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Not responding in AM broadcast band

 

For some strange reason my NanoVNA is now not responding correctly to any antenna designed for the AM broadcast band and instead is showing it as an open. I have confirmed the calibration using the test loads and it looks perfect so I am not sure what is going on here.
 

On 12/5/21 9:43 AM, Nicholas wrote:
For some strange reason my NanoVNA is now not responding correctly to any antenna designed for the AM broadcast band and instead is showing it as an open. I have confirmed the calibration using the test loads and it looks perfect so I am not sure what is going on here.

Could your antenna be picking up significant amounts of signal and overloading the input of the VNA?
 

It was working perfectly in the AM broadcast band before both the S21 and S22 connectors so I don't think that is the problem. The nearest AM radio station to me is about six miles away and I am using the upper edge of the band but that is a reasonable possibility.
 

I checked it elsewhere and it does not seem to be responding at all below 50 MHz so it is not just the AM broadcast band but also HF and to an extent low VHF as well.
 

What frequency range are you calibrated for? Perhaps recalibrate for a narrower range spanning the AM broadcast band, say 200 kHz to 2500 kHz.? Good luck.

Ken - WB?OCV

On Dec 5, 2021 13:52, Nicholas <n.moon@...> wrote:




It was working perfectly in the AM broadcast band before both the S21 and
S22 connectors so I don't think that is the problem. The nearest AM radio
station to me is about six miles away and I am using the upper edge of the
band but that is a reasonable possibility.
 

What frequency range are you calibrated for? Perhaps recalibrate for a narrower range spanning the AM broadcast band, say 200 kHz to 2500 kHz.? Good luck.

Ken - WB?OCV

Show quoted text

On Dec 5, 2021 12:43, Nicholas <n.moon@...> wrote:




For some strange reason my NanoVNA is now not responding correctly to any
antenna designed for the AM broadcast band and instead is showing it as an
open. I have confirmed the calibration using the test loads and it looks
perfect so I am not sure what is going on here.
 

I calibrated it for the upper edge of the AM band, 1600 to 1700 KHz and it worked really well before with the calibration done in this range. For some reason with calibration done in the same range it is not working at all suddenly. I have never exceeded the specifications for transmitter input power being very careful about where I set that for a transmitter under test and I have followed the directions for calibration exactly including some extra steps which would be useful the ISOLAT and THRU calibration steps to be exact. It is just confusing to me why this would suddenly happen.
 

When I have a problem like this, when something working suddenly stops, my
first impression would be that a connection/cable has a problem. So I
check each carefully. Sometimes a cable is broken right inside the
connector, and works fine until it is bumped; or a connector isn't reliable
when disconnected from one setup and moved to another. So I always check
those issues carefully first. Of course, I also check that the nano
input/output isn't broken by doing a simple test that I have seen work
before.
In this case you seem to have indicated (without including details) that it
gives you an 'open' indication after you changed your setup, so it could
well be that it is indeed open because of a bad cable or connection.

On Mon, Dec 6, 2021 at 10:43 AM Nicholas <n.moon@...> wrote:

I calibrated it for the upper edge of the AM band, 1600 to 1700 KHz and it
worked really well before with the calibration done in this range. For some
reason with calibration done in the same range it is not working at all
suddenly. I have never exceeded the specifications for transmitter input
power being very careful about where I set that for a transmitter under
test and I have followed the directions for calibration exactly including
some extra steps which would be useful the ISOLAT and THRU calibration
steps to be exact. It is just confusing to me why this would suddenly
happen.
 

The cable tests correctly at DC with a multimeter with the braids connected and the center conductors connected correctly. The setup I have is exactly the same as when it was working correctly but just out of curiosity, what test are you talking about that might get me closer to a solution.
 

A little embarrassing but I think I found the problem. I checked the SO-238 to BNC male adapter I was using and THE CENTER CONDUCTOR IS MISSING! It likely got lost on a piece of coax that I was using at one point. I am going to find it and put it back in. That would certainly result in an open no matter what antenna I use!
 

On 12/6/21 7:01 PM, Nicholas wrote:
A little embarrassing but I think I found the problem. I checked the SO-238 to BNC male adapter I was using and THE CENTER CONDUCTOR IS MISSING! It likely got lost on a piece of coax that I was using at one point. I am going to find it and put it back in. That would certainly result in an open no matter what antenna I use!

A classic problem is when someone "stretches" a piece of coax to make it reach (or when pulling through a conduit or around something else), and the center conductor pulls back within the shield, taking the center pin with it.
 

I found the center conductor and dielectric and after putting them back in the VNA is responding correctly through out its full range. Sometimes a little poking around can solve a huge problem.
 

Yes! That's the kind of thing I was trying to get you to find. That type
of mechanical issue is almost always the problem when something that was
working suddenly stops, especially after manipulating the setup, even in
innocuous ways. It's often in the least expected place.

On Mon, Dec 6, 2021, 7:14 PM Nicholas <n.moon@...> wrote:

I found the center conductor and dielectric and after putting them back in
the VNA is responding correctly through out its full range. Sometimes a
little poking around can solve a huge problem.
 

Indeed it was and it seems to be giving me good measurements now with the antenna I am testing. With an antenna tuner I can get an impedance of 32 ohms at 1620 KHz which is my desired frequency. On another note, I talked with a friend of mine who is a retired RCA employee with a distinguished past and he was impressed that a piece of test equipment like this can be had for about $100 and I am impressed too. If operated with its limitations in mind this is a fantastic device especially after I flashed one of the better firmwares available in the files section.

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