FWIW - I spent a few minutes looking at the filter and with 4 inductors I
could see in the picture - leading to a 9 th order Elliptic Cauer- Chebyshev
equal termination filter with ripple set to 0.03 db , min atten in stop band
of 40 db , and cutoff freq of 1.78 mhz the highest zero is at 1.6997 mhz
(to kill 1.7 as he said ) . You can play between the cutoff freq and pass
band ripple and tweak the rejection zeros a bit which he apparently did to
kill the 1700 station -
with capacitor Q's of 1000 and inductor Q's of 300 - the filter loss at 1.8
is 0.82 db and at 2 is 0.23 db ( with "perfect elements " the losses are
0.026 and 0.02 db respectively) - the loss gets progressively lower and
lower as you go up in the passband -- the return loss is better than 22.5 db
across from 1.8 to 28 mhz - and the element values are reasonable - cap
values from 7906 to 8176 pfd and inductors from 3.7 to 11 uhy . So its
quite realizable. Loss at 1.8 might be a little high but you could lower the
pass band loss significantly if Fc was not set so close to 1.8 mhz (like 1.6
or so).
Hank K7HP
> We'll see what he has to say about the filter. Turns out he is WB5WPA
> in Allen, TX. No bounce on his email, so we'll just have to wait until he
> replies. I'll post any further info that I get.
>
> 73 John W0UN
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|