nanoVNA, balun, dipole - Page 1
Forum section: electronics / rf-microwave

author: kellogs
author_rank: Regular Contributor
timestamp: February 27, 2025, 05:38:04 pm
content: Trying to measure a Normal Mode Helical Dipole. i.e. a short antenna. Theory predicts some 0.5 ohm, simulation predicts 2.5 ohm. - in open space. I have wound it over a small plastic and mostly empty cylinder.But the nanoVNA shows some 150 - 300 ohm for Re(Z) @ 433.8 MHz (due to hand effect, position varying). Definitely wrong. Also, sweeping over 413 - 453 MHz greatly influences the Re(Z).i have calibrated with the 3 standards as per instructions. So I am thinking of a problem with the balun:https://www.aliexpress.com/item/1005006432112776.html? People are not complaining about it...Or perhaps something is off with how I have connected the antenna to the two inner conductors of the two coax cables - just soldered them to the antenna feed points. Or... ?

author: LM21
author_rank: Regular Contributor
timestamp: February 28, 2025, 06:01:52 am
content: You probably need other kind of balun too. Your transmission line to your antenna may radiate and disturb. A ferrite balun may help with that. Does your antenna need a ground plane.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: February 28, 2025, 08:31:45 am
content: Helical antennas, as used on hand held radios, are not low impedance. Connect a straight dipole comprised of 2 x 1/4 wave long wires (around 160mm each) to a piece of coax, you can fit a ferrite sleeve over the coax at the antenna end to act as a feedline choke, which is the same effect as a balun. See what that looks like on your VNA.I wouldn't waste your time with AliExpress "baluns".SJ

author: A.Z.
author_rank: Super Contributor
timestamp: February 28, 2025, 09:29:17 am
content: Quote from: Solder_Junkie on February 28, 2025, 08:31:45 aman examplehttps://vk2zoi.com/articles/half-wave-flower-pot/https://nomonsuhendar.blogspot.com/2020/12/flower-pot-antenna-calculator.htmloh and a good match (low SWR) has nothing to do with the efficiency and gain of an antenna

author: kellogs
author_rank: Regular Contributor
timestamp: March 01, 2025, 11:04:02 am
content: THrough the aliexpress balun: 222 ohm at around j0 and 441 MHz, did not read VSWR but larger than 3:1Photo:THrough the ferrite balun: 344 ohm at around j0 and 500 MHz (shorter dipole), did not read VSWR but larger than 3:1Photo:Hand body effects were very large. What am i doing wrong ?

author: qbpm
author_rank: Newbie
timestamp: March 01, 2025, 04:12:31 pm
content: Where is your calibration plane? Where did you plug in the calibration kit when calibrating? If you want to measure the antenna impedance, you must connect the calibration set where the antenna terminals are. Otherwise the cable will transduce the impedance. At frequencies on the order of 440 MHz, an impedance Z=(1 + j0) shift of 20 millimetres between the calibration plane and the DUT results in a transformation to Z=(1.1 + j14).

author: LM21
author_rank: Regular Contributor
timestamp: March 02, 2025, 12:52:00 am
content: Quote from: kellogs on March 01, 2025, 11:04:02 amThat often means that you need a ferrite balun between your antenna and the feedline. That is, a high impedance. These have worked here:https://www.digikey.fi/en/products/detail/fair-rite-products-corp/0443164251/8594062orhttps://www.digikey.fi/en/products/detail/laird-signal-integrity-products/28A0592-0A2/668341

author: radiolistener
author_rank: Super Contributor
timestamp: March 02, 2025, 02:28:04 am
content: Quote from: kellogs on March 01, 2025, 11:04:02 amyour feeder is not symmetrically placed relative to the dipole arms. It should depart from the feed point in the center strictly perpendicular to the dipole at least at a distance of λ. If the feeder is not placed perpendicularly, this will lead to common mode currents and distortion of the antenna radiation pattern. You can reduce this effect by placing a lot of RF-chokes along coax cable on that distance, but cannot eliminate completely.

author: kellogs
author_rank: Regular Contributor
timestamp: March 02, 2025, 11:32:00 am
content: THis is hard...The aliexpress balun is 1:1. Does that mean it needs a 50 ohm load at its differential output, or 100, or what (for calibration)?I have done like this right at the solder points:1. Wrapped the two cables together tightly with yellow tape2. Reset NanoVNA, chosen 1GHz center frequency, left the two solder points open and calibrated for Open3. Soldered a short piece of wire at the solder points and calibrated for Shhort4. Sprawled the two solder points just a bit and soldered a 47 ohm resistor between them and calibrated for Load5. Done, Saved6. Soldered my ~ 09 / 1 GHz dipole, ran the two coax cables to it sort of perpendicular, propped one arm of the dipole so it is became relatively aligned with the other arm, and read ~ 105 ohm around resonance.Do these sound right, what next ?Thank you

author: radiolistener
author_rank: Super Contributor
timestamp: March 03, 2025, 12:47:27 am
content: Coax cable shield don't blocks common mode currents. By using it you make it more complicated to modeling and understand what is going on. If you want to reduce effect of CM currents you're need to use RF chokes on the coax feeder to block RF current which flows on its surface. Without this all these cables and other equipment connected to it are working as a part of antenna and it's very hard to predict its behavior due to very complex structure of antenna.and do not use these bending of antenna wire near the feed point, it adding inductance and shifts antenna resonant frequency and its radiation pattern. Which is more important at VHF frequencies and above due to small λ size relative to the size of these elements.You can use coax cable or ladder line to feed half wavelength dipole.Using coax requires to use balun at feed-point and several RF-chokes across feeder at λ distance from antenna. This is because coax is not balanced transmission line - it has one center wire and two shield wires (inner and outer surface of the shield braid).Using ladder line don't requires balun at feed point and don't require RF chokes, but it still requires balun at the point where you connect ladder line to coax cable or unbalanced SMA connector of your equipment.Coax cable is better when your antenna at feed point has low impedance close to to 50 or 75 Ω.Ladder line is better when your antenna has higher impedance at feed point, for example 300 or 450 Ω.If you want to measure impedance of your antenna at feed point, just solder it directly to the SMA connector connected to NanoVNA, with no cable at all. And keep NanoVNA powered from batteries and make sure it is not connected to some other equipment and galvanically isolated from any metal things and wires.if you want to measure through cable, your cable needs to have many RF-chokes to block CM currents on outer surface of cable braid. And make sure that your NanoVNA is calibrated with this cable to get proper calibration plane.

author: kellogs
author_rank: Regular Contributor
timestamp: March 03, 2025, 09:42:47 am
content: Quote from: radiolistener on March 03, 2025, 12:47:27 amIs this observation valid for the differential signals over a pair pair of coaxes and/or for differential signals over a single coax cable ?I would like to make this work with the aliexpress balun that I already have. If the quotation below is correct, then my two pieces of RG-174 need a 2:1 balun; mine is 1:1. Perhaps I can still work with it and adjust the seen impedance by some formula ?Calibration took place right at the solder points, so... I will remove the dipole 90 degree bends and measure again. Hard to work with these small SMA / coax cables.Quotehttps://www.signalintegrity.com/Pubs/news/3_12.htm

author: radiolistener
author_rank: Super Contributor
timestamp: March 03, 2025, 12:16:20 pm
content: Quote from: kellogs on March 03, 2025, 09:42:47 amYes, if you use balanced feeder line it needs to be connected through balun to unbalanced feeder line or port. SMA connector is unbalanced. Coax cable also unbalanced. So if you want to connect your ladder line to coax cable or SMA connector, you're need to use balun. Also put RF-chokes along coax cable, something like this:But as I said before, you can solder the antenna directly to the SMA connector and connect it directly to the NanoVNA without cables. This way, you can minimize the effect of CM currents and obtain a more precise result. However, an important point is that it should be soldered directly to the connector without wires or cables, and no wires should be connected to the NanoVNA - it should be self-powered from its battery. Also keep your body and hands away from NanoVNA and antenna during measurement.

author: LM21
author_rank: Regular Contributor
timestamp: March 04, 2025, 03:51:00 pm
content: As others have said, you don't need ALIexpress balun. It is of no use here.This works at 422MHz. It was at 430MHz when the antenna was in the free air, but now on a chair it was at 422MHz.My VNA shows that the antennaresistance is high at lower frequencies and is around 50-70 ohms at 422MHz. Two pieces of wire on a SMA connector. On the VNA I got a short length of coaxial cable. At other end of that coax is a Female Male SMA adapter. I calibrated my VNA there. Black boxes are baluns or chokes, high resistance at 400MHz. They are from Fairrite.The VNA shows return loss, which works as well as SWR.

author: kellogs
author_rank: Regular Contributor
timestamp: March 04, 2025, 06:47:08 pm
content: Quote from: LM21 on March 04, 2025, 03:51:00 pmI wonder why... a balun is a blaun at the end of the dayQuoteIf I am getting this right, in the picture, at 422.5 MHz there is a phase shift of some 25 deg. That is not the resonance frequency. i wonder what the resistance is at a phase shift of 0 - 2 degrees.QuoteLooks to me like a not so short length of coax. Maybe upward of lambda /2 at the working frequency ? Should not matter anyway as long a it has been accounted for.That SMA adapter - the flange thinggie where you have soldered the antenna, i would like a link to it please.

author: LM21
author_rank: Regular Contributor
timestamp: March 04, 2025, 08:02:10 pm
content: All baluns are not created equal. Some are more equal than others, like frequency range and impedance."If I am getting this right, in the picture, at 422.5 MHz there is a phase shift of some 25 deg. That is not the resonance frequency. i wonder what the resistance is at a phase shift of 0 - 2 degrees."Yep. Return loss shows where the resistance is close to 50 ohms. I probably should measure resistance again. But my frequency range was also too wide, 100-600MHz. A bit narrower range should be better.I buy most of my connectors from Ebay. I have had no quality problems and the the price is right. I found no exact link now, so I may have bought these from elsewhere, like just Aliexpress. Or perhaps exactly that connector is not there any more. Look for SMA Flange connector, or female flange sma, and you'll get like 1000 000 hits.https://www.ebay.com/itm/273674380465

author: LM21
author_rank: Regular Contributor
timestamp: March 04, 2025, 10:39:11 pm
content: This is a screen capture of the dipole when I held it on the air. Taken from Nanovna-saver, or vna_qt, or NanoVNA-App

author: LM21
author_rank: Regular Contributor
timestamp: March 04, 2025, 11:12:29 pm
content: I got curious and had to calibtate the VNA again.This it with SWR. Phase is quite critical and my frequency scale still too wide.

author: A.Z.
author_rank: Super Contributor
timestamp: March 05, 2025, 02:24:34 pm
content: try connecting the "twin coax" between the dipole and the balun as shown herehttps://archive.org/details/spencerpdf/page/n2/mode/1up

author: A.Z.
author_rank: Super Contributor
timestamp: March 05, 2025, 04:14:03 pm
content: or, either, instead of using that "balun" (sic) pick some ferrite beads and build a W2DU style choke/balun which will,most probably,work better than that "poor chinese thing"examplehttps://www.nonstopsystems.com/radio/frank_radio_baluns.htm#chokes

author: LM21
author_rank: Regular Contributor
timestamp: March 05, 2025, 08:01:17 pm
content: A. Z showed real baluns which work with antennas, especially those with ferrites. Ferrite balun should create high impedance between balun ends. That can be done with either large coil or a large resistor. I think that here, at UHF, it is easier to get a high resistance.I chose from this list a core with high resistance at my frequency.https://www.digikey.fi/en/products/filter/cable-ferrites/840

author: A.Z.
author_rank: Super Contributor
timestamp: March 06, 2025, 01:23:30 pm
content: as for dipoleshttps://www.onallbands.com/dipoles-a-domestic-secret-weapon/don't underestimate them, 8 dBi of gain from a dipole (horizontal polarization) are pretty good

author: kellogs
author_rank: Regular Contributor
timestamp: March 06, 2025, 05:25:38 pm
content: Alright, you guys have shared some pretty good stuff.Remember my issue - I am after measuring a NMHA in the end. For now, I need to prove my method + setup to ensure meaningful results. Thinking about proceeding like this:Firstly calibrate at device port (no cable), measure this loaded monopole that I have from an RTL-SDR kit through its magnetic antenna base at 433 MHz (first picture, antenna is on the left):https://www.nooelec.com/store/nesdr-smart.htmlIt should come up as... what, 50 +j0 ? should I mount the magnetic antenna base over a 70 cm diameter metal sheet too?This first step ensures that I can operate the nanoVNA properly and that the nanoVNA works well.Secondly calibrate at antenna (differential) port the following setup should result in a close to 73 + j0 reading; height is around 2.5x lambdas. There is a big ferrite piece just below the antenna port; the two differential coaxial cables to the balun are both surrounded by that ferrite piece.I don't suppose the VNA is too close to the antenna, is it? I'll be close by, reading it, too.This second step will tell whether or not the chinese balun is any good.

author: kellogs
author_rank: Regular Contributor
timestamp: March 07, 2025, 07:57:05 pm
content: Wait a minute, why do I even struggle with a dipole ?...I can just measure another resistor with the nanoVNA, say 100 ohm, differential, after I have calibrated the differential port (currently the feed point for the dipole) at 50 ohm, no ? if it comes more or less 100 ohm -j0 at the frequency of interest then the balun is good.

author: A.Z.
author_rank: Super Contributor
timestamp: March 09, 2025, 10:39:20 pm
content: Quote from: kellogs on March 07, 2025, 07:57:05 pmuse a non inductive resistor.

author: kellogs
author_rank: Regular Contributor
timestamp: March 09, 2025, 10:43:26 pm
content: So I went ahead, measured a few resistors, then a few capacitors. Looks all good; virtually same results with or without a ferrite choke over the two diff coax cables.Caps measurement were a bit / a bit more off depending on the distance between the two solder points - to be expected I guess. All at 433.92 MHz Then the balun is fine, yes ?Now, to the NMHA: built three of them, took four measurements for each: reverse soldered, with /the ferrite choke, :Ant_1: 32 - j118 [67 - j118] reverse: 25 - j118 [36 -j 123]Ant_2: 33 - j133 [68 - j133] reverse: 29 - j136 [42 -j 142]Ant_3: 34 - j119 [80 - j115] reverse: 25 - j125 [31 -j 128]The manufacture process for Ant_2 went not so well, fwiw. They are around 8 cm long and 1.3 cm in diameter.Does anything give, why am i seeing such huge resistance?

author: kellogs
author_rank: Regular Contributor
timestamp: March 14, 2025, 11:46:10 am
content: I had the idea to take measurements at different frequencies, so, for the previously nanoVNA calibrated at 433.92 MHz with a ferrite choke and Ant_2, took more measurementsthe choke[10 - j14] - 485 MHzAnt_2: [68 - j133] - 433.92 MHz[128 - j646] - 385 MHz[29 + j169] - 335 MHz[10.5 + j68] - 285 MHz[7.5 - j60] - 235 MHzMy balun is only rated up to 500 MHz so I did not go any higher.my nanoVNA is calibrated to 433.92 MHz so I did not go any lower.Do these look right for NMHA dipole, 1.3 cm diameter, each arm 4 cm long ?

author: kellogs
author_rank: Regular Contributor
timestamp: March 22, 2025, 08:48:56 pm
content: I might have finally made it.Givens / theoryheight = some 3 lambdas above the groundtip to tip length = lambda/2 ~= 38 cmgap at feedpoint = 1 cm <- should raise impedance (!)first approx resonant freq = c/lambda ~= 394.7 MHzconductor diameter = 12 mmconductor diameter in wavelengths = 12 / 760 ~= .0158length reduction factor for the number above ~= .955 <-- seehttps://en.wikipedia.org/wiki/Dipole_antenna#/media/File:DipoleReductionFactor.jpgsecond approx resonant freq = .955 * 394.7 MHz = 376.9 MHzobservations / measurements nanovNA:kinked wire, some plastic around the antenna, imperfect alignment of the arms, calibration to some 51 ohms, not 5065.5 ohm @ first resonance = 360.55 MHz138 ohm @ second resonance = 365.5 MHz <-- dunno what this is aboutGood enough ?