Topband: Series LC to notch AM broadcast ?

[Grant Saviers]

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 at 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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