A 6m Window Line Slim Jim – eh, it'll do

This weekend is the ARRL's January VHF Contest, and next weekend is Winter Field Day.  I'd always wanted to add the 6 meter band to my quiver of arrows, in the form of a roll-up window line antenna for hiking & camping use.  Either a slim jim or a J-pole, didn't really care which.  There are TON of web sites and videos out there that give you some nominal lengths and then say "Experiment!  Cut to fit!  Spend two days trimming and re-soldering, until you find you've cut too much off, then you'll have to start all over!  It'll be fun!"  No, just no.

So I settled on this one (do not use, reasons in a moment) that purported to give solid numbers based upon actually building the thing, and commenced to cutting and soldering.  TLDR version: The finished product resonated somewhat below the 50 – 54 MHz 6m band, down around 48 MHz.  A couple of hours of cut-measure-cut in which I lopped off 11-1/2" from the top end resulted in this SWR curve:

horizontal axis is frequency in MHz, vertical is SWR; captions in red are the most important frequencies for intended use

Eh, it'll do, but not quite all that I'd hoped for, and I'd really wanted to skip the try-cut-try iterations.  A little more digging this evening turned up this site, which references this slim jim & J-pole calculator, with these slightly better results: 

The blue line is the predicted SWR curve, the red line is the actual measured SWR curve and what I'd been hoping for.  I'm guessing that the "not critical" spacing between the loading stub's wires ("F" in the diagram there) being reduced from 13cm to more like 2.5cm is the reason behind the difference between predicted and measured performance.

Yeah, the author there actually y'know built the thing and measured it, which is a big improvement over lots of other "information" out there on the web.  Just in case that site ever goes away, here are the numbers:

As always, click to embiggen.

In the end I'm OK with what I've built, but if I had it to do over again, I'd go with the second site's numbers.  Also, he actually measured the Velocity Factor (Vf) for the window line he used at 0.90, which yields a significantly shorter antenna than the default bare-wire Vf = 0.96 seemingly used by the first site.  In the end, the shorter antenna similar to what I ended up with after try-cut-try-repeat.  However, mine is not quite as good nor was it quite as easy as the second site.  Wish I'd seen that one first.

Now for the grand irony: The forecast is for rain locally during the forecast and I'll be stuck inside and won't be able to use this new antenna anyway.  At least the extended forecast for next weekend (remember, Winter Field Day) is looking pretty good.