|
How would you do it?
I'm thinking that first, I'd need to construct an adapter for the VNA - sma connector, short length of coax, with clips on the center conductor and shield to clip to the two sides of the ladder line (but, the quality of the connection between the clips and the ladder line wires (among other things) is going to introduce variables, right?)
Second, I should calibrate for the desired frequency range using the adapter as described above, in order to "calibrate out" as much of the peculiarities of the measurement system as I can - I would clip my adapter to my QRP-Labs dummy load for the load part, leave the clips dangling for the open, and clip them together for the through.
What kind of trouble am I going to be getting myself into?
|
How do you normally operate the ladder line? Using a balun or via an antenna tuner that provides balanced output? In case you use a balun simply connect the nanoVNA to the balun and measure. When using an antenna tuner disable all "tuning" and use the tuner as a pure balun. Without a balun (in case your receiver has something buildin) your proposed approach will work but be aware the ladder line impedance is high compare to 50ohm -- NanoVNA Wiki: /g/nanovna-users/wiki/homeNanoVNA Files: /g/nanovna-users/filesErik, PD0EK
|
Erik, thanks for responding.
For the moment, I'm just trying to understand what is going on, impedance-wise, with my antenna system. Not that I am having "problems" that need to be "solved," just curious.
When operating, I will have the following: 130+ feet of copperweld, hung about 60 feet up between two trees, center fed with about 100 feet of 450 ohm ladder line, connected to a 1:1 current balun, short coax run to a ground buss, short coax run from buss to tuner, coax from tuner to radio. I would like to understand what sort of impedance transformation is happening at each connection/through each element.
|
Consider buying one of these and use it between the nanoVNA and the ladder line (without your 1:1 balun). This will allow you to measure with best accuracy even if you have calibrated the nanoVNA with the standard calibration set. Your 1:1 balun may not be optimal as the ladder line is 450 ohm and coax something like 50ohm (?) To understand each impedance transformation connect the nanoVNA instead of the radio and switch to TDR mode. -- NanoVNA Wiki: /g/nanovna-users/wiki/homeNanoVNA Files: /g/nanovna-users/filesErik, PD0EK
|
Erik, if I understand correctly, the purpose of the 1:9 balun is to reduce impedance by a factor of 9. So, presuming that I have a 450 ohm load at the end of the ladder line (which might not necessarily be true, under the circumstances), by connecting the VNA to the 1:9 balun, the VNA should report an impedance of 50 ohms, correct? Any deviation from 450 ohms at the end of the ladder line would show up as a deviation from 50 ohms on the VNA, at 1/9th the deviation at the end of the ladder line, correct? For example, if the actual impedance at the end of the ladder line were 900 ohms, the VNA would report an impedance of 100 ohms.
Yes, the ladder line is (nominally) 450 ohms, and the coax (nominally) 50 ohms. I am working to understand the use of a balanced matching network, the intended purpose of which is to transform the impedance of the ladder line to an impedance appropriate for the coax, but that is for another day.
I will have to study up on "TDR" mode - I get the general idea (impedance as a function of the distance from the measuring device to a specific location in the network), but I am not at all certain I understand it well enough to derive any significant information from its use in the present circumstances.
For the time being, I prefer to measure impedance at the location of each connection. I'd measure at the end of the ladder line, then at the coax side of the 1:1 balun, then at the end of the coax that would be connected to the ground buss, then at the other side of the buss, then at the end of the coax that would be connected to my tuner, then at the other side of the tuner, and finally, at the end of the coax that will connect the tuner to my radio... Whew!
|
From: entilleser@... Measuring impedance at the bottom end of a ladder-line-center-fed wire dipole - How would you do it? I've done it. Simply calibrate your measurement plane at the output of the balun that feeds the ladder line. The attached graph was done with an AIM-4170D but the calibration procedure for it and the NanoVNA are the same. The top graph was done with the calibration plane at the output of the balun and the bottom trace was done with the calibration plane at the input to the balun.
|
W5DXP, thanks for your response...I think... What I mean is that you have provided me with another thing to learn about, that is how to "calibrate your measurement plane at ..." Do you have any reference I could use to learn about this?
Also, in reference to my immediately previous post, I believe I should have said "...a purely resistive 450 ohm load..."
|
From: entilleser@... So, presuming that I have a 450 ohm load at the end of the ladder line (which might not necessarily be true, under the circumstances), by connecting the VNA to the 1:9 balun, the VNA should report an impedance of 50 ohms, correct? ... For example, if the actual impedance at the end of the ladder line were 900 ohms, the VNA would report an impedance of 100 ohms. Concerning ideal TLTs: First sentence above is true. Second sentence is false. A Transmission Line Transformer has an electrical length. If the load on the 9:1 TLT-balun is not the designed-for load of 450 ohms, the transformation will not be 9:1. A TLT is NOT an ordinary (N1/N2)^2 transformer. Another way of saying the same thing is: If the SWR on the 450 ohm ladder line connected to a 9:1 TLT is not 1:1, then the impedance transformation ratio will most probably not be 9:1. As an illustration, here is a graph of the impedance at the output of a 50 ohm 1:1 TLT vs the "transformed" impedance at the input of the 1:1 TLT. Note that the only point at which the transformation ration is close to 1:1 is when the impedance at the output is close to 50 ohms.
|
W5DXP, thanks again, good information.
With regard to your previous post, and calibration at the "measurement plane." II understand correctly, this means conducting the SOLT calibration procedure with a 50 ohm resistive load at the point of interest in the network. In the instance of wishing to measure the impedance at the end of the ladder line on the far side of the 1:1 balun, I would connect the VNA to the balun, and conduct the SOLT calibration at the balun output - in my case, a pair of terminal posts. So, with the VNA connected to the input side of the balun, I would first short the output terminal posts, then leave them open, then install a 50 ohm resistive load between them, and...I wouldn't do the "through" calibration, in this case. After the calibration steps, I would connect my ladder line to the balun's output terminal posts, and would then be looking at the impedance of the feed-line/dipole network at the point where it connects to the balun. Correct? And, if so, what would be the most appropriate way to connect the 50 ohm load across the output terminal posts for the "L" portion of the calibration?
|
When using a 1:9 transformer you should connect the nanoVNA to the "1" side and the ladder line to the "9" side. For calibration use a 450 ohm resistor (or a 470 ohm with a second resistor to make it 450 ohm) connected to the "9" side, and a short (short wire between the two terminals) and a open. Due to the transformer the nanoVNA will see the 450ohm as 50ohm and will easily calibrate Don't bother about the 450 ohm resistor not being perfect. Your antenna is probably worse And yes, the 1:9 transformer/balun will only be realy 1:9 when loaded with 450 ohm but you will still see the difference when other impedances are connected to the "9" side. -- NanoVNA Wiki: /g/nanovna-users/wiki/homeNanoVNA Files: /g/nanovna-users/filesErik, PD0EK
|
Practical measurement using a 1:1 balun, connected the nanoVNA to one side, other side to the ladder line of a half size G5RV. The ladder line does the impedance transformation to the dipole of the G5RV I'm displaying the SWR instead of the impedance but you can display what you want. -- NanoVNA Wiki: /g/nanovna-users/wiki/homeNanoVNA Files: /g/nanovna-users/filesErik, PD0EK
|
For the moment, I'm just trying to understand what is going on, impedance-wise, with my antenna system. Not that I am having "problems" that need to be "solved," just curious.
When operating, I will have the following: 130+ feet of copperweld, hung about 60 feet up between two trees, center fed with about 100 feet of 450 ohm ladder line, connected to a 1:1 current balun, short coax run to a ground buss, short coax run from buss to tuner, coax from tuner to radio. I would like to understand what sort of impedance transformation is happening at each connection/through each element.
First of all _since I DID NOT READ ALL MESSAGES in the discussion, this may be a dupe.
If so - my apology.
Analyzing the above post - here is my reply / attempt to put the whole "impedance issues " in different perspective.
AND ONLY the impedance / balance issues !
130 cubits, sorry I measure in meters, center fed radiator ( antenna) is missing few "dimensions" .
So I will make an assumptions.
Its BASIC intent to RESONATE on FUNDAMENTAL frequency
hence
it is half wavelength long on such fundamental frequency , commonly referred to as (center fed ) dipole antenna.
As such it has FEED POINT IMPEDANCE of roughly 70 Ohms AND it is a BALANCED radiator when fed in center.
Ideally it should be fed using BALANCED feed line of CHARACTERISTIC impedance close to 70 Ohms.
( common "zip line" is OK to use )
In practice it is generally fed with UNBALANCED coaxial feed line of CHARACTERISTIC impedance ranging from 50 to 75 Ohms.
The differences / mismatch in feed point impedance and feed line characteristic impedance are not worth writing home about.
However, the connection of unbalanced feed line to balanced antenna is worth futzing about - hence BALANCED TO UNBALANCED (matching BALUN ) transformer is commonly utilized.
Please note that the (BALUM) transformation ratio IS 1:1 - again ignoring slight differences in impedance.
There are other BENEFITS besides matching the impedance , but my point was to concentrate on impadnces of the SYSTEM from antenna perspective on SINGLE fundamental frequency.
It is a horse of different color when "multiband antenna" is the goal.
|
If you are interested in doing a detailed analysis you may be interested in how simsmith can help you Here is an excellent video explaining how to do impedance matching analysis including where the power goes. -- NanoVNA Wiki: /g/nanovna-users/wiki/homeNanoVNA Files: /g/nanovna-users/filesErik, PD0EK
|
erik, interesting video, which I will review at length another time. For the moment, I am trying to get a handle on an actual, physical system, to give myself a framework for understanding. The information (in various forms) available on the internet is, for me, essentially infinite - I mean that there is far more than I will be able to read/watch/comprehend in the time I have remaining on this earth. I have always had an easier time comprehending when I have a physical model that I can manipulate, therefore my efforts, here, to learn how to characterize the model I have available to me, that is, my antenna system.
This is my answer, also, to the post from vaclav_sal (for which, thank you). The information in your post is valuable, I'm sure, and, when I have attained a reasonable comprehension of my particular antenna system, I hope I will be able to use it. And, by the way, a "multi-band" antenna is my eventual goal - which I hope to reach before I am too old and decrepit to use it.
|
This is one of the big advantages of a small, self contained, unit. One can connect it to a balanced load or coax with little influence from surroundings ofr connection to a power source. During CAL. and making the measurement, let the NANOVNA dangle, do not hold on to it as your body will introduce variables. If you are measuring at HF frequencies, especially on our lower bands, the introduction of a very small instrument intelligently connected to the open wire line will have negligible impact on your ultimate measurements. To form the connection between the VNA and the open wire transmission, I'd first find just the right size bare tinned wire that will snugly fit the center of the s11 SMA connector (CH 0). Then,, make yourself two *very short* clip leads using the small alligator clips. Clip the wire to one end of one clip and run the other between the SMA backshell and one side of your open wire line. Clip the other clip lead that holds the wire in the center of the SMA connector to the other side of the open wire line. Make your measurement. Be sure in both calibration and making the measurement to NOT HOLD the VNA. Measurements at HF are reasonably accurate with this method. 6-Meters on upward, not quite so.... Dave - W?LEV On Sat, Dec 28, 2019 at 1:19 PM entilleser via Groups.Io <entilleser= [email protected]> wrote: How would you do it?
I'm thinking that first, I'd need to construct an adapter for the VNA -
sma connector, short length of coax, with clips on the center conductor and
shield to clip to the two sides of the ladder line (but, the quality of the
connection between the clips and the ladder line wires (among other things)
is going to introduce variables, right?)
Second, I should calibrate for the desired frequency range using the
adapter as described above, in order to "calibrate out" as much of the
peculiarities of the measurement system as I can - I would clip my adapter
to my QRP-Labs dummy load for the load part, leave the clips dangling for
the open, and clip them together for the through.
What kind of trouble am I going to be getting myself into?
-- *Dave - W?LEV* *Just Let Darwin Work* *Just Think*
|
You can calibrate your measurement reference plane at a number of places. You can do your calibration at the SMA terminals of the NanoVNA. You can do your calibration at the ends of coax cables terminated in SO-239 coax connectors as I have. You can do your calibration at the input or output terminals of your balun and I have done that also. Where you do a particular calibration is called the reference plane. If you want to read the impedance at your balun output, then perform your SOL calibration with your Short, Open, and Load connected to the output terminals of your balun. I have done that for my ladder line fed HF antenna system from 3 MHz to 30 MHz. Here's some further information:
|
entilleser@... wrote: ... Correct? And, if so, what would be the most appropriate way to connect the 50 ohm load across the output terminal posts for the "L" portion of the calibration? Yes, you are correct. At HF, it doesn't much matter how you connect the 50 ohm load. I have two 100 ohm carbon resistors in parallel that measure 50.1 ohms that I connect with alligator clips. For most of your measurements at HF, the inductance or capacitance of the test leads doesn't much matter.
|
Making impedance measurements thru a 1:1 balun looking into the rig-end of the 300 ohm twin lead going to my 130 ft dipole is when I realized how poorly the Nano measures impedances above about 300 ohms. I made the same measurements with a GR-1606A (which I did many times before with confidence) and there was a huge difference. One technique called out in the GR-1606's manual to measure impedances above its measurement range is to "Shunt down" the device under test's impedance with a cap of known capacitance (a reference cap). Then remove this capacitive reactance mathematically. To accomplish this convert the initially measured impedance to its parallel equivalent and then convert these parallel equivalents to conductance and susceptance (i.e. admittance). Subtract the "reference" cap's susceptance from the initially determined susceptance and then convert the results of this back to an impedance.....this being the actual Z looking into the balanced line. Sounds like a mess but after some practice it isn't so bad. Maybe there's a Youtube video out there (same basic ideas used for impedance matching that might be in the videos mentioned above).
|
