I was never entirely happy with my 300-ohm ribbon Slim Jim for 2 metres. I made one up starting with the dimensions given on the Southgate Amateur Radio Club website. My SWR analyzer showed a double-dip with minima either side of 145MHz. I ended up pruning a couple of inches off to get one of the dips to occur in the 2m band. My SWR meter told a different story with an SWR of 2:1 at 145MHz. In the end I put it up as it was and it seemed to work, but my transverter wasn't happy with long transmissions on FM and my FT-817 showed three bars of SWR.
I decided to make another one and this time stick to the Southgate measurements. I got a double-dip as before. But I found that when I used the analyzer's PC control program to get an SWR plot on the computer screen I got a different curve with the minimum spot on 145MHz. Confused? You bet I was!
I decided to take screenshots of the two plots and send them to RigExpert for comment. But hunting around their website I found a downloads page that contained, amongst other things, new firmware for the AA-200. My instrument was running firmware version 8.20 but the latest version is 8.31. I installed it (after a struggle trying to start the device while holding three keys down) and now I get SWR plots that are the same on the LCD and the computer screen! And my made-to-Southgate-measurements Slim Jim shows a nice SWR dip right in the FM part of the 2m band.
I shall be putting it up in the attic at the next opportunity!