Hi Tom,
I did explain what sort of load its needed to have the blocking cap
lead j872.6 Ohm equivalent reactance and also that I substantially
agree that PI is mostly ininfluent to produce it.
Anyway, you should have also better read the part of my message where I
explain why this critical and big +j load is not so easily present
across the "transparent" PI.
"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"
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.
73,
Mauri I4JMY
> ---------- Initial message -----------
>
> From : "Tom Rauch" <w8ji@contesting.com>
> To : amps@contesting.com, "i4jmy@iol.it"<i4jmy@iol.it>
> Cc :
> Date : Fri, 29 Sep 2000 07:49:31 -0400
> Subject : RE: [AMPS] Feedback path 3-500Z
>
> The 1000 pF blocking capacitor can easily look like j800 or any
> other value depending on the load at the output of the pi-net.
>
> Do you agree or disagree with that?
>
> I can look at the anode with an impedance measuring set and
> move the frequency of peak anode impedance anywhere I like in LF
> and VLF, because the pi-net is transparent at those frequencies.
>
> Grounding the grid solves all this nonsense.
> 73, Tom W8JI
> w8ji@contesting.com
>
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