Why is my dipole low impedance?
 

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

Why is my dipole low impedance?
 

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

Why is my dipole low impedance?
 

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."

Why is my dipole low impedance?
 

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

MFJ Analyzer.

Craig 'Lumpy' Lemke

www.n0eq.com

Why is my dipole low impedance?
 

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

Roy Lewallen, W7EL

Why is my dipole low impedance?
 

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

Why is my dipole low impedance?
 

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!

Why is my dipole low impedance?
 

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

Why is my dipole low impedance?
 

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

Why is my dipole low impedance?
 

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

Why is my dipole low impedance?
 

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

At this point, I think I'd set aside the MFJ and resort to a VSWR meter.
Once you verify the antenna, you can get to checking out the MFJ. The front
end bridge is very sensitive to overload and damages easily.
Dale W4OP

Why is my dipole low impedance?
 

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

Why is my dipole low impedance?
 

Harry Lippitz wrote:

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?

Nothing seems to make a difference. 12" coax
to the antenna, 50' RG8, 100' RG8, 50' RG58.


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?

I don't think so. I'm receiving (I'm transmitting!)
just fine. No local signals clobber me.


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?

Nope. Several 50 ohm 8/58 coax. Some from cablexperts.com.


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

No, nothing like that.


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

I can't climb this mast but if I bring it down to 10' and
get up there with a ladder, same results.


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

Feel free to ask, no matter how seemingly stupid
the Q might be.


Craig 'Lumpy' Lemke

www.n0eq.com

Why is my dipole low impedance?
 

Thanks for everyone's comments
and questions. Based on what
several suggest, I think the MFJ
analyzer is B/O. I'm still making
contacts like crazy, even if the
meter says I shouldn't be.

I think I'll get the 259 back to
MFJ while it's still under warranty.

Thanks again -


Craig 'Lumpy' Lemke

www.n0eq.com

Why is my dipole low impedance?
 

"Lumpy" wrote in message
...
Harry Lippitz wrote:

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?

Nothing seems to make a difference. 12" coax
to the antenna, 50' RG8, 100' RG8, 50' RG58.


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?

I don't think so. I'm receiving (I'm transmitting!)
just fine. No local signals clobber me.


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?

Nope. Several 50 ohm 8/58 coax. Some from cablexperts.com.


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

No, nothing like that.


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

I can't climb this mast but if I bring it down to 10' and
get up there with a ladder, same results.


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

Feel free to ask, no matter how seemingly stupid
the Q might be.


Craig 'Lumpy' Lemke

www.n0eq.com



Hi Lumpy

I really like your music. You do a terriffic job with that guitar.
I'd suggest that you'd do yourself a favor if you recognize that the input
impedance of your antenna is not being measured properly by your meter.
Instead, try to think of something that is causing the meter to read
improperly.
Could the error be introduced by a stray (off frequency) signal effecting
the meter movement? You know, the antenna picks up the signal and
introduces an error.
Remember how the impedance looking into a length of coax, that is either
opened or shorted, varies all over the place when the length is
uncontrolled.. The only time it's impedance is equal to it's Zo is when
it's length is 1/8th wave ( or odd multiples). And, then, the impedance is
very nearly purely reactancive.

Jerry KD6JDJ

Why is my dipole low impedance?
 

Lumpy wrote:
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

At 2.4 MHz, the line is pretty close to 3/8 wavelength, not 1/4 --
assuming it has solid and not foamed dielectric. At that length with the
other end shorted or open, the impedance would be very nearly 50 ohms,
all reactive, which means the resistance would be zero as you're seeing.
So those measurements are correct. Your meter is showing the correct
values when measuring resistors (purely resistive) and the shorted/open
coax (purely reactive), so there's nothing obviously wrong with the meter.

The remaining possibility for your problem with the antenna is external
RF, as someone else mentioned earlier. I find it nearly impossible to
use my 259B for antenna measurements at my QTH because of strong AM, FM,
and TV signals -- ironically, the "antenna analyzer" is great for a lot
of other chores, but not for actually analyzing antennas. It's pretty
likely that's what your problem is too. When measuring antennas, I have
to use filters which introduce minimal impedance disturbances at the
measurement frequency while providing attenuation of the offending signals.

Roy Lewallen, W7EL

Why is my dipole low impedance?
 

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


Have you measured it with an SWR meter from your radio?


--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

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

Why is my dipole low impedance?
 

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.

Why is my dipole low impedance?
 

"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