[Amps] Design of 2-meter strip lines or parallel lines
Roger
sub1 at rogerhalstead.com
Fri Feb 12 13:30:28 PST 2010
TexasRF at aol.com wrote:
> Roger, I have built my share of VHF, UHF and Microwave amplifiers
> through the years. In the process, I have learned a lot about how they
> work and will share a few "secrets" with you.
I've build quite a few amps for HF, but that's a different world.
>
> The book you are looking for does not exist. You are left with three
> options: 1>copy a known good design (such as the W6PO 8877 2m amp),
IIRC that is a rather boxy cabinet and changing the cabinet/enclosure
changes the amps performance. That and I'm not real fond of the 8877.
Besides, they are very expensive and I have no source for pulls. OTOH
the last Ei mac 8877's I purchased new cost me $305. I had hopped to
build something that will match my low band amp in size and shape, which
is not exactly small, but it's still a desk top even if it does weigh
over 100#. <:-))
> 2>wing it on your own and be prepared for some cut and try (that is
> how the known good designs were developed) or 3> buy a new or used unit.
I already have a Henry 2002A that I need to get on the bench and test as
I've already committed to sell. That uses a single 3CX-800. What I want
is the legal limit, but I was thinking of either a pair of 3CPX800's
which would probably be better than a single one, or a design to use a
pair of the GS-23B's with relatively high current and low voltage. (It's
really a 4CX1600u but with coaxial connections instead of pins)
>
> You ask: why can't I just design the amplifier? The reason is that the
> internal inductive reactance of every tube model is different and get
> this: the inductive reactance changes with frequency. As the inductive
> reactance rises with frequency, the capacitive reactance between tube
> elements lowers with frequency. There is even a frequency that the two
> reactances are equal and look like a short circuit. Above that
> frequency the reactance between the tube elements turn inductive
> instead of capacitive. This is at the tube connection points, the
> capacitance is still there internally.
>
> The tube manufacturers for some reason have never characterized the
> internals of the tubes to allow us to accurately predict the
> inductance at various amateur frequencies. One of the reasons may be
> because the tube internal connections become part of the resonant
> circuits. If your plate resonator is a stripline the Zo of the line is
> very different compared to a coaxial or waveguide design.
>
> At HF, the variation of this reactance is not as severe and we can use
> the input and output capacitance and be close enough for matching
> network designs. Not so at VHF and UHF. Even a small 15 pF capacitor
> has the same problem. For example, one with .25" leads has enough
> series inductance to make the 15 pF work as if it were 1000 pF or so.
> With longer leads, it is no longer a capacitor for the frequency of
> interest but looks like an inductor.
With this I'm familiar.
>
> At these frequencies, you do not get to choose loaded Q as one might
> wish to deal with various plate load impedance variations caused by
> changing the plate voltage or current. You have take what you get as
> the loaded Q can be 60 to 100 or more at 70cm. Even on 2m typical
> loaded Q can be 20 to 30.
and 2-meters is my target. 440 is out of bounds as our power limit here
is only 50 watts out of the rig.
> There is little you can do to lower these numbers other than using
> good common sense: minimize any shunt capacitance, use the highest
> quality resonator design (strip lines are not the best),
What is better for 2-meters.
> Use the lowest plate load impedance possible with the tube you have
> (maximum plate current, less than maximum voltage).
That sounds like a pair of tubes would be better than one of the same type.
> And finally, pay attention to rf current flow in the plate resonator.
> If a strip line is used, nearly all of the current flows on one side
> of the tube anode. There is that inductance mentioned above and every
> inductor has a given Qu and resulting loss resistance. If most of the
> current is flowing on 1/3 of the tube plate connection, there is 3X
> current compared to an optimum design. Losses being related to current
> squared, they are 9X-2 or 7 times an optimum design. These losses are
> subtracted from your output power and can damage the tube or
> fingerstock. Also, in some tubes, such as GS23B, the material being
> heated by the losses will change dimensions enough that the resonant
> frequency drops. With the loaded Q being so high, it takes a few
> seconds for the power output to drop to half or less. Of coarse you
> can keep one hand on the plate tuning control and bring it back but
> who wants to do that? These problems go away with a circular resonator
> design.
>
Circular resonator design on 2-meters in a desktop cabinet of reasonable
size?
> Did I mention that I am down on stripline designs?
>
> By now, you have found the ND2X website with all of the various
> Russian tube designs.
Yes,
> There is a wealth of information there and most of it is good. A few
> of the published designs by YU1AW are from computer models and do not
> work in practice. Beware of any designs designated "lazy builder".
I was wondering about those as they just didn't look right.
> I recently helped W9IIX with a GS35 222 MHz amplifier project started
> from the YU1AW info and it was a ball of flames at turn on.
>
> I will be happy to critique any design you might wish to try. In fact,
> I will offer unending moral support for your VHF or UHF amplifier
> projects.
What? and here I was hoping for monetary support as well<:-))
What about the design used in the old repeater amps with two tubes in
push pull and parallel lines?
>
73
Roger (K8RI)
> 73,
> Gerald K5GW
> eme capable on 6m, 2m, 135cm, 70cm, 23cm, 13cm, 9cm, 6cm and nearly
> ready on 3cm
>
>
>
>
> In a message dated 2/12/2010 3:36:42 A.M. Central Standard Time,
> sub1 at rogerhalstead.com writes:
>
> 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 same is true of the parallel lines. How does the construction
> and
> dimensions change with power.
> How about a pair of GS-23B Parallel lines, or strip line, or a single
> GU-84B/4CX2500, or a pair of GU-74Bs/4CX800's. On HF or even six
> meters
> I can come up with a tank circuit, on two about the only thing I
> can do
> so far is to copy the dimensions of the strip line out of my Henry
> 2002A
> which uses a single 3CX-800. If I went to a pair of them, or the
> 4CX-800s how would that affect the dimensions of the strip line,
> or if I
> replaced the one or two 3CX-800's with a pair of GS-23Bs/4CX1600U
> (IOW
> it's a microwave version of the 4CX1600B which was so popular
> until it
> disappeared although I see Alpha has a few of them yet.
>
> At any rate, this is the kind of information I'm looking for. The
> problem is being able to find it, or construction articles with a
> bit of
> theory, books, or design softtware that doesn't cost a fortune.
>
> 73
>
> Roger (K8RI)
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