[Elmers] NanoVNA Indicates low SWR

[Eugene Dale Tyler]

Hi,

I posted the following on eevblog, but wanted to know if any of the 
SOARA Elmers had seen something like this?

Thanks,
Dale (W6EDT)

---------------------
Hi,

I have read 
https://www.eevblog.com/forum/rf-microwave/difference-in-swr-readings-between-nanovna-and-swr-meter/50/

and this question is slightly different than what was described there.

My test setup:
NanoVNA (black open case, small screen) - purchased in September 2019. 
Sweeping from 144-148 MHz, calibrated with lab quality standards.
Homebrew tape measure Yagi fed with 25 feet of Andrew C400 and assorted 
adapters terminating in a BNC female.

When I measure this antenna using the NanoVNA at 146.565 Mhz, the SWR is 
reported as 1:1.85

This seemed pretty good, so I attached a transmitter and a Bird 43 with 
a 5W element just to verify transmitter output as I usually do in a new 
setup. The output was lower than expected and the reverse power was much 
higher, Now I understand about the 5% full scale accuracy of the Bird, 
but still the reverse power was 1.3W with a forward power of 3.7W.

So, I hooked up a MFJ-259B and it reported SWR of 1:3.5 at 146.565. To 
verify, I hooked up a HP 8753C, sweeping from 130-150MHz, did a one port 
calibration and observed a SWR of 1:3.27 at 146.565 MHz. To eliminate 
the cabling, I replaced the antenna with a 50 ohm terminator at the end 
of the feed line and the result was an SWR of 1:1.12 on the 8753C. The 
MFJ also reported a similar SWR. The NanoVNA reported 1:09.

I then redid the NanoVNA calibration and got nearly identical results to 
the first trial using the antenna and the dummy load. I removed the feed 
line and connected a 150 ohm carbon resistor, which read 1:1.68 on the 
Nano VNA. The 8753C read 1:3.002. An open on the NanoVNA reads 1:2.52, 
with a stable Smith Chart. A short reads 1:infinity and a 50 ohm 
standard reads 1:1.0, both with stable smith charts.

It appears the Nano VNA's reverse detector is inaccurate on my unit.  My 
question: Is this a common problem and can it be corrected, or should I 
throw away this Nano VNA and buy another? I don't recall if I had seen 
this before and with this type of inexpensive tool, one should expect 
high precision, so prior, minor variations of +/- 1:1.2 did not worry 
me. 1:1.18 vs 1:3.27 is not acceptable.

Thanks for any help or ideas you might have.
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