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Some xtra info.

Just for fun I wanted to look closer at my Baofeng UV5R input RX impedance.

(1) Readings varied a LOT if I was using a short (supplied with vna) RG174.
(2) Readings were 100% stable if I connected the HT directly to the vna, via an SMA adapter (calibrated with).
(3) It was essential to power the HT to get a reading, it was different if powered off.
(4) Make sure there's *no dual RX* operating between VHF /UHF because input circuitry params change, do ONE band at a time !
(5) I had a different impedance reading this time, a mere 21 ohms on 2m, then I ran out of time. No UHF measurement completed, sorry.

KB1GMX makes note of using lower power levels to measure the input Z's more accurately for these HT's.
I have no attenuators, so I guess I'll need to educate myself, purchase some, test it and educate myself.

It's called progress and learning ;-)

73 de Andy

Hi betex,

I would like to share also to you my test results when I tried to make a 1/4 wavelength ground plane antenna
at 1.2GHz to test the frequency performance limits of nanoVNA. The graph result is not smooth but it still shows
a minimum SWR at my desired frequency. I hope this can also help you in building your 1/4 wavelength ground
plane antenna at 170MHz.


neb

On Sat, Nov 30, 2019 at 06:51 PM, aparent1/kb1gmx wrote:

FYI measure the antenna that comes with the UV5R, its quite awful.

Mine doesn't seem too bad, better than most that I have seen.
But I think it's luck of the draw what you get supplied with.

I have done a LOT of business with 409shop in HK (excellent dealer !!!), and have many of the Chinese HT's.
So do my friends.

But we can order a rig on the same day or a week later, and occasionally get a different antenna / charger,
so maybe what I got were the better ones.

I have seen just as many awful Icom and Yeasu rubber ducks in my time.

73 de Andy

This is an effect of common mode current on the outer side of braid. The outer surface of the coax cable become a part of antenna. And when you connect usb cable it also become a part of antenna, because it is galvanic connected to outer braid of the coax cable.

In order to eliminate such effect, you're needs to use blaun at antenna feeding point and RF choke on coax cable at half wavelength from antenna.

For S21, the good choice for output attenuator is equal to amplifier gain. And for input attenuator the good value is 20-30 dB attenuator.

For example, if your amplifier has 40 dB gain, then use 20 dB attenuator on the input and 40 dB attenuator on the output.

For S11 you can connect CH0 to input and dummy load on the amplifier output.

For S21 you can use 20-30 dB attenuator on the input and 20-30 dB attenuator on the output of amplifier. You can calibrate NanoVNA with attenutators or just add sum of attenuators value to result. Attenuator value will depends on your amplifier. Just note that you're needs to fit amplifier output to -10 dBm max.

The first part of that statement is correct, but the last part isn't.

It's not coupling from the balanced dipole to the coax that causes shield currents ... it's the simple fact that the antenna is balanced and the coax isn't.? You may get away without a common mode choke, but it is more likely that not having one will result in some amount of feedline radiation, pattern distortion, and/or unwanted pickup.

Horizontal dipoles and yagis typically have the feedline perpendicular to the antenna and they still need a choke for proper performance no matter how high the antenna is (and therefore how far the coax remains perpendicular to the antenna).

73,
Dave ? AB7E

Me strange, nooo. ;)

The RG316DS over the years I've likely used more than 2000ft
of it. Most are two braids but it tends to be more phase stable
than single braid. If you need critical phase stability there is stuff
that will may you cry per foot.

Allison
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PVRC member AA3S Doug Hart gave a presentation on the NanoVNA that is a good intro to some of its measurement capabilities. His TDR example, showing the impedance change from 50 ohm to 93 ohm cable sections, is instructive for new users not familiar with TDR. The link is as follows: .

- Herb

As previously posted, you will need to add a suitable 50 ohm pad to both CH0 and CH1 and do a full calibration with the pads in place. Hence, you must take care not to drive the DUT into compression (monitor bias current), the bias current should not change. As well, you must take care not to drive the vna into compression at CH1. The level of pad unfortunate limits your measurement dynamic range. However, if this is an unmatched device and the measurement frequency is well below the F _tau of the device, that is not an issue, s11 and s22 will be close to unity and most likely capacitive.

You do not mention, how are you applying bias to the DUT. Are you using a bias tee. It may be part of the measurement unless you built a really great bias tee. That is to say it is transparent at the measurement frequency. If not, then it is part of the measured device.

I strongly suggest you do your measurement with a device for which you have data. Measure at a low frequency, say HF and a single device that has a reasonable S21, not to large, say 12-to-15 dB. That way you can minimize some of these complications. I have measured and compared simple 2N3904 devices at Vce of 6 to 9 volts and Icq of 1-5 mA. Small signal s data is available by using LT Spice as the device Spice models are in the program. And Spice provides all the S data output easily. You will also have real bias networks in there or you can put in non ideal bias tee networks and observe the results in Spice. Then go to the vna and do the same. The results will match if you did all the steps correctly!

Alan

Hi Allison, I have been using RG-316 with SMA for years. I also try to avoid RG-174, even though I have a HUGE spool of it made by Alpha,
Just this week, I found some Belden RG-174DS (double shield) and some RG-316DS on ebay. The 174DS has an inner 'foil' shield. I haven't received the 316DS, but it appears to have 2 braids.

Have fun and I really enjoy building cables such as these. Strange, I know...

ron
N4UE

This thread has been locked. It is not adding to the technical knowledge or expertise about nanovnas.

DaveD

Greetings Betex,
What you have there, in the first picture, is not a ground plane; it's a patch antenna closely coupled to a rod antenna. This is causing a severe imbalance in coax currents, causing the coax shield, nanovna, and USB cable to become part of the antenna.
The plane needs to be perpendicular (horizontal) to the vertical rod, and needs to be at least 1/4-wave center-to-edge; in the case of 170 MHz, the plane needs to be at least 33 inches square, and the vertical rod will be around 16.5 inches. The quarter-wave formula is approximately 234/f(MHz)=feet, x12 for inches. Start with it a little longer, and trim by eighth-inches, after you get your test setup built so the nano, USB cable, etc., are not affecting the readings.
However, the SWR of a ground plane antenna with a horizontal plane will never go completely flat to 1:1. It's better to not use a flat plane, but use 4 quarter-wave radial rods and bend them down 45 degrees. There are some youtube videos showing construction of small quarter-wave ground planes, both with flat and bent radials.
Also, any conductor within the near-field of the antenna will have RF inductively coupled to it, messing up your readings. The "near field" is the electromagnetic field of the antenna, generally considered to be about 1 wavelength; at 170 MHz, that's about 66 inches (minimum), so your antenna needs to be over 5-1/2 feet away from anything conductive, with its feed-line extending 5-1/2 feet straight down below the ground plane.
Yes, you can reduce this clearance distance somewhat by using ferrite chokes on the coax, and any others conductors within the near-field distance. An effective 1:1 balun/choke is typically 11 turns of the coax through a toroid (split "snap-on" chokes are generally crap). At VHF, if I remember correctly, ferrite mix 43 is effective; you might want to look at the Amidon web site for application notes, and order some FT-140-43 toroids (or whatever type they recommend for that frequency range).
Finally, there are better and easier antennas for VHF/UHF, particularly the vertical dipole, which you'll need to support from the side with 5-6 feet of something non-conductive (PVC pipe, varnished 2x2 wood, etc.). The half-wave vertical dipole will have about 2.15dBi gain (toward the horizons), and is a lot easier to build and tune than a ground plane. Also, it is inherently balanced, and if you bring the coax away from it perpendicular for at least 1 wavelength, you shouldn't need any chokes. I have some articles on my site, kv5r.com, of building VHF dipole antennas with PVC pipe and aluminum tape.
Hope this gives you some ideas!
73, --KV5R

Nice arrangement.?
Thanks for the updates



On Sat, 30 Nov 2019 at 12:23 PM, Gyula Molnar<gyula.ha3hz@...> wrote: Latest firmwares I've found a picture of.
73, Gyula

Andy,

If the input circuit is overloaded the errors multiply and the reflected
impedance is not correct.

My UV5r measured using a HP4191A at -30dbm gives far more reasonable reading inside
the 1.5:1 SWR circle, about 35ohms low, 70 ohms high depending on frequency.

Up the RF power and its breaks and goes all over the map. Generally all receivers
do this at some input level. So keeping the input power to the radio under -30 dbm
will keep the numbers honest.

FYI measure the antenna that comes with the UV5R, its quite awful.

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This is a tricky case as the power out can overload the device.

The solution I use is attenuate the output, run S21 and use an
external return loss bridge with an amplifier on the return port
to the channel 1 input. With known gains and attenuation its
easy to get return loss, input impedance, and then input SWR

Same methods used for receiver inputs, with care to insure
the input is below overload.


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I use minicircutis series cables as they were purchased for a project
where I used higher end VNAs. so the case is nice 1M cables gets
used.

I also make cable for use and have a set of 30cm RG316 duobriad
cables and 1M cables of same. For projects I ahve a bunch or 1M
RG316 cables terminated one end for direct soldering and SMA
the other.

Short lengths of 316 can be had cheap, being PTFE its more durable.
Local flea I could get 15cm and 30cm cables with SMA for under
1$us each. The vendor has a box of them. Also for those that cringe
at using them solder tailed, matching socket connectors are fairly
cheap.

I try to not use RG174 as it tends to break easily and it melts easily
with soldering.

Also a full box of cross species cable adaptors!
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I am using a cell designed for digital cameras ans is 1200mah
and accepts the 1C charge safely. I prefer to keep the charge
rate at .4C, charge time is not an issue as I have three of
them and a holder. Since I'm not doing a race car, fast charge
is not required and tends to beat up the cells.

I use 18650 cells a lot and LD-2800mah would also work well.
I forget where I got it but there is a battery management board
that does over voltage, under voltage and short circuit protection.

Allison
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W5DXP,

Not a windows user but I've heard that is the case. A handy thing.
With the above in mind I had a moment where I considered porting
Nanosaver to QB45 in a moment of pure madness.

I can run under linux DOSemu and QB45 runs blindingly fast and
all the old stuff as well.

I already have a capture program for Touchstone files and enough
LibreOffice Calc files for grinding them into more useful formats
and plots.

Anyhow I got my NanoVNA a while ago and have beaten it against
commercial (Agilent E8357A PNA and HP8753C VNA) and found it
despite the limited dynamic range remarkably accurate for a lot of
routine S11 and S21 work. Least for the standards I have and tests
performed its better than .1 to .2 db difference to 500mhz. Also
far more portable. Heck of a bargin.

Allison
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Hi dna.dana@... -

Yeah kinda hard to find anything.

You are probably a recent subscriber and likely frustrated by documentation
provided with your nanoVNA, which may be better or worse
but probably different from that provided for other users since July..

Can you suggest some specific changes here that mere users can make,
given Groups.io organization by Messages, #Hashtags, Photos, Files and Wiki?

And if a noob dares ask for help, you probably will get flamed
for not using the search function properly type of answers.

Based on experience with egroups.com, then Yahoo! groups,
flames here seem generally fewer and less hot,
but complaints to this group will improve the nature of neither flames nor flamers.