[AMPS] Feedback path 3-500Z

[Tom Rauch]

> "When an antenna is connected things are theorically more complex 
> because HF antenna impedance can be transformed to high reactive values if
> one has long enough lines (in term of WL and fractions like quarter
> waves). Actually I don’t see very raccomandable to use lines whose lenght
> ranges in the area of 375m (1230 FT), a quarterwave at 200 KHz. Agreed
> that an HF antenna is not 50 +j0 at VLF, the feed point impedance of an HF
> antenna at VLF is much less than one ohm and largely -j reactive (i.e an
> open dipole) or few Ohm and slightly inductive +j  in case of closed
> loops, antennas with baluns, matching devices like hairpin, etc. etc. As
> previously explained, unless the used line is quite long and acts like an
> impedance transformer there is no chance an HF antenna will become so
> higly reactive (+j) to resonate plate load"

It's a big world out there Mauri, with many kinds of loads having 
many kinds of characteristics, I never said it was a frequent even in 
the real world, I just said it could be made to happen and can 
happen. The wrong impedance can occur in hundreds of ways you 
haven't expressed.
 
> My point stay that real chances to have VLF oscillations because of a grid
> to ground low Q parallel resonant circuit at 200 KHz (or lower) are really
> minimal, basically equivalent when grid is directly grounded. Final step
> is if anyone want's to try to start VLF oscillations with an SB220/TL922.
> The circuit must be connected to PA output connector and must have an
> equivalent series impedance of about 5 +j 870, a 5 Ohm in series with a
> 700uH variable inductor could be proper.

First, everyone else decided it was at 200 kHz. Second, everyone 
said it can't happen.

It can happen and does happen. I've seen it happen in another type 
of PA also, where external devices cause an oscillation well below 
the operating frequency!

Adding a part that isn't needed, that doesn't do what it is claimed 
to do, and can cause trouble.... is poor engineering. In this 
example, everything is better without the choke. In this case it not 
only decreases VLF and LF stability, it decreases VHF stability. 

It does not add controlled negative feedback, it adds a mess.

Why ask for problems when a simple resistor in the RF feed to the 
filament will add negative feedback and improve stability, and a 
single resistor in the HV will add much better arc protection?

Does your SB-220 have those needless chokes in it?
73, Tom W8JI
w8ji@contesting.com

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