# Capacitors

Discussion in 'Homebrew and Kit Projects' started by KE0M, Aug 20, 2008.

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1. ### KE0MHam MemberQRZ Page

I really appreciate all of your help and I hope all goes well with my project. I did find some big AM broadcast variables off of oselectronics.com and I hope to build more projects from EMRFD in the future.

Thanks again to all and 73 de
KE0M

2. ### KC0ZJZHam MemberQRZ Page

More notes on the DC receiver in EMRFD pg 1.8.
I've built serveral copies of this circuit, for a few diferent bands including 15 meters. I found it to be a very forgiving/flexible circuit. Don't leave off the 12 ohm resistor and 0.1uF cap on pin 5 of the LM386 (output) or else this chip will howl.

If you don't have the 0.22 uF caps you can parallel two 0.1uF.
Also if you can't get any air variable caps, you can try a plastic variable from an old am radio, that is what I did for my first one. It did make the VFO drift a lot more though.

For the VFO tank, and the input tank you can series/parallel caps to make up the needed value.

Here is an easy formula to help:
L * C = ( 1 / ( 2 * pi * f * 1.0e6) ) ** 2 / 1.0e-18

This gives you the product of L in uH and C in pF for resonance at a choosen frequency in MHz (thats why the constants). note (x)**2 = x squared. This formula is derived from Xc = Xl at resonance.

So for 7.0475 MHz this gives 510.
If your coil is 1.5uH you need 340 pF of total capacitance in parallel.

If you do this at both ends of the tuning range desired, you can get the size of the variable cap you need:
At 7.0, L*C = 517 so for L=1.5uH C = 345pF
At 7.125, L*C = 499 so for L=1.5uH C = 333pF

Cmax - Cmin = 12pF If you don't have a variable this small you can use a larger one and "pad" it down as descrived in EMRFD using a smaller cap in series with the one you have.

For 40 meters you can use L from about 0.7uH up to about 2uH and adjust the capacitance to match.

Roger