I just aquired an MFJ 259b analyzer. I'm using it to measure the
impedance at the input of the 300 ohm twin lead I have feeding a
sloping off center fed dipole. I measured 75 -j236 at 3.94 Mhz. When i
reverse the txmsn line leads where they connect to the 259b I get 175 -
j237. Does anyone have an explanation as to why the resistance value
changes simply by reversing the way the txsmsn line is attached to the
analyzer? I have repeated this numerous times, attaching and
reattaching always with very near the same results. Does the fact that
the antenna has unequal leg lengths somehow explain this? If it
matters, there is a 1:1 current balun between the txmsn line and the
antenna feedpoints.

On 10 May 2007 09:28:52 -0700, dykesc wrote:

Does the fact that
the antenna has unequal leg lengths somehow explain this?

That, and it is sloping (compounding asymmetry).

If it
matters, there is a 1:1 current balun between the txmsn line and the
antenna feedpoints.

It may not be very useful.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On 10 May 2007 09:28:52 -0700, dykesc wrote:


Does the fact that
the antenna has unequal leg lengths somehow explain this?


That, and it is sloping (compounding asymmetry).


If it
matters, there is a 1:1 current balun between the txmsn line and the
antenna feedpoints.


It may not be very useful.

73's
Richard Clark, KB7QHC

Richard

I hate to revisit my main problem with you, since you are normally so
amusing, but if you aren't going to help the poor newbie, could you
please keep quiet and not make his confusion worse?

tom
K0TAR

On Fri, 11 May 2007 19:53:30 -0500, Tom Ring
wrote:

I hate to revisit my main problem with you

but....

As you see it, I just kicked out the crutches from beneath a cripple
newsboy who is struggling in the street and you as the social reformer
prefers to convert this evil sinner instead.

"Won't somebody think of the children!!?"

Does that put us back on the amusement track?

73's
Richard Clark, KB7QHC

Richard Clark wrote:



but....

As you see it, I just kicked out the crutches from beneath a cripple
newsboy who is struggling in the street and you as the social reformer
prefers to convert this evil sinner instead.

"Won't somebody think of the children!!?"

Does that put us back on the amusement track?

73's
Richard Clark, KB7QHC

I hate conversion attempts specifically and football in general, so
you'll never find me trying it.

I am also of the opinion that children are way overrated. They are easy
to make, and not worth nearly as much as an experienced middle aged worker.

tom
K0TAR

dykesc wrote in news:1178814532.207062.89230
@h2g2000hsg.googlegroups.com:

I just aquired an MFJ 259b analyzer. I'm using it to measure the
impedance at the input of the 300 ohm twin lead I have feeding a
sloping off center fed dipole. I measured 75 -j236 at 3.94 Mhz. When i
reverse the txmsn line leads where they connect to the 259b I get 175 -
j237. Does anyone have an explanation as to why the resistance value
changes simply by reversing the way the txsmsn line is attached to the
analyzer? I have repeated this numerous times, attaching and
reattaching always with very near the same results. Does the fact that
the antenna has unequal leg lengths somehow explain this? If it
matters, there is a 1:1 current balun between the txmsn line and the
antenna feedpoints.

You haven't mentioned a balun at the 259B, or any other device to float
the measurement terminals to make a true differential mode impedance
measurment without significantly disturbing the thing you are measuring.

There are issues regarding balance of the feedline with an OCF dipole,
but reversing the meter for different readings suggests that the meter
terminals are not sufficiently isolated from the environment (ground, the
adjacent transmission line, possibly a power cord).

What have you done to make the 259B appear as an isolated impedance
meter?

Owen

You haven't mentioned a balun at the 259B, or any other device to float
the measurement terminals to make a true differential mode impedance
measurment without significantly disturbing the thing you are measuring.

There are issues regarding balance of the feedline with an OCF dipole,
but reversing the meter for different readings suggests that the meter
terminals are not sufficiently isolated from the environment (ground, the
adjacent transmission line, possibly a power cord).

What have you done to make the 259B appear as an isolated impedance
meter?

Owen

Owen,

My recent experience and your post has convinced me that a direct
termination of my balanced transmission line (300 ohm twin lead) to
the 259b is going to be problematic. I am now measuring through the
4:1 current balun in my MFJ tuner. Wish I had a 1:1. At 7.185 Mhz
through the 4:1 balun (tuner bypassed) I get 19 -j48. Assuming an
ideal balun I believe your previous post stated this would be 76 -j192
on the high side. At most even harmonic frequencies I've measured, it
appears the 4:1 balun in the tuner is actually resulting in too low a
resistive term impedance. As I write this I recall some text in the
antenna book about calculating the proper 1/4 wave Zo transmission
line impedance needed to transform to a desired impedance. Will this
work for any odd multiple of a 1/4 wave transmission line? On second
thought this wouldn't work on the harmonics would it? If I set it up
for 20 meters it wouldn't work on 40. The whole deal with the off
center feed is to be able to use it on even harmonics (80, 40, 20
meters). Guess I'll just work on figuring out the best compromise
transmission line, but I'm fairly convinced I can do better than the
300 ohm twin lead.

This is all just for the challenge of understanding the theory and
making it work in application. The tuner is doing fine for all 3 bands
in my current configuration.

Thanks for helping out a Stuggling Crippled Newbie Street Urchin.

On 12 May 2007 21:25:33 -0700, dykesc wrote:

My recent experience and your post has convinced me that a direct
termination of my balanced transmission line (300 ohm twin lead) to
the 259b is going to be problematic.

The way you described it (presuming an efficient choking BalUn) with
battery operation and you remote from it, suggests any issue of
"unbalance" is strictly academic. You can force it to become a real
problem if the case of the 259 is close to ground where the chassis
adds a capacitance to ground, but that is a rapidly diminishing value
as you raise it (couldn't be more than 1 or 2 pF at 6 feet up).

I am now measuring through the
4:1 current balun in my MFJ tuner.

This is extremely unlikely (being a current BalUn) unless it is
specifically specified as one (and even then, many professed 4:1
current BalUns are in fact no such thing).

You have the means to test the assertion, use your 259 to measure the
isolation of the BalUn. This was the subject of a recent thread.

Wish I had a 1:1. At 7.185 Mhz
through the 4:1 balun (tuner bypassed) I get 19 -j48. Assuming an
ideal balun I believe your previous post stated this would be 76 -j192
on the high side. At most even harmonic frequencies I've measured, it
appears the 4:1 balun in the tuner is actually resulting in too low a
resistive term impedance.

Fixation on BalUns has clouded a simpler solution: wind a choke in
the line and dump the ferrites of suspect quality.

As I write this I recall some text in the
antenna book about calculating the proper 1/4 wave Zo transmission
line impedance needed to transform to a desired impedance. Will this
work for any odd multiple of a 1/4 wave transmission line?

Yes, but discrepancies mount up dramatically as you multiply them
(tolerances at 1/4 demand greater precision at 3/4, and even greater
at 5/4). Besides, this doesn't address the odd readings you
experience.

On second
thought this wouldn't work on the harmonics would it? If I set it up
for 20 meters it wouldn't work on 40.

Sub Harmonics wouldn't suffer terribly. You do have a tuner after
all.

The whole deal with the off
center feed is to be able to use it on even harmonics (80, 40, 20
meters).

Off center feeds merely give you different Zs for the same resonances
- something of a shell game where you get to move your problems to
another band (guess what? This is what may be happening.).

Guess I'll just work on figuring out the best compromise
transmission line, but I'm fairly convinced I can do better than the
300 ohm twin lead.

It would be simpler to hang a second, half-length dipole beneath a
full size dipole and forget the off center feed.

This is all just for the challenge of understanding the theory and
making it work in application. The tuner is doing fine for all 3 bands
in my current configuration.

Many antennas work just fine until the operator discovers a new tool
that proves it doesn't (in spite of a wall full of QSL cards).

Thanks for helping out a Stuggling Crippled Newbie Street Urchin.

Wait until you face the sewer rats of Rio.

73's
Richard Clark, KB7QHC

On May 13, 2:07 am, Richard Clark wrote:

The way you described it (presuming an efficient choking BalUn) with
battery operation and you remote from it, suggests any issue of
"unbalance" is strictly academic. You can force it to become a real
problem if the case of the 259 is close to ground where the chassis
adds a capacitance to ground, but that is a rapidly diminishing value
as you raise it (couldn't be more than 1 or 2 pF at 6 feet up).

Richard, I tired measurements again with my twin lead directly
terminated to the 259b. I got better, more consistent results after
taking great care to insure the analyzer and line were well isolated
from ground, other conductors, and myself. I then took the same
measurements with the 4:1 balun between the twin lead and the
analyzer. Unfortunately the results create new concerns. For example
at 7.185 Mhz with the balun in the circuit (tuner in bypass mode) I
got 19 -j48. Again at 7.185 Mhz with the balun out (twin lead directly
terminated to 259b) I got 159 -j443. Doesn't look like 4:1 to me.
Similar spreads in the 80m and 20m bands.

This is extremely unlikely (being a current BalUn) unless it is
specifically specified as one (and even then, many professed 4:1
current BalUns are in fact no such thing).

The MFJ manual for the 993b tuner says the balun is a 4:1 "current"
balun. Haven't looked inside to confirm this.


You have the means to test the assertion, use your 259 to measure the
isolation of the BalUn. This was the subject of a recent thread.


Thanks. I'll search for the thread. Sounds like fun.

Wish I had a 1:1. At 7.185 Mhz
through the 4:1 balun (tuner bypassed) I get 19 -j48. Assuming an
ideal balun I believe your previous post stated this would be 76 -j192
on the high side. At most even harmonic frequencies I've measured, it
appears the 4:1 balun in the tuner is actually resulting in too low a
resistive term impedance.

Fixation on BalUns has clouded a simpler solution: wind a choke in
the line and dump the ferrites of suspect quality.

Would you please elaborate on this? Wind a choke where? In the twin
lead? In the short transmitter to tuner coax line?
Thought I read somewhere that only coax can be used for simple 8 to 10
turn chokes. Balanced lines (i believe because of mutual conductor
inductances) can't be coiled as chokes.


Many antennas work just fine until the operator discovers a new tool
that proves it doesn't (in spite of a wall full of QSL cards).

Partly the reason I'm trying to learn all I can about the
configuration I've currently got. That and I like the technology
aspects of the hobby as much or more than I do operating.

Thanks for helping out a Stuggling Crippled Newbie Street Urchin.

Wait until you face the sewer rats of Rio.

OK I'll bite. Who are the Rio rats?

Thanks for your help. Any thoughts on those measurement results
earlier in the post will sure be appreciated.

73's
Dykes Cupstid AD5VS

On 13 May 2007 16:55:44 -0700, dykesc wrote:

Richard, I tired measurements again with my twin lead directly
terminated to the 259b. I got better, more consistent results after
taking great care to insure the analyzer and line were well isolated
from ground, other conductors, and myself.

Hi Dykes,

It should be comforting that observing standard precautions produces
repeatable results.

I then took the same
measurements with the 4:1 balun between the twin lead and the
analyzer. Unfortunately the results create new concerns. For example
at 7.185 Mhz with the balun in the circuit (tuner in bypass mode) I
got 19 -j48. Again at 7.185 Mhz with the balun out (twin lead directly
terminated to 259b) I got 159 -j443. Doesn't look like 4:1 to me.
Similar spreads in the 80m and 20m bands.

This sounds like you've inserted the entire tuner to obtain the 4:1
BalUn (once you threw the right switches).

If, as you say, education is a principle goal, then build a proper 4:1
current BalUn. It is actually quite simple and requires only two
transmission lines and a several dozen beads. Basically it is two 1:1
current BalUns fed in parallel and loaded in series.

You will be simultaneously checking your system, and testing the
authenticity of the MFJ claim:
The MFJ manual for the 993b tuner says the balun is a 4:1 "current"
balun. Haven't looked inside to confirm this.

You have the means to test the assertion, use your 259 to measure the
isolation of the BalUn. This was the subject of a recent thread.

Thanks. I'll search for the thread. Sounds like fun.

If after a fruitless search (it's easy enough to get slogged down in
the snow drift of useless posts here) you don't find it, ask for help
here. Mentioning you tried the archives will save others from whining
about how much effort they went to answer a stupid question. (I won't
whine, and I never call any question stupid - although I frequently
dope slap some of the denser questioners.)

Fixation on BalUns has clouded a simpler solution: wind a choke in
the line and dump the ferrites of suspect quality.

Would you please elaborate on this? Wind a choke where? In the twin
lead?

Sure, twist it candy cane (or barber shop pole) style and wind it
around a liter bottle with at least its width as separation between
windings.

In the short transmitter to tuner coax line?

Actually for severely unbalanced dipoles (and yours qualifies for
Queen of the May), you may need a choke at the feed point to the
antenna, and then again a quarter wave away from there.

Thought I read somewhere that only coax can be used for simple 8 to 10
turn chokes. Balanced lines (i believe because of mutual conductor
inductances) can't be coiled as chokes.

Even if I'm wrong, it is both cheap and instructive. So few here
actually step up to the bench that I don't take their flabby word that
I'm wrong. You may be the first with authentic achievement to break a
record! You've already lapped the field of these arm-chair analysts.

Many antennas work just fine until the operator discovers a new tool
that proves it doesn't (in spite of a wall full of QSL cards).

Partly the reason I'm trying to learn all I can about the
configuration I've currently got. That and I like the technology
aspects of the hobby as much or more than I do operating.

Where this hobby whose technological demand largely consists of
pushing a credit card across a sales counter, antennas still have the
capacity to stretch the imagination.

Thanks for helping out a Stuggling Crippled Newbie Street Urchin.

Wait until you face the sewer rats of Rio.

OK I'll bite. Who are the Rio rats?

This is an allusion to an SK who compared those who couldn't exercise
their minds as being fodder for the orphans of Rio, who had more will
to succeed than they did. He characterized them as sewer rats gnawing
on our lazy carcasses.

Thanks for your help. Any thoughts on those measurement results
earlier in the post will sure be appreciated.

They will reveal more in comparison to those measurements that follow.

73's
Richard Clark, KB7QHC