Somewhat related was my effort to produce a pair of 80m notches so CW
and SSB could run 80m simultaneously on Field Day (separate antennas).
While the CW guys would be ok with 20KHz BW, not so for the SSB team.
Just below a Rx 40db notch was about the best I could do with a one db
loss Tx.
Anyway, the free filter calculator ELSI was a big help, since plots were
important to minimize out of notch loss that would withstand 100w.
http://tonnesoftware.com/elsie.html
What I also found in playing with off the shelf inductors for this and
matching a passive 4sq Rx antenna (W8JI top loaded design), was that
some of the cheap "20 values" inductor kits on ebay had pretty high Q's
vs Digikey catalog ones. Then silver micas were paralleled to get the
right C value for whatever the L value measured.
I'm sure there is much more expert help available ;).
Grant KZ1W
On 2/20/2022 09:41, jim.thom jim.thom@telus.net wrote:
Has anybody tried using a simple series L-C to notch out ONE offending AM
broadcast station ? I'm talking about wiring from hot side of coax...to
chassis / ground....like via a T connector etc.
On paper, it should work. Did some minor research, and one comment was that
by using higher values of L would result in higher Q..and a deeper notch.
Another comment stated to use some initial values, like what spits out on a
L-C online calculator for practical values..... then multiply one value
by the other...then take the square root of the result. Then you ended up
with 2 x numerically equal values of L + C..... and supposedly the greatest
notch depth.
On software, I tried several values..from one extreme to the other, and
they all resonate on the same freq. Also tried in software, using 2 x
numerically same values..and it too, also resonates on the same freq.
The rationale behind all of this is... in some cases, there is only one
offending AM broadcast station. Typ HP filters offer little rejection
towards the top end of the AM broadcast band..... like 1200-1710 khz.
I would like to try it, but am still confused as to which combo (using
practical values) will result in the deepest notch. It would have to be
wide enough to remove the 20 khz wide AM signal. A fixed coil + variable
cap, or padded variable cap could be used to fine tune the notch freq.
Perhaps 2 or more LC filters could be used in parallel, to notch out 2 or
more offending stations ?
Jim VE7RF
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