I have a NanoVNA-H which I haven't used for 2 years. Just pulled it out of the cupboard and upgraded to latest firmware (1.0.69). The spane is set to 1MHz to 40MHz, and it has been calibrated. It seems to be working of sorts measuring SWR, but the SWR figures are not credible. Attached is a portion of the SWR graph for my 4 band trapped dipole (80-40-20-15-10m). The general shape of the graph is correct, with the minimums in the right place, but the SWR never gets above 1.18. I wish my antenna was that good, but it should be going off the chart literally for most of the time. Any idea why it should give misleading results like this.

What does your coax feed to the antenna contain?  Since SWR is a voltage measurement either the return voltage is getting attenuated or the NanoVNA isn't measuring one of the voltages correctly.
What if you disconnect the feed line at the antenna?  That should be infinite SWR.
Mike W9MDB

Marc,
Perhaps your NanoVNA calibration is bad. If you leave the NanoVNA connector open does it show an extremely high SWR, as it should? If you place the calibration load on the NanoVNA connector does it show 1.0:1 VSWR, as it should?

Another possibility is that your feedline has become very lossy. A feedline with 10 dB loss would produce results similar to what you are seeing.

Jon w2anz

On Thu, Jan 26, 2023 at 09:03 AM, Michael Black wrote:

What if you disconnect the feed line at the antenna?  That should be infinite
SWR.

The return loss (related to VSWR) will be twice the feedline loss, so not infinite VSWR. It should show very high VSWR (low return loss) when the analyzer connector is open- or short-circuited at the point where the calibration was done. however.
73,. Don N2VGU

Feedline is ~30m of RG213. Not really practical to disconnect coax at antenna as it's 15 meters in the air. This NanoVNA has correctly measured the SWR 2 years ago. I think it has died.

I have done the calibration a few times so I think it's OK. Leaving the NanoVNA connector open gives an SWR of 1.183 right across 1MHZ to 40MHz. I think it's broken. I have measured a different antenna, and I get similar results.

Thanks for all your assistance - I'm pretty sure my NanoVNA has a serious fault. A new one is on order. I checked the antenna with a SARK 110, and got exactly the result I expected, so that eliminates the antenna and feed as the problem. My NanoVNA is actually not the H model, but the vanilla one. Have upgraded it to what I believe is the latest firmware - v0.8.0. Development of this branch seems to have ceased in 2020, so time for an upgrade anyway. Have bought the latest 4" H model.

On Thu, Jan 26, 2023 at 01:03 PM, Marc VK3OHM wrote:

I have done the calibration a few times so I think it's OK. Leaving the
NanoVNA connector open gives an SWR of 1.183 right across 1MHZ to 40MHz. I
think it's broken. I have measured a different antenna, and I get similar
results.

I suggest you do a Clear Config before you calibrate. Also remember to Reset before you do the calibration. Check your cal loads and make sure they measure open , 0 ohms and 50 ohms. Then cal and when done the dot should be in the middle of the Smith chart for 50 ohms, on the far left for short and far right for open.

Roger

Thank you Owen Duffy for an off list suggestion. Turns out the 50 ohm load had failed open circuit, so it was not calibrated. Replaced the load and now working perfectly. Who'd have thought one of those could fail?

I would think that someone with a call like yours would have checked the load first! (Just kidding!)

If you clear the calibration (reset), so it's totally uncalibrated, what does it show? It should show S11 fairly low with a load, and close to zero with either open or short. If it does, then check your calibration standards. It's not unheard of for shorts or loads to change - I have a load that decided to become an open one day, close to 50 ohms, then it wasn't the next day. Who knows what happened (probably some sort of tiny crack that developed and finally went open).

Also check your frequency range. Set it to something "moderate" like 1-100 MHz

hah, and I just sent a suggestion - yeah, it happens. And to someone with your callsign, how ironic.

I've had more than one 50 ohm load turn into an open - or what's worse, 50 ohms at high frequencies, but open at low - a break, with sufficient capacitance that at some GHz, the capacitive impedance was negligible - so it was a fine S band load.

If you have a spare 3-10 dB pad around, that's a handy thing to use as a cross check, too.

The standards by their nature are delicate, as the manufacturer attempts to minimize parasitic capacitance/inductance. This often results in mechanically less-than-robust construction.
This mechanical weakness is exacerbated by the fact that many inexpensive standards do not have a place to hold the standard from rotating when mating/demating the connectors; the resulting rotation of the standard can unscrew a threaded center pin, or worse, extend it out so that it damages another connector. Metrology grade calibration kits include gages to ensure that the connector pins are properly positioned and connectors are gaged before beginning a calibration.

"Turn the nut, do not turn the connector body" is the mantra.

A useful habit to develop before calibration is to watch the screen as you connect each standard in turn, to ensure that the expected, or at least different, response is obtained. This will help to pick up gross problems with the cal standards. Another is to closely examine the mating surfaces of the standards for bent, distorted or missing pins or damaged outer conductors. I have even found a male pin which someone left in a female standard.
73, Don N2VGU

Hi Marc,

Just a heads up. It is possible that static on your antenna crisped your NanoVNA.
It is really good practice to short your antenna cable before connecting it to your Nano VNA.
Also be very aware of nearby transmitters, even VHF/UHF ones that could induce high input signals on your HF antenna to the NanoVNA.
A DC block and/or 6/10dB fixed attenuator are also good safety insurance.

73...Bob VK2ZRE

Not to be captain obvious, but on your nanovna calibration, did you reset cal first and click done when finished?
I easily forgot and made this mistake early on and got weird results.

Bryan , N0LUF