When does shortening a dipole DECREASE the resonant frequency?
A couple of us have been working on putting an 80M dipole up at a club member's home, stringing the end support ropes through large trees. Our first attempt found me running out of line in my fishing pole, and my slingshot launches leaving a handful of large bolts hanging somewhere in the top areas of a large maple. We finally got the antenna up, but had to pull the dipole wire itself through the tree. The end of the wire was bent down at almost a 90 degree angle and reached almost to the ground.
On our second attempt 2 weeks later (with new fishline, more weights, etc.), we managed to move the support point for the wire up to 35 or 40 feet, and found that the antenna almost completely straight. Since the antenna had been tuning up at around 3.3 MHz, we knocked about 1.5 feet off each end. BUT when we checked the SWR, the minimal point was now at 3.1 MHz - 200 kHz lower!! What had happened?
My theory is that when the end of the dipole was bent down at 90 degrees the electrical length of the antenna was effectively shortened. When we got the antenna almost completely stretched out, the effective electrical length was no longer shortened (no bend at the end) and electrically it was Longer!
I took over a big roller inductor tuner and got the SWR down on 80M, but we expect to go back after Field Day (6/28-29/03), lower the antenna, and shorten it some more. Hopefully, we can trim it into resonance without too much more trouble.
Any alternative theories?
Shortening an antenna NEVER decreases the resonant frequency!
However, lowering it closer to the ground DOES.
You did both - thus violating a fundamental engineering tenet of trying only one change at a time!
So, you had no way of knowing whether the lowering or shortening did more of the "work" of reducing Fresonant.