Hi all

Testing NanoVNA App + Windows11 with My New H4 , it seems almost working ok despite some bugs and crasching, but my main problem is : i do not found the new Dislord Coax functions, indeed i am using again this days the RG59 75Ohm coax it has less loss coeff then the RG58 50Ohm Coax measured by to the Dislord coax functions on H4, it has a measured 77.70 Ohm =Z0 specific impedance , and then i measure the SWR and Smith with 50-->77.7 Ohm Dislord conversion function, all seems amasing With H4, Wonderfull Dislord functions , but still no success to do it with NanoVNA App and PC+USB .

73's Nizar .

Nizar, the DiSlord measure functions are only in the nanovna firmware, they do not exist in nanovna-app. Nanovna-app uses the raw data from the nanovna and does all of its own calculations. So to use these functions, you will need to read them from the nanovna screen. You can show the nanovna screen on your computer using the little camera icon on the top line of the nanovna-app.

Hi Stan
Thank you very much for clarification info about Nano-VNA App PC program, impedance conversion DiSlord function can be very usefull especially when using 75 Ohm coax's, indeed even my RG58 50 Ohm coax have around 57 Ohm measured impedance, so measurement of SWR with a 50 Ohm calibrated NanoVna will not be accurate enought to adjust antenna resonance, but coax impedance conversion function do the job and make it as easy as for perfect 50 Ohm coax cable.. SWR function measurement is altered if our coax does not have an accurate 50 Ohm impedance without using the DiSlord conversion function .

So My question : is there any other pc program that can include coax impedance conversion function to ensure an accurate SWR measurement and antenna resonance ??
73s Nizar

The resonant frequency of an antenna and the frequency where the SWR is the minimum are different. Resonance occurs when the reactance X = 0 at the antenna feedpoint. When you measure the SWR at the transmitter end of the coax the SWR minimum will be at a slightly different frequency. The characteristics (impedance, loss) of the coaxial transmission line will determine by how much.

Hi Nizar,
I think you are getting hung up on the wrong thing. Yes, without the corrections, the SWR value will be a little bit off, for instance if you use your 75ohm cable to match to a 75ohm load (a dipole), you may get an SWR reading of 1.6 instead of 1.0. With your 56ohm cable the difference would be minor.
But in all cases, the lowest SWR (the dip in the SWR curve) will appear at the same frequency - only the value of the SWR will change a bit - so you can always tune your antenna without doing any of these corrections. Just tune for the lowest SWR dip. And yes, as Roger correctly pointed out, the minimum SWR is not always at the resonant frequency, but it is close to resonance in almost all cases, so tuning for low SWR will give good antenna performance.
Just use the nanovna as you have it, and you will be able to tune some nice antennas. The difference between a 56ohm feedline and a 50ohm feedline will be insignificant for all simple antennas - it will only become relevant if you are doing phased arrays using the feedline. And using a 75 ohm feedline will also be insignificant in the tuning of most antennas. And the difference between an SWR of 1.5 and an SWR of 1.0 is insignificant - the station receiving your transmission will not be able to tell the difference.
So go out and tune some antennas and have fun on the air - the correction factors you are seeking are only a tiny part of the performance of your system, and in my experience, not worth spending time on. I never use them.
Stan KC7XE

Hi Stan

Thank you for your summary of the common sense of an experienced, it is convincing, in fact I have just done the comparative tests of the same dipole antenna with the same coaxial cable Zc = 77.7 Ohm RG59 with and without compensation of calculation of DiSlord of its function Z--> 77.7 Ohm with the same NanoVNA H4 firmware Dislord 1.2.40, the results are as follows:
without Zc compensation: SWR = 1.22 Fr = 14.385 Mhz
With Zc compensation: SWR = 1.16 FR = 14.112 Mhz
there is 0.06 sheeft for SWR and 272 Khz sheeft for Dip frequency

But with DiSlord conversion function it's much better to use and it's a simple and beautiful function .
Thanks
73's Nizar

Nizar,

Are you trying to use 75 ohm coax with a transmitter designed for 50 ohm? If so you should be not be using the DiSlord impedance setting which changes the system impedance to 75 (or 77) ohms. You want the SWR measurement to be based on what the transmitter will see.

Here is an example for you to consider. Take a 1/4 wave vertical antenna with a very good ground system (multiple radials) cut to resonate at 14 MHz. At resonance the feedpoint impedance will be 36 ohms plus the ground resistance - lets say 4 ohms. So if you measured at the antenna feedpoint with the NanoVNA you would see 40 +j0 at resonance and an SWR of 1.25. Next we add 7 meters (1/2 wavelength) of RG-58A/U coax and measure at the far end. We get an impedance of 40.9 +j0.194 and an SWR on 1.24. Now if the length is increased to 10.5 meters (3/4 wavelength) we get an impedance of 62.2 -j1.97 and an SWR on 1.23. So everything looks good.

Next we try 7m of RG-59 as the coaxial transmission line. We get an impedance of 41.8 -j0.02 and an SWR on 1.20 based on 50 ohm Zo and 1.81 based on a Zo of 75 ohms. This looks good as well. However if the length is increased to 10.5 meters (3/4 wavelength) we get an impedance of 134.2 -j2.7 and SWR of 2.69 based on 50 ohm Zo and 1.77 based on a Zo of 75 ohms. This is not desirable to have the SWR based on 50 ohms get worse because we changed the cable length.

Observations
---------------
- When the SWR is calculated based on the impedance of the coaxial cable it goes down as the length of the cable is increased. This is due to loss in the cable. The Return Loss increases with cable length.
- When the 75 ohm cable is used the SWR based on a 50 ohm Zo calculation increased to 2.69 when we went from a half wavelength (7M) to 3/4 wavelength (10M). This also means that a transmitter designed for 50 ohms would see a higher SWR and the foldback circuitry (most solid state Tx use this) would cut back the power output.

Discussion
------------
A transmission line can be viewed as an impedance transformer. At half wavelength with a lossless line you "see" the impedance at the far end and the SWR will be the same. As the length is changed the impedance will change but the SWR will remain constant IF calculated based on the characteristic impedance of the transmission line used. With a lossy line the SWR will decrease as the line length is increased. If a 75 ohm line is used and the measurements are made using a Zo of 50 ohms there will be a considerable change in SWR as the line length is changed.

I am not suggesting that a 75 ohm transmission line cannot be used with a 50 ohm transmitter. On the contrary. Often times a specific line length can be used to transform a high or low feedpoint impedance to close to 50 ohm at the transmitter end. One can also use a "tuner" with 75, 300 or 450 ohm transmission lines to make an impedance match to 50 ohms.

What I am stating is that it is not a good idea to use a 75 ohm cable of unknown length and then trim the antenna to get close to 50 ohms and a low SWR at the transmitter end of the cable. One should also not change the NanoVNA menu setting to a different system impedance in the NanoVNA and then trim the antenna. The reasons are that the radiation efficiency of the antenna could be reduced and what will happen if you change the transmission line length.

All calculations above were done using TLDetails which is freely available on the Internet. Screenshots of my calculations are below.

Roger

Are you trying to use 75 ohm coax with a transmitter designed for 50 ohm?
If so you should be not be using the DiSlord impedance setting which changes the system impedance to 75 (or 77) ohms.
You want the SWR measurement to be based on what the transmitter will see.

The Tx does not see an SWR it sees an impedance, it is the transmission line that has the standing waves and that must be based on the impedance of the line NOT the load that the Tx 'likes to see'. The amount of reflected power will depend on the mismatch between the line impedance (75 ohms) and the impedance of the load. The design impedance of the Tx is irrelevant.

Leaving the settings at 50ohms may well give you an equivalent SWR of the impedance that the Tx sees, but tells you nothing about they standing waves on the 75ohm line. For Example, if a 1:1 SWR was indicated with a 50 ohm setting you might believe that there were no standing waves on the 75 ohm line, which would not be correct.

73
Jeff G8HUL

On Mon, Dec 30, 2024 at 01:58 AM, G8HUL wrote:

The Tx does not see an SWR it sees an impedance, it is the transmission line
that has the standing waves and that must be based on the impedance of the
line NOT the load that the Tx 'likes to see'.

Yes it is the transmission line that has the standing waves on it. However as I stated in my previous post the transmitter power amplifier is designed for a specific load which is 50 ohms for ham transmitters. Modern solid state transmitters contain a circuit that calculates SWR based on the load impedance at the output connector on the Tx. The Tx circuitry cuts back on the output power as the SWR rises.

The amount of reflected power will depend on the mismatch between the line impedance (75 ohms) and the
impedance of the load. The design impedance of the Tx is irrelevant.

Yes the mismatch between the line impedance and the load is what determines the VSWR at the load and the reflected power. However the design impedance of the transmitter is relevant. This TX source impedance will determine how much of the reflected power will be reflected back to the load. If it is 50 ohms none will be reflected back but transmitters are not designed with a 50 ohm source impedance.

Leaving the settings at 50ohms may well give you an equivalent SWR of the
impedance that the Tx sees, but tells you nothing about they standing waves on
the 75ohm line. For Example, if a 1:1 SWR was indicated with a 50 ohm setting
you might believe that there were no standing waves on the 75 ohm line, which
would not be correct.

Yes that is true but the transmitter will be able to output its full rated power with the SWR = 1. And if the transmission line used has low loss there will be little power lost in the transmission line.

Roger

On Dec 30, 2024, at 10:33, Roger Need via groups.io <sailtamarack@...> wrote:

On Mon, Dec 30, 2024 at 01:58 AM, G8HUL wrote:

The Tx does not see an SWR it sees an impedance, it is the transmission line
that has the standing waves and that must be based on the impedance of the
line NOT the load that the Tx 'likes to see'.

Yes it is the transmission line that has the standing waves on it. However as I stated in my previous post the transmitter power amplifier is designed for a specific load which is 50 ohms for ham transmitters. Modern solid state transmitters contain a circuit that calculates SWR based on the load impedance at the output connector on the Tx. The Tx circuitry cuts back on the output power as the SWR rises.

Maybe. It kind of depends on the amplifier - some just fold back when the voltage at the output terminals gets too high (to protect the transistors). That is, the protection circuit on most amps doesn’t actually calculate the SWR - it protects the devices against whatever the limiting parameter is (voltage is the quick death, current or heat is the slow death).

Imagine an amplifier with zero output impedance.