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Re: [Amps] Stripline design

To: "Dr. David Kirkby" <>
Subject: Re: [Amps] Stripline design
From: Paul Decker <>
Date: Fri, 6 Jan 2012 00:06:27 +0000 (UTC)
List-post: <">>
I've had a casual interest in how striplines work for a while, but not to the 
point that I have started a detailed study.  You've provied a great 
explanation.  From what I  can gather, the "stripline" is just being used in 
the same way as for example a coaxial transformer.  The difference is "we" are 
creating the coax out of the equipment case and the bar connected to the tube 
anode.  Based on the bar size and the case size, the impedance is determined.   
This seems very similar in idea to calculating impedance of a coax cable by 
using the 138log(D/d) equation.  If this is roughly true, then increasing the 
strip size, thereby reducing the distance between the strip and the case will 
make the impedance lower, this would be the case of extending the strip on one 

I also think you have hit the nail on the head.  the 2x3cx800 k1fo amp uses the 
same enclosuer so the stripline sides are closer to the case sides, I presume 
this would make less inductance and make it shorter to maintain the right 
inductance.  While the single 3cx800 is narrow so it must compensate for being 

This might seem like an off the wall idea, but perhaps the simpleton approach 
is to calculate the original area of 3.5" x 8.625" = 30.1875.  Then take the 
new length of 3.5 x 9.0 = 31.5, an increase of 1.3125".  To maintain the same 
area, reduce the width by 0.07292"  on each side so the new width is 3.35416".  
 We should have the same impedance because the same insulative area is 

Your first email mentioned that 

"The capaitance reactance of the tube and strays is given by

Xc=1/(2 Pi f C)"

Where does the "C" paramater come from?  Is this one of the paramaters on the 
tube data sheet such as Cin, Cout, Cpk, Chtr.  I would guess Cout.   
Cin   = 20.5/26.0
Cout = 6.0/6.1
Cpk = 0.03/0.05

Cout between the tubes is only 0.1 pF difference, so this seems very small.  

My thought would be to leave the stripline at the same height as original, this 
makes the tube anode connector stand up higher then the original, but there is 
enough room on the top for that.

I do have a grid dip meter I could use, I'm not sure how it would help since 
the stripline likely has to be enclosed with the lid on to give the proper 
results.  How would I couple it to the enclosed stripline?  I would guess I 
would leave the tube out of circuit to use the dip meter.  Would it not be good 
enough to somehow connect a probe and simply measure the impedance perhaps with 
an antenna analyzer or with a VNA (i dont have a vna)?  

I have not found any construction articles on this RF deck so far, I bought it 
off of Bill Olson K1DY half a year ago.  He said it had been sitting collecting 
dust for 20 years and he built it 35+ years ago.  I did some clean up and 
traced out the circuit, threw in some control, metering, and a power supply and 
away it goes.

I will take it apart this weekend and get some of the dimensions as best I can, 
it can never hurt to have those laying around.  

Thanks again,

----- Original Message -----
From: Dr. David Kirkby <>
To: Paul Decker <>
Sent: Thu, 05 Jan 2012 22:31:50 -0000 (UTC)
Subject: Re: [Amps] Stripline design

On 01/ 5/12 09:38 PM, Paul Decker wrote:
> Hi David,
> Thanks for the reply and the great information.  I can put together some 
> specifics and more pictuers of the inside of the deck, and using a copper 
> foil wouldbe an easy way to test it.  The 3cx800 tube height is taller than 
> the 3cx400 so this might play a role.  Perhaps the stripline size difference 
> in the k1fo's amp is due to the cabinet size, he does use a larger rf chamber 
> than what I have.

The case has quite a major effect if it's distance is not much larger than the 
spacing between the chassis and the stripline. I'd say if the distance between 
teh stripline and the side walls of the case are less than 2-3 times the height 
of the stripline, then the walls will have an appreciable effect on the 
impedance of the line.

We need to know where the DC voltage is fed, and where the RF 
coupling/decoupling capacitors are. Scan the original publications of the 

TXLine is a free program to compute the impedance of transmission lines. It 
works with microstrip line, which is your stripline, with Er=1.0. But if the 
chassis walls are close, then TXLine will be inaccurate. ATLC could be your 
friend, as that takes the presence of the chassis, top panel and the two side 
panels, but it's a Unix program, which you may or may not be happy with.

ATLC can't take the end panels into account, as its a 2D simulator, not a 3D 
one. A true 3D simulator is difficult to use and expensive.

Another modeling option might be FEKO lite.

If you can send some detailed diagrams, I could be tempted to try to model this 

using it's Eignemode solver. I've never used that feature, but I reckon that 
might actually be useful.

If I were in your shoes, I'd not worry about the extra length. It will effect 
the system, but in a way which you can easily compensate for by changing the 
width of the line, so from a practical point of view, it's not a major headache.

The theory is a bit more complex, but I've given you some of it. Depending on 
your background, you might prefer to ignore that or research the topic in more 

> Just a quick question, what is a "GDO" you mention near the end of your 
> message?

Grid dip oscillator.

> Get some rest, 73,
> Paul

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