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

Craig,
I wonder what you are using to measure that 6 - 7 ohms?
- 'Doc

wrote:
Craig,
I wonder what you are using to measure that 6 - 7 ohms?

MFJ Analyzer.

Craig 'Lumpy' Lemke

www.n0eq.com

"Lumpy" wrote in message
...
wrote:
Craig,
I wonder what you are using to measure that 6 - 7 ohms?

MFJ Analyzer.

Craig 'Lumpy' Lemke

www.n0eq.com

---------

Check for a short in the connector to the antenna analyzer.

Ed, NM2K

Do you have fresh batteries? MFJ goes crazy when they are barely low.

Buck
N4PGW


On Tue, 20 May 2008 10:51:26 -0700, "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


--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."

Buck wrote:

[MFJ Analyzer]
Do you have fresh batteries?
MFJ goes crazy when they are barely low.

13.something. Opening readout says
"voltage ok".

Craig 'Lumpy' Lemke

www.n0eq.com

What do you read when you connect a 47 - 75 ohm resistor across the
terminals of your measuring device?

Roy Lewallen, W7EL

Roy Lewallen wrote:
What do you read when you connect a
47 - 75 ohm resistor across the
terminals of your measuring device?

Plain resistors read appropriately.
27ohms, 56 ohms etc.

I don't have a good dummy load. I have a 50 ohm
50w resistor but it's wire wound. It varies all
over the range when I sweep the MFJ.

Known good piece of coax, end shorted or open,
reads 50 ohms when the MFJ is tuned to the quarter
wave of the coax.

MFJ seems to read a vertical hamstick
appropriately -

http://digitalcartography.com/n0eq/M...il-600x400.jpg

I just made a contact on 5W QRP from Phoenix (here)
to St Louis. So I have to guess the antenna isn't
open or shorted or something else catastrophic.
That's through an LDG tuner.

But -

Running the dipole into my Yaesu 857 with external
tuner bypassed, shows an SWR high enough to cause
the transciever to self protect shut down.

Engage the tuner, SWR comes down to 1.5:1, and
I'm putting out (at least to the back of the radio)
very close to the selected 5 watts.

Craig 'Lumpy' Lemke

www.n0eq.com

Lumpy wrote:
Roy Lewallen wrote:
What do you read when you connect a
47 - 75 ohm resistor across the
terminals of your measuring device?

Plain resistors read appropriately.
27ohms, 56 ohms etc.

I don't have a good dummy load. I have a 50 ohm
50w resistor but it's wire wound. It varies all
over the range when I sweep the MFJ.

Known good piece of coax, end shorted or open,
reads 50 ohms when the MFJ is tuned to the quarter
wave of the coax.
. . .

Something's seriously wrong there. A shorted quarter wavelength of coax
should read very high impedance when the far end is shorted and very low
when the far end is open. The only time it should read 50 ohms with the
far end shorted or open is if it has many dB of loss. And no even half
decent coax should have anywhere near that much loss in a quarter
wavelength.

So it appears that either your coax is extremely lossy for some reason
(and I can't think of any mechanism which would cause that much loss
unless the coax is specifically designed to be very lossy for some
special reason) or the meter is oscillating or otherwise misbehaving
when connected to coax.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Something's seriously wrong there. A shorted quarter wavelength of
coax should read very high impedance when the far end is shorted and
very low when the far end is open. The only time it should read 50
ohms with the far end shorted or open is if it has many dB of loss.
And no even half decent coax should have anywhere near that much loss
in a quarter wavelength.

Maybe I'm not testing correctly.
100' length of new coax. Tests ok for no shorts or open.

MFJ set to 2.4MHz gives me a Z (on the advanced
menu) of 50 ohms with distal end of the coax
either open or shorted. Actually shorting the
end with a screwdriver shaft drops the Z from
50 to 49. I assume 1 ohm is within normal limits.

Same settings gives me an R (on the main menu)
of zero ohms either open or shorted.

I get exactly the same results on several
pieces of coax, old/new, RG8 or 58. Resistance
is zero, impedance is 50, shorted or open at
the resonant quarter freq.

Craig 'Lumpy' Lemke

www.n0eq.com