>
>> ---------- Initial message -----------
>>
>> From : owner-amps@contesting.com
>> To : amps@contesting.com, "'Maurizio Panicara'" <i4jmy@iol.it>
>> Cc :
>> Date : Fri, 29 Sep 2000 08:22:37 +0100
>> Subject : RE: [AMPS] Feedback path 3-500Z
>>
>>
>> Mauri says:
>>
>> >how, and where, they are connected
>>
>> 1. 50 uH in parallel with the combination of plate tank tuning
>capacitor and
>> series coupling cap and series bypass cap.
>
>Going from PS to plate:
>
>Several uF HV cap filter is in parallel with 1000 pf Bypass, reactance
>is several uF//1000pf, a like a short (-j0) on VLF.
>The 8 uH inductor is in series with the previous found load, 8 uH is
>about j10 Ohm at 200 Khz.
>But +j10 Ohm is in parallel with the second 1000 pF (-j796 Ohm @ 200
>Khz), and equivalent impedance is about j9.7 Ohm.
>The 50 uH inductor is in series, Xl is +j62.8 Ohm at 200 Khz, and in
>series with +j10 Ohm the equivalent is +j72.4 Ohm.
>Until now, on plate is applyed a +j72.4 Ohm reactive load, value thatís
>already a full safety against feedbeck for VLF oscillations unless an
>opposite -j72.4 reactance would resonate it.
>Of course there is a blocking capacitor in series from plate to PI and
>its reactance at 200 KHz is around -j800 Ohm.
>Then, what is after (in series) with blocking cap must be +j872.6 to
>resonate plate at 200 KHz.
>
>In other words, better to say in numbers, we need that PI and antenna
>load looks like a 694.7 uH equivalent inductor to resonate plate at 200
>KHz.
>
>Neglecting the reactance of 1-20 uH PI coil at 200 Khz (because
>changes with band), but having plate and load capacitors that are in
>parallel, what as to be in parallel is an inductance (+J) that has to
>be bigger than 694.7 (exact value depend by settings and switched band)
>
>Assuming no antenna is connected, the HV safety choke parallel to the
>antenna output could be the only guilty device able to rise inductive
>reactance of this lag up to such high value.
>
>Itís worth to point out that HV safety chokes are generally placed
>after the PI network where impedance is already down to nominal 50 Ohm
>and there is no reason for having them too big. First because
>unnecessary, second because they should be able to manage some power to
>be effective if blocking cap fails, third because higher the inductance
>higher the chance the self resonance is within the HF range.
>
>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 donít see very raccomandable to use lines whose lenght
>ranges in the area of 375m (1230 FT), a quarterwave at 200 KHz.
>
>
>> 2.The plate tank capacity is the capacity of the tuning capacitor
>plus the
>> shunting effect (via the tank coil) of the loading capacitor shunted
>by the load
>> reactance (NOT 50 ohm resistive, but the complex load presented by an
>antenna at
>> the low frequency)
>
>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.
>....
>
? For background information, see *The Emperor's New Clothes* by Hans
C. Anderson.
ciao, Mauri
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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