Roger wrote:
> I've brought this up before on the Amps reflector, so far I've not come
> up with any information.
> I'm looking for information (book or software) that doesn't cost a
> fortune, for designing a legal limit 2-meter amp with a fair amount of
> overhead so it can just loaf along using either one or two tubes in a
> strip line or parallel lines.
>
> I've spent hours searching, but most just lead me in a circle.
> I know absolutely zip about 1/2 wave and 1/4 wave strip lines and how to
> calculate their dimensions.
The theory is non-trivial, and unless you have a reasonable understanding of
complex maths, you will struggle to *really* understand this. The closest I've
seen for an explanation was in some old RSGB handbook. By 'old' I mean 30 or so
years ago. Find some of them and you might find a semi-understandable
explanation of the theory.
By 'complex maths', I mean as in j=sqrt(-1), or for mathematicians, rather
than
electrical engineers, i=sqrt(-1). The square root of -1 is called i by
mathematicians and j by electrical engineers - no doubt because using i would
be
too confusing with current.
I did a Master of Science degree in Microwaves and Opto-electronics, so do
understand the theory quite well, but it is non-trivial.
Essentially a transmission line of impedance Zo, of length 'len', at a
frequency
'freq', terminated in an impedance 'Z_load', if you "look into" the other end
of
the transmission line an impedance 'Z'.
i.e. Z is a function of (Zo, len, Z_load, freq)
The maths is non-trivial. I could no doubt point you to a reference which has
this information, as it will be in any standard book on transmission lines.
Whether it would be any use to you is another matter. If you really want it,
I'll dig out a reference. You might at least satisfy youself that it is quite
difficult maths.
If the load is an open (Z_load=infinity) or short circuit (Z_load=0), then the
maths gets somewhat simpler. In that case, the impedance seen looking into the
transmission line 'Z' is purely reactive. If memory servers me correct, it will
follow a 'tan' curve as you change frequency or length.
Make it one length and it can look like a capacitor, make it another length and
it will be like an inductor. Keep the length constant, but change the frequency
you make it change from a capacitor to inductor.
In theory, if you calculate that lot out, know the output capacitance of the
tube + stray capacitance, then you can find suitable values for Zo and length
to
make the reactance seen looking into the transmission line the value you want.
In practice, even though it's not too hard to calculate the impedance of the
transmission line (especially if a coaxial layout is used), you can't actually
terminate it with a perfect short circuit or a perfect open circuit. Neither is
the tube a perfect capacitor.
If you was making an amp for 10 m, then I expect this could all be calculated
out and the results be very close to theory. At that point, the fact your short
circuit is really not a perfect short, but a wide low-inducatance strap would
not matter.
If you look in the Agieent catalog, you will find they sell open and short
circuits!
The 10m amp would be quite big, but you could do it. At VHF/UHF, its going to
be
impossible to get very accurate without access to some very sophisticated
modeling software.
You could no doubt get a trial of something like Microwave Office, but unless
you understand the theroy, that is not going to be much use to you.
I think the reason you are not finding the answers you want, is that this is
basically well outside the knowledge of most hams. And those that do know it,
realise there is not a lot of point in writing it down in great detail in ham
journals, as ultimately you are going to have to do a lot of cut-n-try, unless
you make the amp at low frequencies. In which case, it should be relatively
easy
to calculate this without access to expensive software. But the amp would be
impractically large!
I'm sorry if I can't give you the answers you want, but I hope I've explained
some of the background.
Dave
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