There is something occurring with the SWR detection circuit in the Flex 6000 series. A purely resistive test (as Mike suggests) would probably not exhibit this issue.
I have seen numerous 6300 and 6500 radios display higher than actual SWR at various times, especially on 40m, and into tube amplifier input stages (due to flywheel effect). Some of you may recall the fiasco when "swr protection" functionality was added into SmartSDR a few releases back, then quickly disabled in the next release, due to all the issues it caused.
I can use the 6300 and 6500 and see a high SWR condition (approximately 2-2.5:1) with the radio SWR meter, yet my LP-100A meters (one on the input of the amp, and one on the output) and amplifier SWR meter (whether the amp is in operate or standby) all show a low SWR (provided that is the case with the actual antenna system components at time of measurement). I can then simply rotate my radio selection coax switch to a different radio, such as an Icom or Kenwood radio, and the newly selected radio shows an SWR more in line with the external meters.
I am guessing this is hardware design flaw in the 6000 series. For what it is worth, the Apache Labs 100/200 series radios also suffer from this same condition. At least with the ANAN radios, you can download the PA board schematics and draw your own conclusion. A schematic of the 6000 series would surely help us better understand what is going on with the detection circuit. Questions as to how and why this occurs were never fully answered in the previous threads (when the SWR protection feature was added.
Has the FRS engineering team redesigned the swr detection circuit, or fully tested the new 6400/6600 radios on complex impedance loads, hopefully at the prototype stage?
I will be curious to see if this same issue occurs with the new radios. Maybe someone who has experienced this with a 63/65/6700 and has upgraded to a 64/6600 can report back in due time.
We did a test where I had to make about 1 dozen jumper cables all of various designed lengths. This was to show that when inserted into a 2:1 SWR load, depending where you inserted the SWR meter, the reading was different. Yet, we knew the resistive load was 2:1. And, that was in a controlled environment.
In the case of a mismatched condition, something interesting happens along the transmission line. Before, with the matched antenna, the same voltage existed anywhere along the line. Now as you move along the distance of the line, the voltage will change. It now has peaks and valleys
The above quote is from https://www.arrl.org/files/file/Technology/tis/info/pdf/q1106037.pdf. Well worth reading so that you understand SWR and how it is 'not that simple'.
Others on this forum may have better or remember better than I did, but I believe this to be true.
Also, RF flowing back on a feedline may affect SWR readings if there is RF flowing on it. As an idea, you may want to choke the jumpers to be sure.
Short story, in my whole bunch of years of operating, it was not uncommon to see different SWR readings that my bird watt meter that was different from what was on the radio. There is no doubt in my mind that the radios all displayed the SWR they were reading at that point of the feedline.
My 2 cents.
To my amazement all was well with the world, like a good mentor Helmut would not give the answer but told me that all would be revealed in the ARRL handbook. When doing test with multiple Antenna or TXers it is best to take the PL259 and transfer to the another Radio. If you need to use a coax switch keep all the coax's the same length, same type to back of each transmitter.
We all have coax switching in our Ham Shacks but little regard for the type and quality of coax used in the switching process. Clean up your coax connection between equipment and you may find that some of the interference on some bands is in all that switching mess.
73 .. Mike VA3MD