As of 2022 there are many badly performing clones on the market. V2/3GHz NanoVNA uses parts like ADF4350 and AD8342 which are costly and clones have been cutting costs by using salvaged or reject parts.
See official store and look for V2 Plus4/V2 Plus4 Pro versions only to avoid getting a bad clone. We have stopped selling V2.2 versions since October 2020, so all V2 hardware that are not Plus or Plus4 are not made by us and we can not guarantee performance.
I have a two different unlabeled, sealed baluns that I'd like to characterize with my NanoVNA. They're intended for ham antenna use, with an SO-239 on the unbalanced side, so I'm mainly looking for the impedance ratio. Any pointers would be appreciated.
Eric,
I'd be inclined to measure S11 at the UHF connector while attaching various
values of (preferably carbon comp) resistors on the balanced side, like 50,
100, 200 Ohms. If you do your analysis at a low enough frequency, like <10
MHZ, you could probably just put a 500 Ohm pot on the balanced side and
tune it for 50 Ohms (or lowest VSWR) on the unbalanced side. This would
certainly get you in the ballpark for impedance ratio.
73,
geo - n4ua
On Fri, Sep 23, 2022 at 12:58 PM N3ECC <eric.c.cooper@gmail.com> wrote:
test load them with a fixed resistor of 50 ohms, 200 ohms, 300 ohms ...
and make swr sweeps
greetz sigi dg9bfc
ps if you do not have a 50 oms ... a 47 ohms will do fine ... you just
want to know if it is 1:1 or 1:2 or 1:4
Am 22.09.2022 um 17:22 schrieb N3ECC:
Hi Eric,
I may be way off base here, but perhaps you could SOL calibrate your NanoVNA to cover a range from 3 to 30 MHz, and then hook the CH0 port to the SO-239. Then perhaps connect a 1K potentiometer, or maybe perhaps a 5K pot, across the balanced side and sweep the resistance to see where you get the flattest SWR curve. Once you find the “sweet” spot on the NanoVNA curve, use a multimeter to measure the precise resistance on the pot. That should give you a rough idea what impedance matching ratio your balun is wound for. Otherwise, most DIY baluns/ununs would likely be 1:1, 4:1, 9:1, 49:1, or 64:1, so you could try terminating the ‘balanced’ side with fixed resistors of approximately 50, 200, 450, 2450, and 3200 ohms to see which yields the flattest SWR trace.
I’m basing my conjecture on the recommended process to test how flat a DIY balun is per it’s design. For example, if you put together a 9:1 balun, the recommended termination resistance for testing it is 50 * 9 = 450 ohms. If you connect a resistance of 450 ohms across the balun, the SWR trace should be relatively flat across the desired range. Perhaps one of the other NanoVNA trace parameters (LogMag, Phase, Resistance, Reactance, etc.) may also provide some insights, but I’ve seen YouTube videos of using the SWR trace to measure baluns/ununs.
Good luck and 73.
Ken -- WBØOCV
From: N3ECC
Sent: Friday, September 23, 2022 12:58 PM
To: NanoVNAV2@groups.io
Subject: [nanovnav2] Mystery baluns
I have a two different unlabeled, sealed baluns that I'd like to characterize with my NanoVNA. They're intended for ham antenna use, with an SO-239 on the unbalanced side, so I'm mainly looking for the impedance ratio. Any pointers would be appreciated.
Eric,
You may have to build a fixture so you can attach port 1 center pin to
beginning of one of the wires of the balun and port 2 center pin to the end
of the wire. You should leave the negative side of port 1 and port 2
disconnected.
After calibrating to the end of you fixture to your frequency range of
interest you can read the S21 gain dB. A dB loss over -20 (99%
attenuation) and -30 dB (99.9% attenuation) over most of your frequency
ranges is considered good but the lower the better.
Because just about all nanoVNAs do not measure impedances over 1K ohms very
well, to do further analysis you need to download the S21 Touchstone file
which will graph total impedance and real and imaginary (reactance)
impedance. Ideally, real impedance should exceed imaginary impedance
(reactance) over the entire frequency range. The toroid mix, type of
wiring, spacing of wiring, turns and other factors affect the balun common
mode current impedance. Also, be aware that baluns have trouble handling
high power. On some of my 1000W RF signals I use 3 large 2.9" toroids to
give me effective common mode choking without over heating.
To read the Touchstone files into Excel there are many free Excel
Touchstone file graphing programs.
I suggest you read the following: http://karinya.net/g3txq/chokes.
A final caution. I see many choke testing schemes that use resistors.
Most non-surface mount resistors in the 3-30 MHz range have varying
resistance due to paralytics (inductance and capacitance) that may
materially affect your measurements. This is very pronounced above 100 MHz.
Hank
On Fri, Sep 23, 2022 at 11:58 AM N3ECC <eric.c.cooper@gmail.com> wrote:
Oops – I left something out. Not just flat, but as close as you can get to a 1:1 SWR across the sweep range
From: Ken Sejkora
Sent: Friday, September 23, 2022 02:16 PM
To: NanoVNAV2@groups.io
Cc: N3ECC
Subject: Re: [nanovnav2] Mystery baluns
Hi Eric,
I may be way off base here, but perhaps you could SOL calibrate your NanoVNA to cover a range from 3 to 30 MHz, and then hook the CH0 port to the SO-239. Then perhaps connect a 1K potentiometer, or maybe perhaps a 5K pot, across the balanced side and sweep the resistance to see where you get the flattest SWR curve. Once you find the “sweet” spot on the NanoVNA curve, use a multimeter to measure the precise resistance on the pot. That should give you a rough idea what impedance matching ratio your balun is wound for. Otherwise, most DIY baluns/ununs would likely be 1:1, 4:1, 9:1, 49:1, or 64:1, so you could try terminating the ‘balanced’ side with fixed resistors of approximately 50, 200, 450, 2450, and 3200 ohms to see which yields the flattest SWR trace.
I’m basing my conjecture on the recommended process to test how flat a DIY balun is per it’s design. For example, if you put together a 9:1 balun, the recommended termination resistance for testing it is 50 * 9 = 450 ohms. If you connect a resistance of 450 ohms across the balun, the SWR trace should be relatively flat across the desired range. Perhaps one of the other NanoVNA trace parameters (LogMag, Phase, Resistance, Reactance, etc.) may also provide some insights, but I’ve seen YouTube videos of using the SWR trace to measure baluns/ununs.
Good luck and 73.
Ken -- WBØOCV
From: N3ECC
Sent: Friday, September 23, 2022 12:58 PM
To: NanoVNAV2@groups.io
Subject: [nanovnav2] Mystery baluns
I have a two different unlabeled, sealed baluns that I'd like to characterize with my NanoVNA. They're intended for ham antenna use, with an SO-239 on the unbalanced side, so I'm mainly looking for the impedance ratio. Any pointers would be appreciated.
I do not believe that method measures choking impedance.
However, that method will measure SWR in a 50 ohm system if a 50 ohm dummy
load is used. Resistors have paralytics that cause stated resistance to be
inaccurate most of the time.
On Fri, Sep 23, 2022 at 1:16 PM Siegfried Jackstien <
siegfried.jackstien@freenet.de> wrote:
Ken,
SWR measurements of a RF choke balun have nothing to do with the balun
being able to attenuate common mode current. The main purpose of a choke
balun is to balance the differential current by attenuating the common mode
current. For example, you can wrap two insulated wires around a piece of
wood and the SWR will not be 1:1 into a 50 ohm dummy load because the wires
are not 50 ohm. If your wrap a piece of 50 ohm coax around the same piece
of wood into a dummy 50 ohm load the SWR will read closer to 1:1 because
the coax itself has approximately 50 ohm characteristic impedance.
There are three sources of current coming back down the feedline from the
antenna to the transmitter: (1) the center pin of the coax, (2) the inside
of the coax shield and (3) the outside of the shield. The outside of the
shield carries the common mode current. The current on the inside of the
shield and the center pin are, for the most part, 180 degrees out of
phase. However, a smaller amount of current (common mode) travels on the
outside of the shield along with noise picked up along the way. Noise
generally is comparatively minor that reaches the inside of the shield and
center pin of the coax. When you are reading the SWR your antenna your
analyzer is measuring the total current going out vs the current
returning. If 1.4 amps is going out the center pin of the coax and 1.4
combined (inside and outside of the shield) amps are returning, the SWR is
1:1. But, the analyzer does not know there is, for example, 200 mA
returning on the outside of the shield ready to screw up your signal with
noise and possible RF burns.
So, the transmitter current coming back is out of balance because the
impedance of your antenna and it's surroundings are out of balance. The
impedance of each half of the dipole in this example are not the same. It
still could be 50 ohms for a 1:1 SWR but the complex impedance on each half
of the antenna could be different but combined it is 50 + j0 ohms.
A good common mode choke presents 3-5K ohms of impedance over all
frequencies of interest to the common mode current and this impedance
forces the current on the outside of the shield to jump to the inside of
the shield balancing the current so that the inside of the shield and
center pin of the coax carries the same equal, but opposite, current. If
the common mode current is not attenuated it will jump somewhere and we
need it to jump back to the inside of the shield.
The SWR of a common mode choke should be kept as close to 1:1 as possible
to keep from returning current to the transmitter. This is not a big deal
but it is hard to build a common mode choke with an SWR of less than 1.2 to
1. The big deal is eliminating common mode current and its effect on noise
and RF in the shack.
A nanoVNA can measure S21 attenuation on most any transmission line. A
good common mode choke should have a minimum of -30dB or higher attenuation
(-20dB is not enough attenuation) across all your frequencies of interest.
To graph impedance (total, real and net reactive), you need to export an
S21 Touchstone file and then import into the program you will use to
manipulate the file. There are many free programs on the Internet that do
this.
To measure common mode impedance the center pin (using a fixture) of the
NanoVNA should be attached to the choke wire that carries the return
current (the shield on a coax). The ground on either the S11 or S21 port
on the nanoVNA are not connected to the choke.
If anyone disagrees with what I have said I would appreciate your thoughts
and reasoning. This is a complex topic so input, especially from RF
Engineers, would be greatly appreciated.
Hank
On Sat, Sep 24, 2022 at 12:30 PM Ken Sejkora <kjsejkora@comcast.net> wrote:
If you want the impedance ratio, or better yet, what the turns ratio is follow the fixed resistor methods mentioned and do it for 80 or 40 Mtrs. The lower the frequency the strays will affect the results. Since you know the Balun ( or is it an unun? ) is likely 1 of several popular flavors coming close to a 1:1 SWR is close enough
Or, use a screwdriver.
1. When you make measurements you can connect the unbalanced side to one port of your instrument. Make the connection to the balanced side of the balun being tested to the other port of your unbalanced instrument in such a way that the balance is not disturbed, such as through an isolation transformer.
2. Alternatively, you can connect a signal generator to one side of the balun. Connect both inputs of a dual-trace oscilloscope to the input and output. The ratio of those two signals is proportional to the turns ratio of the balun. This will not work for frequencies beyond your scope bandwidth.
3. If you have a balun of known values, test it to verify your results.
See:
https://electronics.halibut.com/product/common-mode-current-choke-test-rig/
--
73,
-de John NI0K rural Debs, MN
John Galbreath via groups.io wrote on 9/24/2022 10:34 PM:
To reply to this topic, join https://groups.io/g/NanoVNAV2
