Trying to copy a stripline design without copying it EXACTLY will
usually lead to a lot of cut and try and quite possibly low efficiency.
To design a stripline for any tube (or tubes), take the output C of the
tube(s), add stray C from the chassis etc., add the C required for your
flapper tune cap and the C required for the output coupling/loading cap
if you use one. Calculate the reactance of that total. (at the operating
frequency)
Using the formulas or on-line tools, calculate the impedance of the
stripline you intend to use. That means knowing what the width is and
the distance to the top and bottom of the anode compartment. Once you
know what the line impedance is going to be, use a smith chart or
on-line tool to calculate the length of line needed to cancel the
reactance of the "C's" noted above. A shorter than 1/4 wave line will be
inductive. When the inductive reactance of the line is equal to the
capacitive reactance of the tubs, etc., it is "tuned".
With high C tubes, the resulting line is very short. A bit of time spent
with a smith chart will show why this in not a good thing. To get around
that, use two or three 1/4 wave lines in parallel (W6PO example)
Some designers try to use a 1/2 wave or 3/4 wave line. Try to avoid
these as the loaded Q skyrockets. Imagine if you will a tank with a Q of
400 or so.
Using a link to tap on the line to couple the output makes the line
length requirement slightly different than using cap coupling. Usually
makes the line a tiny bit longer. Allow room for "adjustment". If you
are lucky, you will get it in one. If you are like me, the chassis will
be all cobbled up before it's right.
With a 1/4 wave line, decent construction, and a tube that's running
under the proper conditions, VHF amps will easily do 60% efficiency and
UHF amps near that. At UHF silver helps.
Larry - W7IUV
Larry - W7IUV
DN07dg
http://w7iuv.com
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