开云体育

After running multiple calibrations, when hooked up to an antenna and sweeping the 14.000MHz to 14.350MHz (20m band in the US) my NanoVNA-H is never showing a good SWR. The SWR sweep from my transceiver shows a nearly flat 1.0:1 for the entire band. However, with the NanoVNA hooked up to a rubber duck antenna from my HT, sweeping 144MHz to 148MHz, I see SWRs that are within the expected range.

Things I have done:
Setting the sweep range appropriately before running each calibration
Resetting the calibration and disabling the correction before running each calibration
Updated the firmware to 20200118 / 0.4.5-4
Tested the 14MHz calibration on different antennas, the only commonality between the antennas was the series of adapters I had to use to be able to hook up to the SMAs on the NanoVNA

I'm kinda stumped by this, and haven't really found anything in searching through old messages. Any help or ideas are greatly appreciated! Would love to get this sorted so I can tune antennas without having to lug my HF rig to the park :D

How did you hook up the nanovna to your 20m antenna (wiring)?

For those living in Urban areas, try connecting a Power Meter to that HF beam.You typically seem about 0-10 milliwatts coming back down the coax fromother services.???? Unlike your 100 watt HF rig, the Nano just can't putout enough power to over come these signals, or has the pre-selectionto filter them out.???? Kent WA5VJB

re: wiring - I have tried two different HF antennas. For my G5RV Jr., I hooked up to my feedline from the shack (50' LMR400). It does make sense that the NanoVNA doesn't have the oomph to get a good reading there (my HF rig is only 20W ;) ). The other antenna is a 20m 1/4 wave vertical, and I hooked up to that via a 9" section of RG58A/U.

re: environment - I am fairly urban, I have a fairly consistent S4/S5 noise floor at home, so it does make sense that the noise level might be too high to be overcome. If I measure with my Fluke across the shield and conductor I'm seeing 0.045v. I did take the 1/4 wave vertical over to the schoolyard near us to try and test it once. There is several hundred feet of clear space around where I was, and got the same type of result as I see at home. I guess the next attempt would be to head out to the woods and see if the behavior is the same with miles of space around me.

A real-life anecdotal experience:

At the time we lived in the forest some 12 miles due west and a couple of
ridges of solid granite from Fort Collins, Colorado. We could not see any
cities or towns from the antennas. Even the local fire department
considered us "to he.. and gone". I had a dipole cut for the FM BC band,
88 to 108 MHz, tacked to the ceiling of my radio room on the second floor
of the house. Across the canyon and some line-of-site 1.5 miles distant to
our east, we had a collection of transmitters on Buckhorn Peak, four of
which were in the FM band and all running max legal power (several were
over, but we won't go there!). Fortunately, no TV transmitters. The rest
were public service and pager links. When I measured the total power from
that FM BC dipole on my ceiling I consistently obtained readings between -4
and +1 dBm. 0 dBm is 1 milliwatt. That, in a 50-ohm non-reactive system,
is 0.224 vrms or about 0.640 vp-p. I had to install FM SBF's on everything
I had that operated above 50 MHz. At times, I could even weakly demodulate
the strongest FM transmitter(s) on a crystal set (this isn't supposed to
happen!!)! And we were isolated in the forest.....??!!

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

I was trying to prune an 80 meter dipole temporary hung in my yard. I also could not figure out why my swr readings were all over the place and changing from scan to scan. It wasn't until after I went into my shack that I heard a fellow ham across town on the air. He is located at least a half mile away and below a low ridge line from me. A little patience and when he went QRT all readings settled into what was expected and I finished tuning up my field day antenna. Bottom line, check for RF interference of significant levels, if strong enough not necessarily close to your frequency of interest. It may be necessary to include a low-pass, band-pass, or notch filter between your antenna and nano to eliminate interference.

Hi Ted, in most areas the highest RF Voltage levels are in the FM Broadcast band.Most run about 1/3 rd Megawatt ERP.??? Time to break out that Drake Low Pass filterwhen working with HF antennas.?? 73 Kent WA5VJB/G8EMY

Hi Tyler,

After reading your issue, I wonder if it is perhaps possible that your transceiver has taken care for a good vswr of your antenna by its build in antenna tuner?
Its just an idea.
Hope this helps you further.

Kees, PE0CWK

Op 29-7-2020 om 09:17 schreef Tyler:

Kees : I had the tuner disabled when running the SWR sweep from the transceiver.

Ted/Kent : I'll look into buying/building a LPF for this, see if I can get some better rejection ahead of the NanoVNA

Got a Spectrum Analzyer??
Would be very interesting to see where the strongest signals are.?
Especially if the coax run is looking like a good end feed antenna
in the AM broadcast band.??? Considering their bridge circuit, I
wonder just how much reject the nano's have of 50-60 Hz? coupled
signals from the AC mains?

I think this topic will be a nice addition to my next antenna column.

This is always a problem with using a VNA to look at active circuits,?
or in this case, active antennas.

Kent WA5VJB

Unfortunately a spectrum analyzer isn't something I have access to

Tyler,

I suggest you check out your NanoVNA to see how well it measures VSWR after a calibration using your short, open and load. Do you get tight dots at the far left, center and far right? If so try a resistive load below 50 ohms and one over 50 ohms. For example 25 ohms and 100 ohms should give you a VSWR of 2.

Roger

Tyler,

On the NanoVNA Facebook group we have has several users report the same VSWR reading 1 problem and in most cases it was the result of an improper calibration. Sometimes this occurs because Reset is not pushed before calibrating. I always press it twice to make sure.

Roger

On 7/29/20 1:11 PM, KENT BRITAIN wrote:

Got a Spectrum Analzyer??
Would be very interesting to see where the strongest signals are.
Especially if the coax run is looking like a good end feed antenna
in the AM broadcast band.??? Considering their bridge circuit, I
wonder just how much reject the nano's have of 50-60 Hz? coupled
signals from the AC mains?

The bridge is just a group of resistors, R9,10,11, and 12.

however, what you're really interested in is the filtering components that go into the mixer RF input. and that's 100nF into the input impedance of the SA612AD. The data sheet gives that as 1.5k and 3pF in parallel up to 50 MHz.

At power line frequencies, 100nF is about 26k, so it's a 17:1 voltage divider (ignoring the 3pF, which is infinite at 60Hz), so, about 40 dB rejection.


At broadcast band, 1 MHz, though, there's not much attenuation and the signal passes pretty well. But there, you're relying on the selectivity of the filter after the mixer. As long as the mixer doesn't saturate, you're ok. The IP3 on the mixer is pretty low (-13dBm) and the mixer starts to compress at about -30dBm. (Of course, maybe you want compression and harmonics.. that's how you get a 200MHz part to work at 900 MHz, after all)

The bridge output to the U7 mixer has a 390:50 voltage divider, which knocks it down about 19dB. So, at a first guess, the receiver mixer starts to compress at -10dBm on the bridge.


If your antenna has 100s of millivolts of AM BCB interference with a 50 ohm load on it, you're going to have problems - 1mVrms is -47 dBm, and still in the linear region. 10mV is -27 dBm and getting close. 100mV and you're definitely living dangerously.


The Port 1 input has a voltage divider of 50:480, about -20.5 dB.

BTW, hooking up a high z meter or scope to an antenna doesn't tell you much about what voltage you'll see with a load on it. The antenna's source impedance well away from resonance is probably pretty high (mostly reactive, but that still forms a voltage divider with the 50 ohm load)

I think this topic will be a nice addition to my next antenna column.
This is always a problem with using a VNA to look at active circuits,
or in this case, active antennas.
Kent WA5VJB
On Wednesday, July 29, 2020, 3:04:50 PM CDT, Tyler <twardnw@...> wrote:
Kees : I had the tuner disabled when running the SWR sweep from the transceiver.
Ted/Kent : I'll look into buying/building a LPF for this, see if I can get some better rejection ahead of the NanoVNA

Tyler,
How about an oscilloscope? If you have one that does even 20MHz that could give you some idea how much RF you are faced with. A 50 ohm termination in parallel with the oscilloscope would knock down some of the capacitively coupled 60Hz that might be there. The Fluke 87 probably does not respond too well above 1 MHz.
--John Gord

In last two weeks I did a lot of measurements on wires 9-20m long (end fed antennas with ununs). No problems with nanovna-h4 SWR measurements in 2-30MHz range in dense urban area. As an example - below you may see a measurement on a 20m long EFHW erected into 15m height, aprox 30m off a telecom tower.