[Amps] Stripline design

[Dr. David Kirkby]

Roger wrote:
> I'm looking at building a 2-meter amp using a stripline with a pair of 
> 3-CX800's or 4CX-800's, but I've not found any real data on the design 
> (dimensions) of the actual strip line.  Another possibility is building 
> the 6 'n 2 amp in the ARRL classic amp design handbook. I also have a 
> Henry 2002A using a single 3CX-800 with a strip line.
> 
> Unlike using the tube characteristics and designing a standard Pi or 
> Pi-L network for a given Q, I've found no information on strip lines in 
> any of the hand books.  Yes, there were a couple of amps using one and 
> the dimensions were given, but if the design information was there I 
> missed it.
> 
> The only thing I found on the net were the download sites that appear 
> free until you give them your information and then they want you to 
> pay.  I'd probably sign up if they told me the charges up front. 
> 
> 73
> 
> Roger (K8RI)


The theory and the practice do not always agree too much, but some of the RSGB 
and ARRL manuals do cover the theory

Basically you need to make a transmission which has an equal and opposite 
reactance to the tube. The reactance of a length l of a transmission line of 
impedance Zo is easily calculated - the equations can be found in any decent 
book on transmission lines. If you want some references, I can dig you out a few 
- I've got several on my book shelves which cover this.

You will need a reasonable understanding of maths to follow the derivation, but 
if you only want the result, it is quite simple. It has a coth() (hyperbolic 
cotangent) in the formula if memory serves me correct, but a handhold calculator 
can compute that.

The impedance of things like coax lines are easily found from theory. For 
non-standard shapes, you might like to look at

http://atlc.sourceforge.net/

which is free and open-source finite difference software. I wrote that software 
- there is no need to sign your life away.

It needs a Unix or Linux or Mac OS X system to run on. (That said, somebody has 
compiled it to run on a Sony Playstation!) I can't be bothered with Windows. At 
the time I wrote it, the software was very expensive in terms of CPU time 
needed, but that will not be an issue now.

I believe the results are very accurate for enclosed air-spaced lines. For 
multiple dielectrics, with open structures, the accuracy drops a bit.

The problem is that while one can do a lot in theory, the practical aspects 
means that some things can't be modeled (at least with software I'm aware of).

You can't consider a tube to be electrically small at 2m. If you have a shoring 
link at the end, you can't consider it has zero length, but neither can you 
model its effect accurately.

So basically, while you can do all the theory, in practice it will come down to 
cut-n-try.