[PVRCNC] antenna leaves me scratching my head :: comments?

[Steve Jackson]

Today, I erected a 40M 1/4 wave ground mounted vertical out in the woods
behind the house, with the help of W4KIL.

It's made of 8 sections of military aluminum mast, of the sort sold by
Henry K4TMC, plus a 'stinger' on top.

The riveted-together sections, net of the inter-section overlap, are
44.5625" long, each, and have an outside diameter of 1.8125"

Eight sections together therefore produces a main radiator of 356.5" in
length.

There is a 7.375" long coupler section which ends in a 3/8"-24 TPI threaded
brass insert.  Screwed into that is a 60.125" stainless whip of an average
thickness of 0.125"

Total height:  416.625"  or ~34.72 feet.

I modeled this antenna in EZNEC.  (Wasn't hard.)  Predictably, with a 37
ohm feed, on paper the whole enchilada rings at 7.125 MHz.

But today, when I tilted it up for the first time, with only one radial, I
use my MFJ-269 analyzer to 'look' at the antenna.

Fed at the very bottom, against a few 10' ground stakes and one
laid-on-the-ground 1/4 wave radial, the MFJ showed the antenna 'dip' at
around 5.7 MHz.  Not 7.1 MHz.

Odd.

So, I tilted it back down, and unscrewed the 5' whip, figuring that would
show a marked change, and prove that something else wasn't awry.

Not really; the 'dip' moved only to around 6 MHz.


Question for the group:  What do you think might be the cause of this?