Topband: Series LC to notch AM broadcast ?

[Jeff Blaine]

The secret to using this sort of solution is to apply just enough of it 
to kill off the mixing products that are the result.  The good news is 
that you get (generally speaking) a 3:1 payback - so 1 db attenuation of 
the BC station will knock the mixing product down by 3 dB.  So 45 dB (as 
the article provides) is not needed to get meaningful mitigation.

The L/C ratio does set the Q of the notch.  And a series R can cut down 
on the notch depth, as well as reducing the SWR impact. There is a 
balancing act between these factors but generally the highest Q, and 
maximum R combo that is stable enough over the temp range, will do the job.

Have fun!

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 2/20/22 8:42 PM, donroden at hiwaay.net wrote:
> We had a 1550khz in town and a 1450khz moved 2 blocks away from 1550.
>
> Yep.... mix products on 1650khz and 1350khz. ( and to some extent 
> 1500khz )
>
> a simple 1550 notch on the top of the 1450 transmitter eliminated the 
> mix products and didn't bother the 1450 transmitter.
>
> Don W4DNR
>
> On 2022-02-20 12:14 pm, Nick Hall-Patch via Topband wrote:
>
>> Hello Jim,
>>
>> Better, I think, is this notch, found at the IRCA Reprints:
>>
>> <https://www.ircaonline.org/editor_upload/File/reprints/irca-reprint-index.pdf>https://www.ircaonline.org/editor_upload/File/reprints/irca-reprint-index.pdf 
>>
>>
>> Look for Reprint A-063, An RF Notch Filter by VE6JY.
>>
>> You really should only need to notch the carrier of the offending AM 
>> station, as  that's where the bulk of the overload will be coming  
>> from.  It's a pretty amazing device, in my estimation.
>>
>> 73
>>
>> Nick
>>
>> VE7DXR
>>
>> At 17:41 2022-02-20, 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
>>> _________________
>>> Searchable Archives: http://www.contesting.com/_topband - Topband 
>>> Reflector
>>
>> Nick Hall-Patch
>> Victoria, BC
>> Canada _________________
>> Searchable Archives: http://www.contesting.com/_topband - Topband 
>> Reflector
> _________________
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