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

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."

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

MFJ Analyzer.

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

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

In article ,
Lumpy wrote:

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

Silly question - have you calibrated your MFJ meter against a
known-good load recently?

The fact that you get the same impedance reading when you add 50 feet
of coax (which is pretty close to a quarter-wavelength) is a red flag.
Since the added cable is roughly 1/4 wavelength, you'd expect to see a
*big* impedance transformation - a load of 6 ohms at one end, would be
transformed up to around 450 ohms at the other end.

Since you're getting the same reading with, and without the
quarter-wavelength cable extension, I'd be very prone to suspect that
your measurement tool (the MFJ meter) is frotzed.

The MFJ-259 and -269 meters use "zero bias" Schottky diodes as part of
their power-measurement circuitry. These diodes are relatively
delicate, and can be damaged if the meter inputs are exposed to more
than a relatively small amount of RF. Even keying up a transmitter on
one antenna, while having the meter hooked to another (nearby) antenna
at the same time, can hit the diodes with a high enough voltage to pop
'em. Reportedly, the -269 meter can also be damaged if you turn the
UHF feature on or off at the wrong time or with the controls in the
wrong settings.

Check your meter with several loads - a 50-ohm dummy load, 25- and
100-ohm test loads, open circuit, and short circuit. See whether it
reads close to the correct value or not.


--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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

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

Stupid questions:

1 - Are you using a short test jumper cable or adapter connector between the
test instrument and the end of the coax? Could the test lead or connector be
at fault?

2 - Is there any possibility of a STRONG signal coming back down the cable
to stuff up your test instrument readings? Do you live near a source of RF,
such as local broadcast radio?

3 - Is there any possibility both feeder cables are defective? Perhaps you
are using coaxial cable not meant for this purpose? TV coax, or LAN cable?

4 - Have you wiped out your neighbours TV viewing or something? Checked the
cables for sabotage (pins or nails)?

5 - Have you climbed the mast and checked the antenna directly at the feed
point, without a feeder cable?

You have probably thought of all these, but I had to ask. Regards Harry