Inductor for TA7642 AM Radio

Thread Starter

SamR

Joined Mar 19, 2019
1,133
I have some TA7642 TO-92's and have been looking at circuits such as the attached. Most show the tank and maybe even the variable capacitor rating but not the inductor size. Would I be wrong in taking the midpoint of the AM band say ~1MHz as the resonant frequency, midpoint of the variable capacitor range to solve for the inductor size? Once I have that size I may want to follow up with winding the coil. Any help will be much appreciated.

 

Ramussons

Joined May 3, 2013
797
I would suggest 1.2 MHz as the "mid" point (instead of 1 MHz) for an AM band 500 KHz to 1500 KHz.
The frequency change is not "linear", but as "parabolic" to the change of capacitance (and inductance).

f ~ root( L/C)
 
Last edited:

Ylli

Joined Nov 13, 2015
760
Commonly, a 240 uH inductor is used with a 365 pF variable capacitor. If your variable cap has a max C of 150 pf, then you will need an inductor of about 585 uH. Depending on the lower limit of the 150 pF variable, and circuit strays, you may not be able to tune as high as the top of the AM band (1600 KHz). I'd suggest using the 240 uH coil and using a 365 pF variable cap.
 

Thread Starter

SamR

Joined Mar 19, 2019
1,133
Thx @Ylli that should get me in the 550 - 1600kHz area. Ordered a range of inductors to play with so should have a 220uH soon. The 12 - 365pF variable air cap I ordered came in and actually measures ~40 - 400pf. If I were to "roll my own" inductor any suggestions as to the size of ferrite rod and wire gauge? Or any other coil winding hints for inductors? I did read the ARRL PDF on building coils. I am assuming the ferrite core would be preferable to air core? I could drag out my antique HeathKit roller inductor tuner but that thing is a bit large.
 

Ylli

Joined Nov 13, 2015
760
Don't know how true it is, as I haven't done much with ferrite rod inductors, but I have read that for maximum Q/minimum losses:
Do not wind the coil directly on the ferrite rod. You want the winding to be about 1 wire diameter off the ferrite. So a couple of wraps of paper, and the windings on top of that. Make mechanically secure with some bees wax.
 

Thread Starter

SamR

Joined Mar 19, 2019
1,133
K Thx guys. I ordered a couple of 10 x 100mm ferrite rods and some 0.07mm x 28 str litz wire. Didn't know about not wrapping directly on rod so thx. Was just going to use Kapton to keep it all in place at the ends of the winding. I've seen Peter Viz's weird site and don't plan on using wood and nails and brass screws so... There are some other interesting ICs I may play with but in no hurry and this should keep me busy for a while. Thx again guys!

EDIT: Also going to put a LM386 and speaker on the output...
 
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