开云体育

Hi,
just received my nanoVNA H4 and after a few satisfactory tests, decided to use it to fix my 3 element 50Mhz yagi which presents a little too high SWR on FT8 frequency (50.313Mhz).

The SWR reading at the radio was around 2.8 and that put in protection the rig.
To have an idea of how work on the antenna I run a scan with NanoVNA (after a calibration from 48 to 54 Mhz) and the results can be seen on the attached NanoVNA saver file..
The SWR at 50.313 Mhz were 2.6 quite similar to the 2.8 measured on the rig , but when I moved to 51.450 Mhz , the predicted lower SWR frequency, SWR jumped to infinite!

Quite shocked I resort to the old usual way of tuning an antenna and sampling with the transmitter a range of frequencies, found that the SWR trend was the opposite way from the predected and the real SWR minimum (1:1) was between 49.6 and 49.7 slowly rising to 2:1 at 50.1 Mhz. and then higher and higher as the frequency increase.

I did the measurements a couple of time before writing and I wont to believe to NanoVNA but instead of helping was misleading, and I have no idea of what I did wrong.

Have a nice evening,

73 de I0MOM Marco in sunny Rome

Hi Marco,

your measured file looks like measured directly at the antenna. But running antenna with you rig, you need a cable with connectors. So take a look at that cable and connectors and make a vna-plot with this cable. Maybe there are faults in the cable / connectors.

73, Günter, DK5DN

Am 04.11.2020 um 18:40 schrieb I0MOM via groups.io:

Hi Gunter,

I made the measurements in the shack, the antenna was connected to the VNA via 28 meters of RG-213 Coaxial cable.
As you suggested I’ll try to make direct measurement directly at the antenna without the coax and see what does change.

Thanks for You interest,

73 de I0MOM Marco
Inviato da iPad


Inviato da Posta per Windows 10

On Thu, 5 Nov 2020 at 08:20, I0MOM via groups.io <morelli=
[email protected]> wrote:

Hi Gunter,

I made the measurements in the shack, the antenna was connected to the VNA
via 28 meters of RG-213 Coaxial cable.
As you suggested I’ll try to make direct measurement directly at the
antenna without the coax and see what does change.

Thanks for You interest,

73 de I0MOM Marco
Inviato da iPad

The usual technique would be to calibrate to have the nanoVNA in the shack,
but perform the calibration on the end of the coax. Then when you connect
the antenna to the coax, you will make an accurate measurement.

Of course, if the coax is faulty, all bets are off.

Dave

Thank you Dave,
Got Your point.
I removed the antenna from the roof and brought to the shack and measurements withouth the cable were really close to the real SWR as seen from TX.

Next step would be VNA calibration at the end of the coaxial cable - antenna site.
Not an easy task!

73 de I0MOM Marco

Inviato da iPad

I have excellent results operating NanoVNASaver remotely via VNC from my iPad (and iPhone for that matter). I have not tried it with my Android pad, but since I use VNC clients from the same developers on both, it should work fine.

This is very convenient for calibrating out the cable length at the point of antenna attachment. Saving that calibration also allows later damage testing from the shack.

I do the calibration first and then reconnect the cable and proceed. I get good agreement between readings taken this way and results immediately afterwards at the antenna connection point (Using the more normal calibration).

M

I hate to to reply to myself, but I got this PM:

Hi
Very interesting!
You mean you had someone with a computer and nanovna at the other end of the cable and you
were controlling them remotely via VNC from your iPad ?
Regards,
Corneliu

and I thought the reply might interest others...

To clarify what I do:

Back at my operating position:
1. I connect my MacBook Pro to the nanovna by usb.
2. I make sure that I have screen sharing enabled on the MB Pro.
3. I connect the nanovna to the antenna cable instead of the transceiver.
4. I start NanoVNA Saver on the Macbook
5. I do as much setup as I can on the Macbook before going outside
6. I connect to my Macbook via a VNC application running on my iPad.
7. I go outside and drop the center of my dipole array and disconnect the cable
8. I calibrate the cable by operating the Macbook running NanoVNASaver from the iPad
9. I reconnect the cable to the dipole array and hoist it back into normal position
10. I either go back inside to do more scans in comfort on the MacBook or -
11. Stay outside and trim things while observing the results on the iPad

At other times I've mixed the hardware up by using my Windows 10 box on the shack end to run NanoVNA saver and by using different VNC apps running on my iPhone, iPad and Lenovo android tablet. All work well, but bigger, brighter screens work better outdoors - if you have somewhere to set them down - The phone is better if climbing is involved, but clumsier to use.

hope this helps

M