[AMPS] Feedback path 3-500Z

measures 2@vc.net
Fri, 29 Sep 2000 11:43:36 -0700


>
>> ---------- 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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