st class Radio Operators license ( with ship radar indorsment ).
If ne1 is interested I could scan the page and post or E-mail it somehow. Is is a very old book (1942) called The “Radio” Handbook. I found it on the Island of Guam when the Military had me over there about 20 years ago. A couple years ago I found a second copy same pub date in a thrift junk shop in Maryland. It is for Hamradio dudes and us madscientist types.
(Edited by drybilgeltd at 9:30 am on Mar. 9, 2002)
— Posted by Hergor on 5:10 am on Mar. 10, 2002
if you could send me some pages it’d been nice… thanks. 🙂
— Posted by drybilgeltd on 8:54 pm on Mar. 12, 2002
I have a scanner but have not posted or E-Mailed any scans. This BUZZBOX that some people call a computer did not come with any usefull instructions for the programs that are on it.If someone would give me instructions on how to post scans to this site or E-Mail scans, I would be very gratefull. My E-mail is email@example.com
— Posted by Hergor on 4:28 am on Mar. 13, 2002
you could save the scanned image on your computer as .jpg or .gif (.jpg is better for photos or illustrations while .gif is for schematics and scetches) then open a new e-mail and click on ‘insert’ or something at the top menu bar and then on ‘attachment’ (this is the important thing) then select the scanned image and click on ‘ok’. send. voil…
sorry, i got your IM but forgot to answer it… 🙁
— Posted by drybilgeltd on 7:59 pm on Mar. 13, 2002
Thanks Hergor. I found coil formulas in another old book called Radio Engineers’ Handbook pub date 1943 McGraw-Hill Book Company, Inc. It is one of my bible’s, the other one is King James Verson. Depending on the coil shape round, rectangular or other cross section shape what the actual formula is. Best way to handle that math is with a real computer, my Commodor 128 in basic with what is called a defined function.
Send me your e-mail and I’ll send the scans.