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Hi, I am sure that almost 3 months ago, when I measured my Log Periodic antenna Create-5130-1N, its SWR was almost perfectly flat in all the 50-1300 MHz range. However I am now seeing strong and variable spikes in a whide band around 300 MHz which, if I properly understood, might be probably fixed by changing the setting CONFIG - EXPERT SETTINGS - THRESHOLD. However I am also seeing the same problem around 500 MHz as per attached images. Please note:
1) I didn't change anything in my NanoVNA-H4, apart updating the firmware
2) After updating the firmware I re-calibrated my unit
3) I am seeing the same spikes even by measuring my 2m/6m/70cm vertical antenna (Diamond V2000A)
4) No problem if I measure a 2m/70cm portable antenna

What can I do? Thank you for your comments and help.

si5351 generator overheating and not stable work near top limit
Need reduce threshold to 295MHz, save config, recalibrate

This solve problems in 290-310MHz

Others threholds - only fixed inside firmware
Try add radiator on SI5351 generator

Did it, but unfortunately it didn't work: same condition at 300MHz.

Is really the SI5351 generator overheating? The problem appears even when the device is just switched on.

Additional question: I live close to the centre of Milan and I receive very strong signals, especially from FM broadcast stations, receiving levels around + 6dBm (yes: 0,44 V !!!) or so. Can they affect the masurements?

You reset calibration after change threshold? Old calibration contain bad data

On 7/7/23 2:08 PM, Andrea I2UEA wrote:

Did it, but unfortunately it didn't work: same condition at 300MHz.
Is really the SI5351 generator overheating? The problem appears even when the device is just switched on.
Additional question: I live close to the centre of Milan and I receive very strong signals, especially from FM broadcast stations, receiving levels around + 6dBm (yes: 0,44 V !!!) or so. Can they affect the masurements?

Yes, very much so
There are two effects to be aware of:
1) if the signal is close to the test frequency, it makes it through the receiver. Broadcast FM is pretty wideband - the VNA's receiver is about 1 kHz wide, so if you're sweeping across that band, you'll likely get interference

2) saturation of the receiver. I can't remember what the max input before saturation is, but considering that the VNA puts out around 0 dBm, and with a short, all that comes back to the receiver, 6dBm probably won't saturate the mixer.

Hi, I had similar problem, in any way at 300MHz (at 299 152 500 to be exact). Big spike. Hence I searched this forum to get some wiser :-) I just did what DiSlord said in his comment to you and the spike is now gone. My threshold was set at 300 000 100. So a strong station would be a too much of coincidence. I live in The Netherlands, but okay, we have FM stations too :-D By the way, I have no idea what this threshold is :-D

A number of firmware updates will change the chip type. Go into expert
settings and change your chip type to the other choice.

On Mon, Feb 19, 2024, 18:13 ErnestB via groups.io <ernest072=
[email protected]> wrote:

Hi, I had similar problem, in any way at 300MHz (at 299 152 500 to be
exact). Big spike. Hence I searched this forum to get some wiser :-) I just
did what DiSlord said in his comment to you and the spike is now gone. My
threshold was set at 300 000 100. So a strong station would be a too much
of coincidence. I live in The Netherlands, but okay, we have FM stations
too :-D By the way, I have no idea what this threshold is :-D

Hi Richard,
Thanks for the info. I have HW_ver. 4.3_MS, and have recently updated the FW to ver. 1.2.20. I had the whole time the MS5351. Lowering this threshold removed the spike at 300MHz. But as a check I switched to Si5351 and the spike was there again, regardless of this lowered threshold. So, no idea what is the real problem, but spike is luckily gone, as I was already concerned about the unit...

I hope Andrea solves his problem too.

Hello Ernest,

The original "edy555" design for the NanoVNA only went up to around 300MHz. As this design was released as open source, several developers ran with the idea and developed the design further. But the main improvement was to the firmware. Hugen (of NanoVNA-H and -H4 fame) and DiSlord worked out how to use harmonics of the Si5351A Clock Generator chip. This pushed the operation out to 900MHz, then with a bit more development out to 1200MHz and even 1500MHz using the 4th and 5th harmonic.
The 5351 clock generator is only specced to 200MHz but is "overclocked" to 300MHz in the NanoVNA. Some clock chips can't quite make it to 300MHz and hence the instability spur noticed at around 300MHz and its harmonics at 600, 900, 1200 and 1500MHz.
Most recent FW versions have reduced the "Threshold" to 290MHz in an attempt to overcome this problem.
So the "Threshold" is the maximum frequency the 5351 clock chip is driven to before switching to harmonic mode.
Below is an excerpt from a previous post I made on this subject and explains the difference between the Si5351A and the MS5351M clock chips.

/On the subject of MS vs Si 5351 Clock Chips, the reason supplies got scarce is that Silicon Labs, the developers of the Si5351A/B/C sold that part of the company to Skyworks and the chip became scarce during the changeover. The Skyworks parts use the Si5351A/B/C numbering system, but only mark the part "5351". There is no "Si" or "A/B/C" variant marking. Meanwhile, Chinese company Ruimeng Technology made a knockoff of the Si5351A but only in the MSOP10 leaded version used in the "original Version 1" NanoVNA design. Their Data Sheet states they mark the device "MS5351M". So it should be a simple matter to determine the manufacturer by opening the case and looking at the chip markings. Both parts are specified from 2.5KHz to 200MHz, but the NanoVNA design pushes the device out to up to 300MHz. Both Si and MS MSOP10 parts have 3 clock outputs but Ruimeng state only 2 should be used if both are operating above 112.5MHz, presumably to limit thermal rise. That is OK, as the chip generates the "Stimulus" frequency plus the LO signal for the mixers, the IF being at 5KHz./

Hope this explains the "Threshold" setting for you.

73...Bob VK2ZRE

Hi Bob,
Your explanation is most appreciated, so I get to know better my equipment and better understand the limitations. Okay, I understand now better that the FW ver. that I have goes up even to the 9th harmonic.

So we could also bring down the clk freq back to 200 to reduce the "heat"? And if so do we get the ranges that are multiple of 200? (instead of 300 and does FW support that?
Do I understand it clear, so it uses both even and odd harmonics? (odd being the natural higher harmonics of a square wave and even probably intermodulation)?

Cheers..
Ernest

Hi Ernest,

I am pretty sure they FW uses the odd harmonics, but you will still see the instability at both the odd and even harmonics if you do a scan from 100KHz to 1.5GHz.
If you lower the Threshold too far, you will limit the upper frequency limit because the reduction in energy at each higher harmonic will affect the performance adversely.
So, as long as the clock chip operates happily at say, 280MHz or 290MHz then you are getting optimum performance.
So leave the Threshold at the default value if your NanoVNA is working OK.

HTH...Bob VK2ZRE

Hi Bob,

It is now at 295 and working good. Default it was slightly (100Hz) above 300MHz.