<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
Hi,<br>
<br>
I posted the following on eevblog, but wanted to know if any of the
SOARA Elmers had seen something like this?<br>
<br>
Thanks,<br>
Dale (W6EDT)<br>
<br>
---------------------<br>
Hi,<br>
<br>
I have read <a
href="https://www.eevblog.com/forum/rf-microwave/difference-in-swr-readings-between-nanovna-and-swr-meter/50/"
class="bbc_link" target="_blank">https://www.eevblog.com/forum/rf-microwave/difference-in-swr-readings-between-nanovna-and-swr-meter/50/</a><br>
<br>
and this question is slightly different than what was described
there.<br>
<br>
My test setup:<br>
NanoVNA (black open case, small screen) - purchased in September
2019. Sweeping from 144-148 MHz, calibrated with lab quality
standards.<br>
Homebrew tape measure Yagi fed with 25 feet of Andrew C400 and
assorted adapters terminating in a BNC female.<br>
<br>
When I measure this antenna using the NanoVNA at 146.565 Mhz, the
SWR is reported as 1:1.85<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
Thanks for any help or ideas you might have.
</body>
</html>