[Hallicrafters] Inductance measurements in capacitors.

[James Liles]

Good morning and hope everyone had an enjoyable holiday season.  Snow was great everywhere --- not sure this antique body will be able to survive the slopes much longer. 

My last post regarding Orange dip, Yellow tubular, and ceramic disc capacitors somehow totally misaligned the number chart that I was using to quantify the effect of inductance within a capacitor and the wording is not there???  Must have fallen asleep at the keyboard too much family fun or something!!  

I started this thread dealing only with the measurement of capacitive inductance because a conversation regarding the various capacitor forms is unbearable for the overly emotional folks which I guess is a somewhat normal.  Would like to have had a courteous dialog regarding this subject and hope to at some point in time but I guess now is not that time.  Moving on.  

Thanks again for your interest.  There were too many requests for a direct response so I wrote the technique in the last post and its here as well.  If you find it difficult to understand I’ll re-write it and refine the wording and if you have any questions please drop a note to this thread or if your uncomfortable doing so, send it directly to me; there are no dumb questions.

Carl has agreed to replicate the measurements that I recorded using his HP Q Meter.  We will compare the results and if there are any significant differences, I’ll refine the setup or procedures.  





What you will need:


1 - 50pf to 550pf variable cap
1 - 12pf or so piston trimmer or easy to turn trimmer
1 - coil wound over a 3/8 inch dowel pencil or what have you, equaling 1.0uh
1 - coil wound over a 3/8 inch dowel pencil or what have you, equaling   .4uh
1 - signal generator
1 - scope
1 - bit of patience
1 - flattened 3 inch piece of RG58 coax shield

The caps that I tested had 3/4 " leads --- probably should have made an 
adapter to accommodate 1/4 " leads --- that’s to come later.

The procedure cannot be original as there seems to be no other way to accomplish this measurement.

For the yellow .01ufd cap, I attached one end of the 1uh coil to one side of 
the large variable and trimmer cap, attaching the other side of the coil 
and capacitors to separate binding posts.  between the binding posts I added 
the .01 ufd test cap and the flattened coax shield in parallel.  We will remove and re-attach the coax shield throughout the test to add and remove the test cap.  Then add a one turn coupling loop to one side of the coil, not over it.

Placed the scope probe close to the coil end or cap and use the shorting 
strap to shunt the test cap.  Now set the signal generator to 5Mc and 
resonate the circuit.  Next remove the shunt on the test cap logically 
placing it in series with the coil and caps and re-resonate.  If you had to 
add capacity to re-resonate, INCREASE  the frequency and if you had to 
reduce capacity to re-resonate REDUCE frequency.  Now , re-attach the shunt 
and resonate. Then remove the shunt and resonate.  And again, if you added 
capacity , increase the frequency and if you reduced the capacity to 
resonate reduce the frequency.  At the point that the cap is self resonant 
(Series) removing and adding the shunt will have no effect on the resonant 
point but will indicate a slight drop in amplitude on the scope because the 
cap is series resonant and behaves like a pure resistance in series with the 
tuned circuit.

For the Yellow tubular cap I measured 9Mc using 3/4" leads --- need to try 
1/4"leads but that’s for later.

Now I can calculate the inductive reactance and inductance as it will be 
equal to the capacitive reactance.

What we intend to accomplish with this process is to move the frequency to a 
point where the test cap is purely resistive/resonant.

Capacitive reactance for the .01 yellow cap at self resonance is approximately 1.76 ohms --- inductive reactance 1.76 
ohms and the inductance  .031uh  calculated.


Conclusion:  Numbers with the yellow .01uf polypropylene cap


                                                              Ohms               Ohms            Ohms
                     Effective inductance        Capacitive      Inductive     Effective
At 1Mc      the capacitor will have          15.9                   .19            15.71C
     5Mc                                                      3.18                 .97              2.21C
     9MC                                                      1.76              1.76      purely resistive
     From this point the effective nature of the capacitor will behave like an inductor
     10Mc                     .0056uh                  1.59               1.94                .35L
     15Mc                     .0196uh                  1.06               2.92              1.86L
     20Mc                     .0247uh                    .079             3.89              3.10L

If you do the measurements with this configuration, please forward the results to me, would like to compare the numbers for the .01 and .001 caps.

I believe that this will be an interesting experiment with inexpensive home brew components.  

Kindest regards Jim K9AXN