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W3LPL Receiving Bandpass Filters

The following is extracted from a July/August 1985 NCJ article (or was it Sept/Oct 1981?? The page I have pasted in my notebook has dates from both issues, neither of which I have any more. The original article was written by N3RD and K3ND.

Maybe later I will draw up some pretty .gifs of these, or someone could scan the originals. But for now bear with my cheap text drawing.

             C1           C2           C3           C4
IN ]---------||-----------||-----------||-----------||--------[ OUT
                  |   |        |   |        |   |   
                  |   |        |   |        |   |   
                  |   |        |   |        |   |   
                 ===  $ L1    ===  $ L2    ===  $ L3
               C5 |   $     C6 |   $     C7 |   $   
                  |   |        |   |        |   |   
                  |   |        |   |        |   |   
                  ---------------------------------------Ground

Band   C1    C2    C3    C4    C5    C6    C7    L1,2,3   Core   Turns
---- ----  ----  ----  ----  ----  ----  ----   -------   ----   -----
160   600   100   100   400   720  1200  1000    5.3uH    T50-2    30
 80   300    50    50   200   360   600   500    2.7      T50-2    17
 40   150    24    24   100   180   300   240    1.4      T50-6    17
 20    75    12    12    51    91   150   120    .7       T50-10   15
 15    51     8    10    36    62   110    80    .45      T50-10   10
 10    38     6     6    26    46    75    60    .35      T50-10    7

All capacitors are Silver Mica.
Toroid core numbers are Amidon designation.
Wire for coils is #24 enameled

Claimed Characteristics:

Other nice thing I have found. They have a very high input impedance outside of their passband. This makes them useful for splitting Beverage antennas between multiple receivers.

Construction is simple, use small aluminum miniboxes and a perf board prototyping board. Use Phono jacks for In and Out, keep leads short. The recommended layout has the string of C1-C4 going up the middle of the board with L1 and L3 on one side and L2 on the other side of them to maximize the distance between the inductors. The whole thing can be fit on a board about 2 1/2" long and 1 3/4" wide.

Tune by spreading turns on the toroids until the bandpass is centered in the middle of the band. I had to adjust the turns in some of the coils to get them to line up right, may be due to variations in capacitors or toroid cores. A spectrum analyzer would be nice. I had good results using a sweep generator and O-Scope and a reciever as a marker generator for the scope.


The following came from an internet e-mail message and appears to be an updated design:

                       
                      W3LPL RECEIVING BANDPASS FILTERS

Receive only filters optimized for minimal loss and very high rejection of 
frequencies below 75% of the filter center frequency.  

These filters use very high "Q" high impedance resonators consisting
of powdered iron core inductors and silver mica capacitors.  The resonators
are lightly top coupled to each other with minimum value silver mica
capacitors.  

C1/C2 and C6/C7 form capacitive voltage dividers to match the input and
output resonators to 50 ohms.

Be sure to use good quality RF connectors (UHF, BNC or N; not phono!)
Do not substitute cheap inductors or capacitors!  The components specified
below are not difficult to find, nor are they expensive.
               
50 ohms --- C1 ------------ C3 ------------ C5 ------------- C7 --- 50 ohms
                  |    |           |    |          |    |
                  |    |           |    |          |    |
                  C2   L           C4   L          C6   L
                  |    |           |    |          |    |
                  |____|           |____|          |____|
                    |                |               |
                    |                |               |
                  Ground           Ground          Ground
                  
                  
                             TABLE OF VALUES
 
                                         INDUCTOR  (all three identical)
Band   C1   C2   C3   C4   C5   C6   C7  turns  wire size  powdered iron core
 10    36   47    7   75    6   56   27    8       16      T50-10  BLACK
 15    51   62   10  110    8   75   36   10       18      T50-10  BLACK
 20    75   91   15  150   12  120   51   13       20      T50-10  BLACK
 40   150  180   27  300   24  240  100   18       20      T50-6   YELLOW
 80   300  360   51  620   47  470  200   23       22      T50-2   RED
160   600  720  100 1240  100 1000  400   28       24      T50-2   RED

Tune filters by squeezing or spreading turns and/or adding or subtracting
turns

Nominal 3 dB bandwidth:  10% of center freq

Nominal midband loss:  1 dB

Attenuation is greater than 40 dB above 125% of center freq

Attenuation is greater than 60 dB below 75% of center freq




David Robbins, K1TTT K1TTT@arrl.net