Quote:
Originally Posted by ms660
""Ok I think I have a better understanding of this now, in the circuit is one wire connected 1/4 of the way up the coil in order to provide a tap? If that's correct then I could adjust the coil to fall within the MW range? I will probably not bother with the ferrite rod if that's the case. Sorry I'm very new to all this but I'm sure I'll pick it up eventually.""
Yes the cathode is connected 25% up from the ground end of the coil in the schematic.
The coil in the schematic will not cover the MW band, here's why, the coils inductance is given as 5uH (5 microhenries) the tuning capacitor has a maximum value of 365pF (365 picofarads) the minimum value isn't given but it will be around 30pF or so when all's done, plug those values into a resonant frequency calculator like what's in the link below, the two different resonant frequency results for a capacitance of 365pF and a capacitance of 30pF will be the approx tuning range limits of the tuned circuit using the values given above, eg: approx 3.7MHz to 12.9MHz:
http://www.pronine.ca/lcf.htm
For MW you will need a larger inductance, to get an idea of what value you can use the same calculator, you already know the approx minimum and maximum capacitance values.
For a single layer air cored coil you can use the calculator in the link below, for instance 131 turns of 26 SWG enamelled wire close wound on a 35mm diameter former will give an inductance of approx 273uH which would put you in the ball park for MW coverage using the 365pF variable capacitor:
http://www.66pacific.com/calculators...alculator.aspx
Lawrence. |
Ok so say I got a 500pf variable capacitor instead, would that work for MW? I would like to understand the calculations but I have never been good at theory or mathematics in general! I suppose the best way is to experiment with coils until I understand. I imagine the higher the maximum the capacitance the better though, within reason of course. I will try to find a variable capacitor with a higher value.
EDIT- Upon rereading your post i realised I was clicking the wrong link for the calculator, I have done the calculation now and it doesn't seem to make much difference having a higher capacitance. I will need to modify the coil it seems.