TopBand: BC Band Filter

[km1h@juno.com]

OK Guys, I opened my big yap so here is the filter that I use.



         680     450        450        450         680
-------l l--l------l l-----l----l l----l.-----l l----l------l l--------- 
  For TX use the caps should be 500V silver mica 5% tolerance
               l                  l              l                l      
                    For RX only  NPO monolythic or disc ceramic 5% 3are
OK
              L1              L2         L3             L4
              gnd           gnd        gnd          gnd


L1, L4  	   34T  #28 enamel on a   T50-2     7.8 uh
L2, L3, L4    32T       "     "            "        T50-2     7.3 uh

You can also use a T68-2  if  the T50 runs too hot. In that case you want
29 and 31T respectively of #26 enamel.

I have to add a caveat here; I have never run this filter under power. 
The original data shows .2 dB IL at 1800KHz and it works fine in receive.

The inductance readings are with the windings evenly spaced and occupying
80 % of the core. It does not take much variation to change the
inductance. I would suggest that you try and measure L .  

Those with L meter access can also experiment with the next larger wire
size and even a T80-2 core. The more adventuresome can even try a   -1
mix core which would require less turns. 

A final caution. Bring up TX power slowly and watch the SWR. Check for
component heating. 

GL...........Carl
 

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