Lumpy wrote:
I'm a little stumped.
I'm sure I'm missing something obvious
but I just can't see it.

Built a 40m wire dipole, 33ft per side, symmetrical.
Center mounted on an 18' pole. Ends of the
legs drop about 2 ft to 16' above ground.
Slack/dip in the wire legs is pretty minimal.
The mechanical connections of my antenna allow
me to get it pretty tight.

Without any balun, running coax to the center
feed point of the dipole, I get an impedance
of around 6-7 ohms.

Different lengths of coax, different pieces of coax,
make no difference. All the coax pieces I've
tried test ok for shorts and continuity. Two
of the test coax pieces are brand new.

Dipole legs aren't touching anything, including
each other. Nearest piece of metal is at least
a half wave away from the end of one leg.

Mast is an aluminum tubing tripod in sections.
The sections aren't particularly bonded, the
section connectors are plastic/nylon. The
telescopic sections of the tripod/mast are
4' long each. The mast has rubber feet on
it's tripod legs, it is not bonded to earth.

Same 6-7 ohms shows up on the MFJ meter whether
the mast is at 10 ft or 18 ft.

Same 6-7 ohms shows up whether I stand at
the base of the mast with a 17' piece of
coax attached, or if I add a 50 or 100 ft
length of coax and stand virtually anywhere.

Why isn't my dipole showing a more expectable
impedance? Any guesses?

Craig 'Lumpy' Lemke

www.n0eq.com

If I look in the antenna books, the feedpoint impedance of a half-wave
dipole less than 0.1 wavelengths above a perfect ground can be less
than 10 ohms.

You are, say, 17 feet above ground. That's more like 0.14 wavelengths,
and the charts in the antenna handbooks show that as you get up to 0.2
or 0.3 wavelengths that you'd expect to be way above ten ohms. And
your ground I'm assuming is not "perfect", with a zero-area call it
might be better than average, but it's probably not salt-water.

So maybe the numbers don't add up perfectly to match the charts in the
books, but I think the lesson is that a very low dipole (you're just
"pretty low") dipole over perfect ground (maybe you're "very good")
can have a very low feedpoint impedance, less than 10 ohms in some
circumstances.

If your ground really is that good, you're a prime candidate for
verticals or an array of verticals. I used to live in the midwest next
to the Mississippi, and the ground conductivity was really very very
good (if not as good as sal****er) and I was very happy with my 4BTV's
performance.

Tim.