Balun measurements

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK2FXXX, Feb 13, 2011.

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  1. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    Gday QRZ readers.
    Last night I had some time to do some tests on a toroid type guanella (current)
    (choke)balun.
    The toroid was bought from an Australian electronics supplier about 2 years ago. A pair cost about $ 7 Au. It was bought on the chance it may be useful as a lower hf balun.
    It is as follows: Part no LO1238 size 35mm dia ,21mm ID , 13mm thick.
    Material is L15.
    I wound 8 turns of 1mm diameter enamel coated wire bi-fillear ?wound, three quarters around the toroid.
    The balun was measured using the following:
    A 50.1 ohm load using 2 x 100ohm metal film Rs in parallel
    A VK3AQZ noise bridge.
    Icom 718.in AM mode.
    First I measured the reactance of the load connected as close to the bridge as possible. through the frequencies below.
    It measured flat (50 +j0)
    from 1.9mhz -10.5mhz .
    At
    14.2 mhz = 50 +j2
    21.2 mhz = 50 +j4.8
    28.5 mhz =j50 +j6.6

    I then connected the 50 ohm load to the balun ,and then to the bridge through a 450 mm (aprox 18inch) patch lead.
    The impedance's were as follows:

    1.9mhz : 50+j5.7
    3.6mhz : 50+j7.5
    7.1mhz : 52+j10.3
    10.5mhz : 55+j11.5
    14.2mhz : 58+j16
    21.2 mhz: 73+j18.7
    28.5 mhz: 88+j12.7

    The swr was then plotted on a smith chart program ,the results were:
    1.9mhz =1.1:1
    3.6mhz =1.2:1
    7.1mhz= 1.2:1
    10.5mhz= 1.3:1
    14.2mhz = 1.4:1
    21.2mhz =1.6:1
    28.5mhz =1.8:1
    I then ran the balun into a 300w dummy load for 1 minute cw at aprox 100w .on all frequencies above .I then felt the balun.I noticed no heat in the balun.Maybe a little at 28.5 mhz. Not real scientific I know but should give me some idea of loss.
    Now the questions
    1. Is this a good balun?

    2. Is the test method good,constructive criticism is welcome

    3. Is there other tests I can do with simple equipment,specifically choking impedance effectiveness tests.I intend to make a common mode current meter as on Toms site.

    4. what is L15 material,does this mean 15 uh per turn.or is there no real standard for ferrite material ID.I cant find much on L15 on the net

    5. would the highest freq of use be 14.2mhz
    Thanks for any help.
    Brendan
     
    Last edited: Feb 13, 2011
  2. KH6AQ

    KH6AQ Ham Member QRZ Page

    You have not measured the balun common-mode impedance. This is what it's all about.

    Connect the balun input leads together. Now connect the balun output leads together. Measure the impedance between the two sets of leads.
     
  3. K1BQT

    K1BQT Ham Member QRZ Page

    Off the top of my head, I wouldn't call that "good". I'm not familiar with L15 material, but it must have pretty high permeability to introduce that much mismatch above 14 MHz with only 8 turns. The common-mode suppression wasn't actually measured, so it's pretty hard to know how effective it really might be.

    I know I'll get criticized for suggesting this test, but you might try it. If you have a calibrated signal generator, connect it to your receiver through a patch cable and set the generator output for exactly S9. Next, short both ends of your balun (to put the windings in common mode) and insert it in series with the center conductor of the patch cable (keep the ground side of the test cable in tact). See how many dB you have to increase the output of the generator to bring the signal back up to S9 (your reference level). Do this for each band, and you'll have some idea how effective the balun is (at least over a 50-Ohm path). It's a poor excuse for a S21 scattering parameter test, but it will give you some basis for evaluation. In my book, 30 dB of suppression is "very good".
     
  4. K1BQT

    K1BQT Ham Member QRZ Page

    Also, if you want to check for balance using your bridge, instead of using a single 50-Ohm resistor for a load, use two 25 Ohm resistors in series (22 is probably close enough) and make a center tap between them. Take your reading and then run a short wire from the shield side of the balun input to the center tap of the load attached to the output. If the SWR disruption is minimal to none, your transition from an unbalanced feed to a balanced load is good.
     
  5. VK1OD

    VK1OD Ham Member QRZ Page

    Hi Brendan,

    The L15 is a medium µ material, µi is about 1100. That said, µi of itself is not adequate for designing an RF inductor (as is a common mode choke or Guanella 1:1 balun). My measured Al value for that core is 1460nH for 1t, but again, it is not a very useful single figure for RF applications for that core.

    I have characterised the LO1238 core using a VNA to measure S parameters for a single turn inductor. I have written some programs to model a balun given amonst other things, that file of S parameters. The article A model of a Guanella 1:1 choke balun using a Jaycar LO1238 core describes a balun made with 8 turns of RG174, including the common mode impedance model which was calibrated to the measured effects of distributed capacitance in a prototype.

    In your case, you have constructed a transmission line from a pair of enameled copper wires. Depending on the insulation thickness, and how tightly you managed to keep the wires paired, you probably have a transmission line of about 400mm length and Zo somewhere from 80 to 120Ω. The effect of that line will be to transform the impedance of an ideal 50+j0Ω load somewhat, depending on the Zo of your line section. That transformed impedance will set the VSWR on your 450mm 50Ω fly lead, and it will transform the impedance further. These impedance transformations will be most evident at the high frequency end because the line sections are electrically longer wrt wavelength.

    If we assumed that your load was exactly 50+j0Ω, the impedance looking into 14.4° of 100Ω line (400mm at 28.5MHz) would be about 52+j19 and VSWR(50)=1.45. The coax section would transform that impedance to 69+j11Ω.

    I know these are not exactly the numbers you measured, but it demonstrates what happens, and the actual results depends on the detail of some of your components. You probably also have some significant instrument error in your results.

    Is it a good balun?

    Well, if you wanted a near perfect 1:1 transformation, then it is not very good. The insertion VSWR is a bit high at some frequencies. That is easily fixed, you wind it with coax and insertion VSWR will be much lower.

    If you intended to use it with an ATU, insertion VSWR is not a very important parameter as you can tune out any undesired transformation in the ATU. But, if it was for use with an ATU, you might expect very high differential voltages under some conditions and you would want better insulation than just enameled wire.

    That deals with what you measured, but it is my view that the most important parameter for a general purpose HF balun used with an ATU is its choking impedance. The instrument you have used for impedance measurement will be challenged. The choking impedance should be similar to that in my article for the 8 turn coax implementation, and IMHO, that is quite good for such a balun.

    So, my advice is that if you wish to use this with an ATU, improve the wire insulation. If you need low insertion VSWR or will only use low power (eg your 10W limit), wind it with thin coax.

    Owen
     
  6. K1BQT

    K1BQT Ham Member QRZ Page

    1-mm wire IS pretty small. You'll likely come a lot closer to 50 ohms using #16 enameled wire (or 1.5mm?). If you're only running 100W, RG-174 is also a good choice -- as Owen suggests.
     
  7. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    thanks

    Ok .
    Now I know what to do to measure common mode impedance,thanks Dave WX7G
    Unfortunately my bridge goes to 500ohm R. Looking at Owens article that is the R at low hf.I may be able to measure at 1.8mhz.

    I did read your article Owen, a long time ago. I re-read it today ,and Im glad somebody with the tools and knowledge has measured the toroid as a balun. Good work. As you know ,hf ferrite is a little difficult to get here. It appears Jaycar no longer lists these particular toroids for sale!? I believe I saw some still available at my local store.
    One of those AIM 4170 analyzers would be good for these measurements?

    K1BQT.thanks for the Ideas ,I have a signal generator here with a variable att.Maybe a stepped att would be good? I have plans to build one of these soon.

    I have 4 cores of this type here.
    The intention was to replace the burnt 4:1 dual core balun in my little Emtron ATU ,with an outboard 1:1 balun. By the way I purchased the ATU like this,knowing the balun was Burnt.

    From what I understand now ,the wires for a Balun in an ATU should be well insulated as high voltages will be encountered,.
    I have some 2 mm building wire here that should work ok?
    As Owen point out swr excursion` s dont matter in an atu balun so the two wire method would be preferable for 400w cw high impedance balun loads.
    My current power limit is 10w,however when I upgrade the ATU balun will need to handle 400w CW.

    I also Intend to Build a Balun for a dipole to coax connection ,Rg174u is ok to 100 w with a low swr?
    It looks like a balun for a dipole at 400w would be best made with rg58 and low swr, however rg58 can only be looped to a relatively large diameter circle,meaning a larger core would be necessary?

    Thanks everybody for the replies
    Brendan
     
  8. VK1OD

    VK1OD Ham Member QRZ Page

    Brendan,

    It is quite difficult to make a high voltage twisted pair with Zo below 100Ω, so don't try... use coax if you need low insertion VSWR.

    Plain enameled wire will not give very high breakdown rating. Thermaleze is better, but very expensive here in Oz. Some people sleeve wires (at least one) in PTFE sleeving which is usually readily obtainable on Ebay at low cost... but it spaces the wires and drives Zo up. The option I have used in my current project is PTFE insulated stranded wire, again usually available on Ebay at low cost. The PTFE insulation is tough, high voltage rated, and withstands high temperatures without deforming.

    I was not aware that the LO1238 was no longer available. I must admit that it has been a while since I received email from constructors... so that perhaps explains it! I have tested it at 100W continuous on a matched asymmetric load and it works fine.

    The core used in HF current balun project is obtainable from Neosid, I paid about $30 for two including postage IIRC. It is suitable for higher power rating, and the prototype you see in the pic has been tested at 600W pulsed tone at 10% duty cycle on an asymmetric 50Ω load for a half hour with no untoward signs.

    There is no standardised method for testing these baluns, and bench tests are of very limited value. The dissipation in this type of balun is mainly due to common mode current, and that depends so much on the particular antenna system in which it is deployed.

    Owen
     
  9. VK2TIL

    VK2TIL Ham Member QRZ Page

    You have done some very good experiments with the test equipment that you have.

    Lou's noise bridge is a very nice instrument; it is, of course, not a mega-$$$ VNA but it will provide you with some results to ponder-over.

    W7ZOI tells us that any measurement is better than no measurement at all; it's important, though, to understand the limitations of the instrumentation.

    Even the best noise bridge will get a little "wooly" above about 20MHz; results at 30MHz are a little "approximate" but still useful for antennas where a bit of "slop" doesn't usually matter much.

    But I repeat; Lou's bridge is as good as they get.

    Some comments to supplement the good advice you have been given;

    The impedance of the "transmission line" used in the balun is very important; in a balun that sits between two 50-ohm impedances the line impedance (Zo) should be 50 ohms.

    That's why 'OD suggests using 50-ohm co-ax.

    It's not really possible to make 50-ohm line with large wires.

    Parallel lines made with the kind of wire you are using tend to have Zo around 100 ohms, give-or-take a bit; that Zo works nicely for 50:200 (4:1) transformations but is not good for 50:50 (1:1) transformations.

    The optimum line Zo is the geometric mean of the two impedances; the GM of 50:50 is just 50 whilst the GM of 50:200 is 100.

    That, incidentally, explains why there is no such thing as a "4:1 balun/transformer"; it's necessary to specify the two impedances as a balun/transformer for 50:200 should be designed differently to a 50:12.5 device.

    Zo 50 lines can be made from enamelled wire; about 0.3/0.4 mm ecw twisted at about 15 tpi makes a line of about Zo 50.

    This is in the small signal/instrument category; it's not what you are doing so the message is; use small co-ax.

    If you do use co-ax you will make a "W2DU" balun as distinct from a "Guanella" balun; Gustav Guanella used twin-line.

    I think the principle is a little different when co-ax is used and is more akin to W2DU's principle.

    But that's just something for you to think-about; make your balun with co-ax and it will work very well.
     
  10. VK2FXXX

    VK2FXXX Ham Member QRZ Page

    burnt balun

    Owen
    Just looking at the 4:1 2 core voltage balun from the atu,which is burnt ,it is made from enamel coated wire . The coating has been melted away in a small section on one of the cores.Great design!
    As I intend to make a balun for this atu I will definitely use thick insulation on the wires.
    The two wire impedance will probably be around 120ohms but this shouldnt be a problem in an atu balun.
    A balun in a low swr situation I would build with rg 174 or rg58 ,depending on power output,or possibly an w2du type.
    Thanks again
    Brendan
     
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