65-67mm on this antenna is right in the middle of the coil. (picture of the original with a ruler, above).
I could solder a copper wire directly to the threaded stub (or to a nut), since this seems to be the best antenna.
Looking at the HeyWhat’sThat map, my current pattern looks like the one that I can expect.
Today is 21,230 <50, 2,889 50-100, 288 100-150 NM. The 50-100 is 13%. It was 17% earlier today. With the original antenna, it was around 7%, but that number varies with air traffic, which is quite variable. My sky is not full of airplanes. The area around SFO is in the shadow of some mountains for me. I have seen 20 at a time. I have seen none, or two. Comparing to Flightaware/Live shows that I’m not missing much in the coverage pattern that is becoming familiar. I’ve started watching the Age column in SkyView, to know what the maximum range is of a flight before it disappears.