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I am having difficulty constructing a set of phase matched cables using my NanoVNA SAA-2N and the tutorial available at this link .

The cable I am using is Times Microwave LMR-400-LLPX with a nominal length of 20 feet. Manufacturer's specification for time delay is 1.34 nS/ft.

My cables need to be phase matched +/- 5 degrees at 1.1GHz.

The difficulty that I'm having is with the following results returned by the NanoVNA.
Cable 1 measured time delay = 54.35 nS
Cable 2 measured time delay = 52.93 nS
Surprisingly, both values are significantly greater than the calculated delay of 26.8 nS = 1.34 nS/ft x 20 ft
Furthermore, based on the formula in the tutorial, the phase shift appears to be 562 degrees

What am I doing wrong?

S11 or S21 delay?

On S11 2x delay (to cable end and back)
On S21 1x delay (from one port to another(

On 11/15/22 8:21 AM, dalerheaume via groups.io wrote:

I am having difficulty constructing a set of phase matched cables using my NanoVNA SAA-2N and the tutorial available at this link .
The cable I am using is Times Microwave LMR-400-LLPX with a nominal length of 20 feet. Manufacturer's specification for time delay is 1.34 nS/ft.
My cables need to be phase matched +/- 5 degrees at 1.1GHz.
The difficulty that I'm having is with the following results returned by the NanoVNA.
Cable 1 measured time delay = 54.35 nS
Cable 2 measured time delay = 52.93 nS
Surprisingly, both values are significantly greater than the calculated delay of 26.8 nS = 1.34 nS/ft x 20 ft
Furthermore, based on the formula in the tutorial, the phase shift appears to be 562 degrees
What am I doing wrong?

You're seeing the round trip time of ~50 ns

What does the S21 measure, phase wise?

Rather than fool with TDR.. (or are you using TDR to measure the distance to the cut end, before installing the connector?)

You can also measure S11 phase at your frequency of interest, it's twice the one way phase.

at 1.1 GHz, roughly 1 ns per cycle, or ~8" for 360 degrees.
If you want 5 degrees, that's about 1/10th of an inch.

Are your cables at the same temperature? Do they have any other mechanical variations (many cables "off the reel" have periodic variations in dimensions).

I think you got it correct (almost)... But you see a double delay cause its up and back down the cable... 26.8x2=53.6...
And... You need them the same length / delay... If both are 52..or 53 does not matter but they need to be the same.. Right?!
So... Cut the longer a tiny bit shorter... And you have it...
Dg9bfc sigi

Am 15.11.2022 17:21 schrieb "dalerheaume via groups.io" <dalerheaume@...>:

On 11/15/22 9:59 AM, Siegfried Jackstien wrote:

I think you got it correct (almost)... But you see a double delay cause its up and back down the cable... 26.8x2=53.6...
And... You need them the same length / delay... If both are 52..or 53 does not matter but they need to be the same.. Right?!
So... Cut the longer a tiny bit shorter... And you have it...
Dg9bfc sigi

They also make cool little length adjusters, at least in SMA. I've not seen one in N or UHF.

They're not cheap, at least new, but they might show up surplus.

Someone with a big box of adapters might also find barrels and other adapters that differ in length by a few mm, which can be useful for phase trimming at microwave frequencies.

Sorry...S11 delay. Cable terminated at one end only to allow for trimming.

On Tue, Nov 15, 2022 at 12:50 PM, Jim Lux wrote:

You're seeing the round trip time of ~50 ns

What does the S21 measure, phase wise?

Rather than fool with TDR.. (or are you using TDR to measure the
distance to the cut end, before installing the connector?)

You can also measure S11 phase at your frequency of interest, it's twice
the one way phase.

at 1.1 GHz, roughly 1 ns per cycle, or ~8" for 360 degrees.
If you want 5 degrees, that's about 1/10th of an inch.

Are your cables at the same temperature? Do they have any other
mechanical variations (many cables "off the reel" have periodic
variations in dimensions).

THANK YOU! That helps tremendously.

On 11/15/22 10:56 AM, dalerheaume via groups.io wrote:

Sorry...S11 delay. Cable terminated at one end only to allow for trimming.

Probably want end "open" so you get a reflection to measure the length, not terminated in 50 ohms.

Some very important observations in previous responses:
1 - to phase match, you don't need any particular value, just to make them
the same for two cables
2 - at your target frequency, 5 degrees is about 1/10 of an inch, a very
exacting measurement over 20 feet of cable: just stress in the cable or
connector issues could easily change its electrical length by that much.
And the velocity factor variability in the cables could readily introduce
that much uncertainty.
3 - you do want to leave the far end of the cable open when using the
nanovna for measurements

Another important measurement issue:
The nanovna 'tdr' measurements can't measure within 0.1 inch at 20 feet.
This is because it is not a true TDR - it uses frequency scanning at
discrete frequency points, then an FFT to compute the corresponding
length. The 'bin' size of the FFT depends on the size of the frequency
scan steps (with an inverse relationship). In TDR mode you can see this as
you move the cursor across the display. The 'length' measurement will be
in steps of so many mm or cm. That is the finest granularity that the TDR
can do for the given frequency range. You can make it more accurate by
expanding the frequency span - but only to a point

One way to deal with this in your situation is to measure one cable,
trimming it by about 1/10 inch at a time just until the nanovna measurement
pops from one 'bin' to the next - meaning you are measuring at the edge of
that bin. Then do the same with the other cable, and you should be very
close. (Assuming there are not significant velocity factor differences in
the two lengths of cable).

Another perhaps better way is to not use the TDR mode, but (if your
firmware has it) the Measure / Cable function. Leave the cable open at the
end. Set the top frequency of the vna such that the smith chart trace is
just over a half-circle. Then note the length of the cable shown. Trim
the other cable to show that same length. You can ignore the velocity
factor and whether or not the length is correct to measured length, since
you only need them to match electrically. This method is usually more
accurate, because it measures the quarter-wavelength resonance of the cable
by finding the phase crossing, using excellent interpolation between
measured frequency points.

Good luck.

Stan

Hi Stan,

Thank you very much for your very informative and thorough response.

I'm finally back at this and would appreciate your thoughts on what I did to achieve phase matching although somewhat different from what you shared at the end of your reply.

I setup the stimulus close to my target frequency of 1.1 GHz.
Start Stimulus 1.098 GHz
Stop Stimulus 1.108 GHz
The smith circle is just over half a circle in this case.

I formatted S11 as SMITH LIN to measure phase angle.

I physically measured the following two cables then measured the phase angle with marker set at 1.1 GHz:
CBL-1 20FT 0.50IN -33.1deg
CBL-2 20FT 0.63IN -54.8deg

After trimming CBL-2 by approximately 0.25IN, I measured the phase angle for CBL-2 at -35.8deg...a difference of 2.7 from CBL-1 and within the specification of +/-5 degrees.

Is this method acceptable, also, given that the phase measurement is reflected, is 1 degree phase difference between cables actually 2 degrees.

Thanks,
Dale

Yes, that technique should work fine.


On Tue, Nov 29, 2022 at 10:03 AM dalerheaume via groups.io <dalerheaume=
[email protected]> wrote:

Hi Stan,

Thank you very much for your very informative and thorough response.

I'm finally back at this and would appreciate your thoughts on what I did
to achieve phase matching although somewhat different from what you shared
at the end of your reply.

I setup the stimulus close to my target frequency of 1.1 GHz.
Start Stimulus 1.098 GHz
Stop Stimulus 1.108 GHz
The smith circle is just over half a circle in this case.

I formatted S11 as SMITH LIN to measure phase angle.

I physically measured the following two cables then measured the phase
angle with marker set at 1.1 GHz:
CBL-1 20FT 0.50IN -33.1deg
CBL-2 20FT 0.63IN -54.8deg

After trimming CBL-2 by approximately 0.25IN, I measured the phase angle
for CBL-2 at -35.8deg...a difference of 2.7 from CBL-1 and within the
specification of +/-5 degrees.

Is this method acceptable, also, given that the phase measurement is
reflected, is 1 degree phase difference between cables actually 2 degrees.

Thanks,
Dale

On Tue, Nov 15, 2022 at 02:52 PM, Stan Dye wrote:

Another perhaps better way is to not use the TDR mode, but (if your
firmware has it) the Measure / Cable function. Leave the cable open at

the

end. Set the top frequency of the vna such that the smith chart trace is
just over a half-circle. Then note the length of the cable shown. Trim
the other cable to show that same length. You can ignore the velocity
factor and whether or not the length is correct to measured length, since
you only need them to match electrically. This method is usually more
accurate, because it measures the quarter-wavelength resonance of the

cable

by finding the phase crossing, using excellent interpolation between
measured frequency points.

Good luck.

Stan

On Tue, Nov 29, 2022 at 11:03 AM, <dalerheaume@...> wrote:

...given that the phase measurement is
reflected, is 1 degree phase difference between cables actually 2 degrees.

Other way around. 2 degrees measured in reflection (out and back) is 1 deg one way.