Phasing wire beams with 2180

[killy]

SO after reading more on VNA and their funciions,
i see that the 2180 is not a 2x 4170. The 2180 is what
is referred to as a T/R network analyzer
(transmission/reflection).
A true 2x4170 VNA would provide RF power and receiver
on both ports and would be referred to as a "S parameter"
network analyzer, providing increased measurement
accuracy by using the full 2 port error correction techniques.

Personnally this "limitation" will not affect my use of the
2180, which is exclusively for horizontal multi element
phasing and related measurements.

My applications are for 80M CW,
5 element phased inverted v's and
3 wire element phased horizontal dipoles.

Now one thing we know (well i found out from ON4UN book)
is that if you try to use an electrical 135 degree length of
coax, hoping to get 135 degrees phase shift when connected
to an element, you will probably not get 135 degrees shift,
UNLESS the element it feeds has the same impedance as the coax.

Since my ground is solid granite, i have found so far with the
existing antennas, the "real" ground to be somewhere around
20 to 40 feet below this rocky hilltop, depending on frequency.

When modeling the arrays in this environment, the variables
(mystic ground level, drooping and sagging wires, varying
support structures heights) all contribute to a substantial
difference between model values and reality.
Modeling gives you the current magnitude and phase required
to max out the gain (i dont care about f/b here). But in order
to get those values with the real elements and their actual
impedances, you must make physical measurements at the feedpoints.

This is where i plan to use the 2180, 2 elements at a time.
Port A goes to one element and port B goes to a coax "T" in line
with the other side of the phasing coax to the second element.
Now i know that second element will not be 50 ohms; so the
B port 50 ohm impedance is sure to affect the readings.

From my very limited knowledge of the 2180 and VNAs in general,
i should be able to see the Z and phase at these feed points, only
after i have added a properly designed LC network to make that feed
point, 50 ohms.

The main reason for this posting is to state that i have found very
little in the use of the 2180 (or 2 ports VNAs) for antenna
measurements and phasing in particular, in the litterature or
on the web.

If anyone reading this far has first hand information or reference
to litterature/web sites they would like to share, I would be most
appreciative for the input.
Thanks

roy, va2gu

[Bob]

Hi Roy,

The input impedance of Port B on the VNA2180 is very close to 50 ohms over the whole range. The return loss is better than 30 dB. I've uploaded a screen shot showing Zin measured with an AIM4170.
www.w5big.com/Port_B_Zin.gif

The output impedance of Port A is close to 50 ohms in the HF band and then goes up in the VHF range. Here's a screen shot of Port A, again measured with an AIM4170. The output signal from Port A was set to zero amplitude for this measurement:
www.w5big.com/Port_A_Zout.gif

73/ Bob

[killy]

OKAY so i'm learning.
First tks Bob for the A and B port scans.
Second i'd like to make a statement, a tribute to Bob W5BIG.
On Christmas eve, i sent a query to Bob, thinking he'd be back
at it after the holidays. Well, on Christmas day i already had
a full reply. So if any one of you has any doubt about his
dedication to the VNA products, better rethink.

Now for the pot!!
I have put up two 80m dipole elements (yes it's cold, but we
hams are crazy, right?) and playing with the initial tuning and
measurements, learning to use the 2180 in the process.
I dont think that port B is going to be of any use in tuning the
array.
The RF is directly feeding element 1 and through a 135 degree
phasing line, feeding element 2.
The RF is coming from port A and the other side of the delay
line has a T going to port B and direct to element 2.

Looking at the 2180 schematic i saw it had receive functions
and had (wrongly) presumed that sending RF from port A into
element 1 and connecting port B to that T, port B would be able
to read Z, etc.
Okay, port B is not sending any output RF, but can it not read
and interpret the same parameters that port A is able to do?
Is that possibly in the offing, or am i going at this whole
thing the wrong way?
Seems that part of the software for doing the receive functions in
port A could be reused for port B. Is that major rewrite?

tks again
roy
va2gu

[Bob]

The response of Port B is proportional to the current flowing into it. The response of Port A is proportional to the voltage, so in a sense, the ratio of the voltage to the current could be interpreted as an impedance.

If a plain resistor (or two terminal complex impedance circuit) is connected between Ports A and B, the impedance can be determined. For this special case, the type of circuit (two terminal) is known.

In the general case, when the external circuit is considered to be a two-port network, the ratio of Port A voltage to Port B current is not a unique impedance. There are an infinite number of circuits that would produce the same ratio. I don't know if this "impedance" would be useful for your application or not. Perhaps others can comment on this.

--73/Bob

[musiharry]

Dear guys,
I'm sorry if I misplaced post. I am a newcomer here. I bought AIM-4170 last year and hoping to replace the task of my HP8508 Vector Voltmeter which is too heavy to carry. My question is how to cut several transmission cables with same phase using this AIM-4170? I work with the constant frequency in VHF band up to 112 MHz. I usually make one cable with any length as reference (with HP8508) and record the phase and then cut some other cable following the reference value of phase which has been recorded.
Thank you.

[killy]

1/4 wave measuring: shorted or open ended?

Okay, i will be phasing a 5 element delta loop on 80m and will be using
stubs to replace the caps needed for the L networks. I want to follow the
exact procedure to get the correct phasing/currents to the elements.

I am practicing inside before the wx warms up and finding inconsistencies
in the readings, whether i am doing the test for 1/4 wave using a short or
open at the end of the coax.
THIS IS NOT A FAULT OR DEFECT IN THE 2180. BUT RATHER A CONSEQUENCE
OF MEASUREMENT TECHNIQUES.

YOU DONT HAVE TO READ THE FOLLOWING, IF FROM EXPERIENCE YOU CAN
ANSWER THE FOLLOWING QUESTION.

When measuring the "phase zero crossing" frequency of a piece of
coax (be it at 90* or 180* crossing), should one use the freq. obtained
when the free coax end is shorted or open? The diff. is as much as
300kc and almost 4 feet!! This may be caused by the measuring
wave being reflected or absorbed at the end, with the consequential
additional losses; but for 3.5mc this should not be that great....

From my past experience, it looks like the open end test is closer to the
"real thing".

I believe that when you use lengths of coax where the phase angle goes
through zero crossing at a very steep slope, many factors get into the
act to modify the real zero crossing.

I have calibrated the 2180 with the supplied standards, using no
additionnal cables/adapters.

I have cut a 45' 6.5" 50 ohms coax with an educated guess of 0.66 VF.
I have peeled back 1/2 inch of coax and just twist (short out) the shield
around the center conductor. I have put a PL259 on the other end to go
to the 2180. Am running the "distance to fault" and this gives
me 44.55 feet.
I have to enter 0.675VF to obtain my physical length of 45.54ft in the
"DISTANCE TO FAULT" field.
Now i open the end, leaving all the cable at the same place and shape
on the floor and rerun the "distance to fault" using that value of
0.676VF. I now get 43.86; i now have to use VF of 0.705 to get the
physical length of 45.54ft in the display window.

Now i run the 1/4 wave stub length program with the free end still
open and same place/layout on the floor and using the VF(.705) above.
The program recalculates the VF and gives me 0.645 and tells me for
3525 i am 8.7" too long.
I short the free end the same way as before and rerun the program.
Now it tells me i am 2.89' too short.

If i run the "scan A" with coax end shorted, phase is zero at 3732
and 7501;
With an open, phase is zero at 3469 and 7139.

So the question remains for field cuttings, should one use the short
or open conditions to get the real thing?

I use the Autek RF1 analyst and for the short, it shows high Z (946
ohms) at 3750 which agrees with the 2180.
For the open, it shows 4 ohms at 3445 again agreeing with 2180.

Tks if you've read so far. But this instrument is sure a joy to play
with.

va2gu

[Bob]

I get a difference of about 0.5% when measuring length with a short and an open.

I suggest mounting a female connector (or a male connector with a female-female adapter) on the far end of the coax so you can easily screw on the open and short cal loads for this test. That will make the stray capacitance and inductance more consistent for both the short and the open tests.

You can experiment indoors with a physically scaled down antenna to get a feel for how dimensions affect the impedance.

--73/Bob

[w0qe]

Something is definitely wrong with the test setup. I just took a chunk of coax with wasn't quite as long as yours which had N connectors on both ends. Carefully calibrating the 2180 and keeping track of the reference planes etc. resulted in the quarter wavelength data for the phase = 0 degrees:

Freq. with coax shorted at far end = 4.4076MHz (step size = 1KHz)
This load results in an open at the 2180

Freq with coax open at far end = 4.4052MHz (step size = 1KHz)
This load results in a short at the 2180

These frequencies are closer than 0.1%.

However changing the step size to 1 MHz results in the following:

Freq with coax open at far end = 4.4981MHz (step size = 1MHz)
This load results in a short at the 2180

I then redid the measurements on an HP VNA with the same results. The VF of coax does change slightly with frequency but it is very stable at a constant frequency with varying loads.

73, Larry W0QE

[killy]

YEP , thanks both Bob and Larry. But my wi-fi was out of commission so i could not get on the net ant reply to my own questions before you did. But i'm glad because you provide great input to me and maybe others on the thread.
SO i have answered those questions, tks to the 2 ports of the 2180.
Putting that piece of coax from port A to port B, you can see the
"lousiness" of that coax; the losses in this FOAM (USELESS) dielectric
coax becomes apparent and is the cause for the difference in the
two readings, short/open ended when doing the 1/4w stub meas and
the distance to fault measure.
But it should also be noted that if you have to locate a fault in a buried
"lousy" piece of coax like this one, do not expect to put the shovel right at the
specified fault location.

So i restested by putting a piece of "good" coax with solid pe dielectric,
the difference becomes now barely noticeable.

That piece of foam coax has been sitting arount for over 15 years and
has trapped moisture. This is why i NEVER use foam coax in my LOW
FREQUENCY antenna experiments. I dont even use the so called SSPE
or semi solid poly ethylene. Only the solid pe, which usually is
translucent.
I was going to put that piece of coax to some useful use or do without it.

Now you know what i've done with that foam dielectric coax.......

va2gu

[killy]

The climbing part is done! The five 80m loops are up and tuning has
begun today.
Used the 2180 to "tune out" the 165' odd length RG8 coax going to the first loop and found its caracteristics as if i was right there!
With standard calibration, i get 22+20.3J, and "at the antenna"
202.6+10.6J; just what the doctor ordered!

BOB:
Two little quirks happened.
1. After doing the custom calibr. and running a first SCAN A, the readings
on the right side of the curves were gone; never mind moving the cursor, or putting "vertical freq. lines" nothing doing. Tried shutting off everything and running with std calib, no go; tried loading older scans, nothing.
Reset two more times (shutting off 2180, disconnect USB cable and restarting the program every time (using ver 4.20)) and then it came back up. Does that happen to others too? Is there something one can do to just call those readings from the pull down menu?

2. Still do not have the graticules (hor/vert) ref. lines, to line up the
curves and read the values.

Now the fun begins, finishing putting up each loop, taking down the others and measuring the self Z and the coupled Z, etc, etc, then calculating the mutual Z, then use the modeled currents mag and phase to get the drive Z of each loop.
With that in hand, the phasing network can be designed.

Thanks to the 2180, this becomes doable!!!
More later.
VA2GU

[w0qe]

Some suggestions:

1.) Try version 4.30 which is the latest Bob has posted. I beta test the software and use it regularly and haven't seen the problem you mention. What operating system?

2.) The symptom you describe occurs when there is no cursor visible but clicking in the graph area makes the data on the right side appear. However you said that didn't work for you. Is it possible that the 2180 is still scanning? Is the front panel red LED on? Did you inadvertently set a frequency step size to be very small or is the averaging or settling time set to be very long? Also dragging the window to be a smaller size produces the symptom you describe but again clicking in the window fixes the display.

3.) If you have edited the .cfg file try the default one that Bob provides. I can attest to program issues if the file contains unexpected data.

The .scn files contain the basic data but there is no pull down window to do what you want.

73,
Larry, W0QE

[killy]

TKS LARRY,

loaded up the 430 version and graticule lines back on and in last 4 days of intensive use, never lost
the rh side data. I use XP pro for OS.

Have been playing with those long wire loops, and with winds blowing hard, had to put the 2180 in recycle mode. Sure is great to see the data swing from 220 ohms to 245 ohms and Xj go from 67 to 84 with swinging wires. You can take an average and get pretty close to the "no winds" readings.
This allows you to continue the tuning process even in windy days (80% of the time here).

Had to do custom calibration three times, to reach the far end loops with different lengths coax, .never minding how long it was (keeping it out of the quarter wave multiples though!!).
But the 2180 is so sensitive, if you happen to move a part of that long coax from the snow top to a tree branch, the values get affected. But you got it all there.

va2gu

[killy]

INSTRUMENT AFFECTING THE ACTUAL VALUES BY JUST BEING CONNECTED?

OKAY, for the purists and the non so-purists, it is very nice to be able
to do custom calibration and calibrate out a piece of feeder line going
to your antenna.
Now we know that most (if not all antennas) are not purely resistive;
and even if they were, the following point applies equally well.

When you put the VNA at the ant. terminals, the actual antenna
impedance will change to reflect the parallel combination of the
complex ant. Z and the measuring source Z, in this case the VNA port A

complex Z, as seen by the antenna terminals.

And now you try to measure the effect of coupling between that antenna

and another element.
Let's not confuse the COUPLED impedance and the MUTUAL impedance

(MUTUAL cannot be measured, but only calculated).

As we know, the MUTUAL impedance (Zm12) between 2 elements is

proportional to both elements SELF impedances (Z11 and Z22) and the

measured COUPLED impedance Z12, as per the equation
Zm12= +/-sqrt(Z22x(Z11-Z12)).

So changing any of those Z, will give you a different Zm12, which in
turn give the wrong DRIVE impedance, used to calculate the phasing

networks.

Just like the observer, actually influencing the experiment by just being
there, observing......

Am I missing something here? Or is anyone worrying about this?

I will appreciate any kind of feedback, technical in nature or not.

tks

va2gu

[dl4raj]

Killy,

an antenna's impedance (I'm NOT talking about a parasitic element)
in not changed by being measured by an antenna analyzer.
It's just the same as if you were operating the antenna
in transmitting mode with your TX.
When you transmit or measure the antenna is the load.
When you receive the antenna is the source and the
receiver is the load.
The only antenna parameter which is influenced by adding
a source is the antenna Q.
The circuit Q is lowered by the source's R.

73
Clemens
DL4RAJ

[killy]

....mmmmmm.....

TKS CLEMENS for the comment.

I am reflecting more about what you said, "....in the receive mode, the
antenna is going to send back to the VNA a reading which is going to be
affected by the same factor as the transmitted signal was affected when it
entered the antenna terminals....".

In essence what you're saying is that the VNA in tx mode (with its Z of 50
ohms in parallel with my 220 ohms+24j) may change the "Zant original" to
a "Zant new", but the resulting current coming out of the antenna back to
the receiver, will be a current changed by the same "Zant new"..... mmmm.
So in math form, Vori/Iori = Vnew/Inew and Zant ori = Zant new?

What i will do when it stops snowing, is calibrate out the coax piece and
measure the ant z; then i put a 150 ohms non inductive resistor in series
with the ctr conductor and recalibrate out the same coax piece with the
VNA. Then remeasure the ant z.

With my limited knowledge in these matters, but learning a lot thanks to
all you guys, I will be "gladly" disappointed if the reading is the same in
both cases.
I say gladly because this last matter will be of no more concern.

What makes me hesitant a bit, is that when you have a tx of most any
sort, the output tuning is not looking at what you told it what the Z was,
but the tx is telling you (via swr, etc) the L and C providing as close to a
conjugate match as the network will allow.
Since the tx is happy, all is well.

But in my case, i am not at the tx just yet. I have to calculate the phasing
networks in between.

These five loops are each going to present different Z (all non 50 ohms
and reactive by the time they are coupled) to their respective phasing
networks.

So if i tell my phasing network that it has a certain Z and i calculate a
network to give me the phase and magnitude of current i want at the
element, the network better see that certain Z, or the phase and
magnitude will be off.

Supposed to be sunny tomorrow! So stay tuned.

va2gu