Measuring on a commercial AM-filter from ITT, type 931 AM with my DG8SAQ VNWA

From start the filter specifics was unknown, so I made an experimental measuring with a 47 OHM resistor in series
with a 1 K Trim Pot and made a sweep.
I found that the best and flat top-of-filter was close to, but a little less than 940 Ohm.

Then I figured the transformation between VNWA (50) and the filter (900 Ohm): SQR of 900/50 = ~ 4,25.

I made a Toroid-transformer, 4-filar ( 4 wires wrapped closely with a drillingmashine), type: 4C65 ferrite,
9 windings total, then connected as shown.

One side to the other. Then I connected the filter in this way:
2 pc. trimmer at 25 pF is added.

Setting the frequency to 10,7 MHz, plus/minus 250 KHz and CALIBRATING the VNWA,
I connected the WNVA:

You will possibly need to make a screen between filter input and output to avoid overhearing. I did by hand.

And here is the result:

  • Centerfrequens: 10,7 MHz
  • Width:
  • 3 db = 6 KHz
  • 50 db = 10 KHz
  • 90 db = 16,5 KHz
  • more than 90 db deep - good shapefactor.
  • about 2.1  db loss in center
  • ribble les than 1 db
  • quit good SWR

In the 1970'ties I used that filter in a homemade 2 meter SSB-rig and it was working perfectly.



A close-up picture:

The data says:

That is it!!!

OZ6YM, dec. 2011.