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This is my first post after lurking since October, so I'm quite new to the Nanovna. The Nanovna measurements of SWR are considerably lower than what I'm seeing when I transmit on-the-air. What does this indicate? Calibration problems, or errors in my other equipment? Any insight would be greatly appreciated. Thanks.
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Ron, K7UV

What are you using to measure VSWR while transmitting. If you use a meter
that simply changes scales with no re-normalization for any specific power,
they are quite prone to read improperly when less than full scale power is
sourced from the transmitter. Cross-needle meters do not have this
problem, in general.

Dave - W?LEV

On Fri, Nov 15, 2019 at 3:44 PM Ron - An Old Ham in Utah <k7uv@...>
wrote:

This is my first post after lurking since October, so I'm quite new to the
Nanovna. The Nanovna measurements of SWR are considerably lower than what
I'm seeing when I transmit on-the-air. What does this indicate? Calibration
problems, or errors in my other equipment? Any insight would be greatly
appreciated. Thanks.
--
Ron, K7UV

--

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

Hi Ron
If it is the SWR indication build into your radio you are referring to, and your radio has an internal antenna tuner (or I would rather call it a impedance matching network), then the radio is matching to the actual antenna impedance when tuning so the PA stage see a close to 50ohm load impedance. Thus you can newer compare a NanoVNA SWR measurement of the antenna with a SWR indication in the radio display.
Kind regards
Kurt

-----Oprindelig meddelelse-----
Fra: [email protected] <[email protected]> P? vegne af Ron - An Old Ham in Utah
Sendt: 15. november 2019 16:45
Til: [email protected]
Emne: [nanovna-users] SWR...Nano versus Transmitting

This is my first post after lurking since October, so I'm quite new to the Nanovna. The Nanovna measurements of SWR are considerably lower than what I'm seeing when I transmit on-the-air. What does this indicate? Calibration problems, or errors in my other equipment? Any insight would be greatly appreciated. Thanks.
--
Ron, K7UV

Good afternoon!

Depending on how everything in your rig is connected and/or grounded, switching from a rig that is connected to the powerline ground to the NanoVNA that is floating could change things so that both of your readings are correct. If that sounds possible, you could tie the shield of the transmission line to the ground/chassis/etc of your xmtr while the NanoVNA is connected. That would eliminate that as a possible issue.

Just a thought...

Tom
AE5I

Thanks. I'm using a cross-needle meter without any tuner, so I'm matching
the same antenna as measured with the Nano.

I'm using an external full-range tuner with a straight through (direct without using tuner function) reading to use the cross-needle SWR/PWR reading, so no, I'm not using the rig meter for SWR measurements. The frequency for lowest SWR as well as the actual SWR level are off. Comparing the NANOVNA readings versus the SWR measurement show 1.3:1 on the nano device and 2.6:1 with the SWR meter when I'm transmitting. Perhaps this discrepancy is to be expected between devices and exceeds measurable accuracy? I would think the nano device reading would be more accurate, but while transmitting, the SWR is high enough to cut back the power level from that with a lower SWR. Usually, my transmitter self-protects when the SWR is 2.0:1 or higher. Thanks and 73.

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Ron, K7UV

Hi Ron,
Just to understand:
You are disconnecting the cable from the back of the radio and connecting
it to the vna? Is that correct? You are not using a different jumper or
removing any of the existing items that are inline when connected to the
radio, correct?

If you are, that might change what you are reading.

Thanks,

Marco

On Fri, Nov 15, 2019, 9:38 PM Ron - An Old Ham in Utah <k7uv@...>
wrote:

I'm using an external full-range tuner with a straight through (direct
without using tuner function) reading to use the cross-needle SWR/PWR
reading, so no, I'm not using the rig meter for SWR measurements. The
frequency for lowest SWR as well as the actual SWR level are off. Comparing
the NANOVNA readings versus the SWR measurement show 1.3:1 on the nano
device and 2.6:1 with the SWR meter when I'm transmitting. Perhaps this
discrepancy is to be expected between devices and exceeds measurable
accuracy? I would think the nano device reading would be more accurate, but
while transmitting, the SWR is high enough to cut back the power level from
that with a lower SWR. Usually, my transmitter self-protects when the SWR
is 2.0:1 or higher. Thanks and 73.

--
Ron, K7UV

@Ron, K7UV: When you lay your hand on the SWR meter enclosure, do the meter needles move?

This is because SWR meters have high measurement error.

Ron,

Have you calibrated he nano with a calibration standard of the same style that your antenna connector is? I.E if your cable connected to the radio has a male UHF connector, your nano needs to be calibrated with a male uhf set. If you calibrating with the SMA set and then attaching an adaptor or adaptor cable to connect to the antenna cable the readings will be off. A VNA no matter if it cost $50 or $100k always needs to be calibrated at the plane of measurement. Here, the plane of measurement is the point you attaching your antenna cable. Since the VNA looks not only at forward and reflected power but also at phase not calibrating it right will cause errors in either direction...i.e. either read too good or too bad. You can make male calibration sets yourself that are adequate and only have minute errors vs commercial sets. U-tube has quite a few instruction videos how to do it.

Thanks for the reminder and suggestions. Yes, I have calibrated using my
cables to connect with my antenna. This users group is great!

Roger!

What band are you doing your testing on? Also, is your xmtr running on AC power or battery?

Tom AE5I

Howdy Ron,
Properly calibrated, the nano should give you much better accuracy than even a very good SWR meter, because a VNA calibrates at many points (101 for the nano) across a frequency span, versus an SWR meter that is calibrated at one point only. And the VNA uses a resistance bridge that is much flatter than the directional coupler used in typical SWR meters. Some things to consider.

1a. You can perform a calibration on the nano thru your existing cables and connectors, e.g., the input cable, the bypassed tuner/meter; and on its output side, PL259 short, open, and load. For the load, use a 100-watt+ dummy load, so you can apply radio TX power and calibrate your cross-needle meter. Then replace the radio with the nano, and calibrate it with the exact same setup. This will give you a calibration through the tuner box and let the nano see what the radio sees.
OR 1b: Cal your cross-needle with a 50-ohm dummy load; Cal your nano at a SMA-SO239 adapter+same dummy load (and PL259 short and open); then move the antenna cable between them; i.e., in both cases the ant coax replaces the dummy load. This will measure antenna without the tuner being in the system.

But also consider:
2. SWR meter indication may rise with increased power, due to all the those bare wires in the tuner (several inches long even in bypass) having some inductive and capacitive coupling within the tuner. On tuner/meter+dummy load, crank the radio's power & frequency up & down. There will be some non-linearity caused both by the directional coupler (probably a Bruene), and some caused by unshielded wires in the tuner. My 3 tuner/meters will vary from about 1.3:1 on 80, 1:1 on 20 (where I calibrate), to 1.7:1 on 10 meters, on a dummy load, and rise a little more with increasing power.

3. Similarly, on antenna, SWR may rise a little with increased power, due to antenna coupling with ground and other conductors in its near-field environment. Set the radio's freq at the antenna's resonance then crank the power up; if increasing power raises the SWR (more than it did with the dummy load), then you know you have some antenna coupling effect (RF induced in nearby conductors causing re-radiation back to antenna, changing its feed-point impedance -- this effect shouldn't increase with power level, but alas, in practice, it sometimes does).

Anyway, my points are that you need to calibrate the nano with the same (as close as possible) setup as the SWR meter uses, and also consider that the power output of the nano is something like 0.1 milliwatt, causing very little coupling effects; versus the 10-1500 watts the radio/amp puts out, causing considerable coupling effects. In addition to all the other possible errors! Like Bruene coupler vs. resistance bridge, grounded meter vs. floating, etc etc.

73, --kv5r

The band that the primary problem is 40 meters; my xmtr is running on a
conventional power supply, not battery powered.

if your transmitter has high distortions and out-of-band emission, you will get different VSWR with VSWR meter. Because VSWR meter shows VSWR for all frequencies in the signal. While NanoVNA shows VSWR for specific frequency.

For example, if you're transmitting at 3650 kHz and your transmitter has 90W emission at 3650 and 10W at 7300 kHz, your VSWR meter will show you average between 90W at 3650 kHz and 10W at 7300 kHz.

But when you use NanoVNA it will show you VSWR for 3650 kHz with no out-of-band emissions.

I'm with you Ron, this thing is not right. It show my 80m dipole as having a good match across the entire 75/80 band, lol. It is cut to favor about 3.9, and three analyzers (Comet, MFJ and Rigexpert) along with meter in my Expert linear all agree! But they must all be wrong. I did not expect much from a $50 unit, but I thought it might be useful to take in the field for quick measurements on portable ops. But I don't trust it at all yet. Need to figure out what I am doing wrong.

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Paul W1ip

Paul

Are you calibrated for and looking wideband 50 kHz - 900 MHz or calibrated and looking narrowband 3.5 - 4.0 MHz? Confining the 101 data points to the band of interest yields the most detail.

Jay W1VD

To further communicate the point made by Paul, W1IP, the following may be
of some enlightenment.

If I take 101 points, the max for the NANOVNA and spread the evenly of the
max frequency range, that yields only 101 / (900 - 0.05) = 0.11 points per
MHz. You don't even cover the whole 75/80 meter band with one point!!!
You get only 0.1 points for the whole half MHz!! Of course, it will read
the same over the whole band.

Now, calibrate over 3.5 to 4 MHz: 101 / (4.0 - 3.5) = 202 points per MHz.
Of course, you only use the max of 101 points which covers the whole 75/80
meter band with good resolution. Now you have all 101 points devoted to
reflecting the measurement of the entire band - MUCH more accurate.

Please, before badding the instrument, read and digest the tutorials! No,
this isn't an HP 8753C, but it's also not your typical MFJ or Comet piece
of gear. Learn how to use a VNA. Don't feel bad as most electrical
engineers (even at the PhD level) don't understand how to properly use a
VNA or crawl around on a Smith Chart. Believe me as I worked with one at
HP who had absolutely no idea what an S-Parameter represented or what
return loss indicated. You'll never go back to MFJ!

Dave - W?LEV

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

For best results always calibrate the same frequency range as you will be measuring.
With 900MHz range and only 101 data points the nanoVNA will interpolate the calibration data leading to relevant errors, SWR measurement is very susceptible to this.
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Erik, PD0EK