Great info !! Thanks.
I just checked with a dummy load, and the nanoVNA shows flat 1:1 SWR across the band, as expected.
But why the HF antenna which reads 1:1.02 on 3x different SWR meters (all high quality ones) is reading SWR 1:2.8 in the nanoVNA is a mystery to me.
My rig has also inbuilt SWR meter, and it reads 1:1 on the 7Mhz with the HF antenna, so I have been using the HF antenna straight into the rig with just PWR/SWR meter all the time for ages with full 100W most of the time, and making loads of good contacts with good reports.
Have you done the full check, which entails also checking the transmitter plus SWR meters into the dummy load?
Also, try checking the SWR of your antenna at full power, if your SWR meters are rated for it.(do it fast, as other hams may look askance at a massive carrier up for too long
)
Write that down, then check at the lowest power level you can reduce the radio to.
Write that down, also.
Compare the two------ does it confirm the point made by
radiolistener?
Another test you can try, this time on the nanoVNA, is to use it to check the SWR with a 75
load, then a 150
load
They should measure 1.5:1 & 3.0:1 respectively.
I tend towards
Kalvin's suggestion of interference from a strong external carrier.
Back in the day, we occasionally needed to measure the return loss of the antenna bays of our TV site, over their designed range of frequencies.
To do this, we used a Polyskop SWOB, which produced a swept RF signal, which was applied to the item under test, then detected the reflected signal & compared the two, to show return loss on a CRT screen.
We had to wait for the TV Station next door to close down for the night, (stations did that back then) as the Polyskop was "as broad as a bull's bum", & would display their signal as errors.