OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

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.

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

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

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

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

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

Owen Duffy wrote in
:

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

I did mean to add that in this case, the impedance at the end of the
isolated feed line is not necessarily (and not likely to be) the same as
when it is connected to your transmitter. Again the same issue arises about
the path to ground for common mode current, and the influence that has on
the antenna feed point impedance transformed by the transmission line.

You need to think about the purpose of the measurement.

Owen

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

On May 10, 4:14 pm, Owen Duffy wrote:
Owen Duffy wrote :

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

I did mean to add that in this case, the impedance at the end of the
isolated feed line is not necessarily (and not likely to be) the same as
when it is connected to your transmitter. Again the same issue arises about
the path to ground for common mode current, and the influence that has on
the antenna feed point impedance transformed by the transmission line.

You need to think about the purpose of the measurement.

Owen

Owen I am operating the 259b on battery power and keeping the meter
away from all conductors including myself (hands). I have searched
posts on the internet about using unbalanced analyzers to measure
balanced line input impedances with little success other than one post
which spoke to the need to measure impedance in 3 connection
configurations and then mathematically solving for the final impedance
value. The 3 configurations were balanced line lead A to ground with B
gounded. B to ground with A grounded. And A &B connected together to
ground. (Ground was defined as the common on the analyzer.) The author
then mentioned some mathematical determination of impedance which he
didn't elaborate on. Are you familiar with this method? What is the
mathematical solution once you have the data?

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

On May 10, 4:14 pm, Owen Duffy wrote:
Owen Duffy wrote :

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

I did mean to add that in this case, the impedance at the end of the
isolated feed line is not necessarily (and not likely to be) the same as
when it is connected to your transmitter. Again the same issue arises about
the path to ground for common mode current, and the influence that has on
the antenna feed point impedance transformed by the transmission line.

You need to think about the purpose of the measurement.

Owen

One more question Owen. If I measure the complex impedance on the low
side of a 4:1 current balun. Is the impedance on the high side simply
4 times the low side? Can I just multiply the resistive term by 4 and
the complex term by 4?

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

dykesc wrote in
ups.com:

On May 10, 4:14 pm, Owen Duffy wrote:
Owen Duffy wrote
:

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

I did mean to add that in this case, the impedance at the end of the
isolated feed line is not necessarily (and not likely to be) the same
as when it is connected to your transmitter. Again the same issue
arises about the path to ground for common mode current, and the
influence that has on the antenna feed point impedance transformed by
the transmission line.

You need to think about the purpose of the measurement.

Owen

One more question Owen. If I measure the complex impedance on the low
side of a 4:1 current balun. Is the impedance on the high side simply
4 times the low side? Can I just multiply the resistive term by 4 and
the complex term by 4?


If the balun was ideal, you could do that. To the extent that it isn't,
error will be introduced.

I think some of this comes back to the question "what do you want to
know".

If you want to know the load as connected to your transmitter, simulate
that connection including baluns, earth connections etc.

If you want to make an isolated measurement for some reason, I would have
expected that the 259B on batteries at low HF frequencies and supported
clear of other objects would be sufficiently isolated to not get the
differences you observed.

My suspicion is that if you follow the calculation path you described,
the inherent range / accuracy of the 259B will be a problem, and the
precision introduced by the measurement approach will be lost due to
instrument error, indeed you might be worse off.

Owen

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

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

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

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

OCF Sloping Dipole Txmsn Line Input Resistance Measurement
 

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