[TowerTalk] Hairpin Match Calculator

[Jim Lux]

On Fri, 12 Jan 2024 03:09:25 -0800, Brian Beezley <k6sti at att.net> wrote:

"NEC does use a simplified model for the "wire" - it takes into account
dielectrics and skin effect (I'm in the middle of figuring out what ZINT
does, which is the core of the "resistance and inductance of a segment",
so I can make a Python version that's not just a copy of the Fortran)."


Jim, carefully check the NEC resistance calculations. Many years ago a
knowledgeable German ham found an error and brought it to the attention
of some NEC person. He was never happy with the acknowledgment he
received. I assume the correction made its way into the code, but I
never checked.

>> The code in NEC2 and NEC4.2 is essentially the same - 4.2 has more precision in some of the series approximations.




I found a free FEM program (code free, help costs) but it looks like a
big effort to get it going. Not sure it's worth the trouble just for the
boom problem.


>> Yes - that is always the challenge with FEM codes - the actual calculations are pretty straightforward to implement, but a user interface that makes entering the geometry is more challenging (hence front ends for NEC codes).  There’s also the whole making sure your boundary conditions are right, and whether the gridding is correct.  At JPL, we’ve spent a fair amount of time making software that can take models from the mechanical engineers (typically as .step files from NX) and turn them into models suitable for EM codes.  It sort of works, but we still build big mockups of things like rovers to test on an antenna range.


A benefit of writing the hairpin code is that I'm now aware of subtle
magnetic interactions everywhere. Some not so subtle.

>> R.W. Hamming: “The purpose of computation is insight, not numbers”


For the hairpin Q calculation, I had been meaning to check loop
radiation resistance. I finally got around to it yesterday. Radiation
loss is below 1% of conductor loss for most designs, but I did find a
few cases where it was 5-10%, and one case where it was 13% for a
2-meter hairpin. More significant than I expected. So I added the code
and posted a new version. I did the same thing to my coil program for a
better comparison of coil and hairpin losses. I always wondered whether
a large, efficient coil that replaced a hairpin might radiate, but so
far I haven't found a case where the loss is anything but tiny.

Brian

_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk at contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk