Ok, here is why a....? C1 -? resonant choke -? C2? cap is? 1 step forward and? ?6 steps backwards.??
Ditto? with a? classic? ( henry radio style)? ?.. resonant choke -? Cap? config.?
When you resonate the choke, at 120 hz,? ?you now have a glorified? 120 hz? notch filter.? ? You no longer have a? good? LP filter, with a low freq? cut off.? ? Reason is.... the small cap that's in parallel with the choke (used to resonate the choke)? will? simply pass all the? harmonics, like? 240/360/480/ 600? hz? ?etc, etc.??
Typ value parallel? resonating caps have low? XC? at? >240 hz.? ?The ONLY thing that is killing the harmonics is the cap on either side of the choke ( in the case of a C-? resonant choke - C filter).......? or the output cap ( in the case of a? ...Resonant choke -? C filter.? ?Ok, now you know why? FM broadcast? TX..that use the? classic? ?resonant choke +? cap setup.... also follow it up with a? 2nd choke? +? 2nd cap.? ?The? 2nd choke (NOT resonated)? ?+? 2nd cap is? there strictly to kill the harmonics.? ?Now in essence? they end up with a resonant choke notch filter..... followed by a? ?C-LC? filter.??
With a simple? ?C- choke - C? ?filter.... it forms a bonafide? real deal? LP? filter..with a real low freq? cut off.? ?In my case, the? cut off freq is real low, like 3.7 hz.? ? ?Attenuation at 120 hz? is? -131 db ... and lower still on the? harmonics? at? 240/360/480/600/ etc.? ? ? Max? gross overkill,? I like it.? ? So do the glass tube? audio geeks.
I can see why the FM broadcast folks? want? loads of? 120..and also? 240/360/480? etc? ?harmonic redux? (this is on single phase? B+? supplies).? ? FM is about the only mode I can think of, that sucks max plate current.... with no modulation / audio / program? material /? music? applied to the TX.? ?IE:? 100 kw? ERP? and during pauses? etc, if there is any 120/240/360? etc products, they? will be heard on the RX end.? ?Esp if the? 100 kw erp station is fairly local,? full quieting, and? S meter? pegged.?
A few folks? tried? 75M? ?FM late one night..that was an eye opener.? ?Locally,? FM is used on 11 M band !?
later.... Jim? ?VE7RF
