[AMRadio] Help with Baur 707 transmitter

JAMES HANLON knjhanlon at msn.com
Sun Dec 28 19:23:20 EST 2014

Don,

I found a listing on the J W Miller 43A224CB1 coil.  Its form is 1/2 inch diameter and 1.5 inches long.   It's nominal inductance is 220 micro henries, max inductance 275 micro henries +/- 5%, min inductance is 138 micro henries +/-10%.  Its minimum Q at minimum inductance is 128 measured at 790 kc.  Its minimum Q at maximum inductance is 168 at 790 kc.  Its max dc resistance is 3.72 ohms.

All of that doesn't help answer your question of "how many turns?"

If I use a formula for the inductance of a single layer solenoid coil from the Editors and Engineers Radio Handbook, I calculate the following number of turns.

For an inductance of 128 microhenries (the minimum, assuming running in the slug will increase the inductance) and for a coil length of 1/2 inch, the number of turns is 122.  For a coil length of 1 inch, the number of turns is 158.  For a coil length of 1.5 inch, the number of turns is 188.

Getting that many turns within the specified length is pretty near impossible.  That suggests that the original coil must have been a multi-layer solenoid.

Checking the capacitance necessary to resonate with the values of inductance specified at 1600 kc, I get:

For L = 138 microhenry, C = 71.7 pf .
For L = 220 microhenry, C = 45 pf .
For L = 275 microhenry, C = 36 pf .

My guess is that this coil is being resonated by whatever tube plus stray capacitance may be in the circuit and that the slug is being adjusted to bring it into resonance.

You could always wind whatever size inductor onto that form that you could fit with whatever wire you have available and then resonate with some additional capacitance.  Or ...

I may be able to help a little.  I found two coils in my stash that appear to be wound on the Miller Jumbo Adjustable Coil Forms.  They are ceramic forms, 1/2 inch diameter, 1.5 inch length, with 1/4 inch mounting screws and an adjustable slug that I'm guessing is powdered iron.  Both forms have multiple layer coils on them.  I can do several things with them that might help.

I can measure the inductance of each of these coils with my Boonton Q meter.  On the off chance that one of them is what you need I could send it to you.  If their inductance is too large I could take off some turns until the inductance falls within the range of the coil you are looking for and then send that to you.  If sending a coil to you would take too much time, I could just continue to unwind the coil that I measured as having your wanted inductance and tell you how many turns and layers it had.  That would give you a starting point at winding a coil yourself, but if you did not have the same size and insulation wire as is on my coils you might turn up with a bit different inductance.

So, what if anything would you like me to do?

Jim Hanlon, W8KGI