Wednesday, October 28, 2020

Balun musings (2)

 After my doubtful experiment with the PA0FRI 1:4 airwound balun I decided to make the balun shown at DH1TW's site. Just because I had a mix#61 ferrite core still in my "junkbox". And because I wanted to see if I could measure it with the basic test. I also found two 100 Ohm resistors you need to test the thing.

It is my intention to use a balun between my vertical on top of my tower and the CG3000 autotuner at the feedpoint. I don't expect to get a better signal at least that is not my intention. What I would like to see is a better noise/signal ratio and a better and faster/better SWR match on some bands.





This is the basic balun test:



Basically you need to make a test clip from the outer side of the analyser (earth) and measure points A/B/C. The resistors are half the value of Zout. That will say two 25 Ohm resistors for a 1:1 balun or 100 Ohm two resistors for a 1:4 balun.

If you have a current balun, like the PA0FRI balun, the SWR should be low on all 3 points. 

If you have a voltagebalun like I made the SWR should be high(er) on points A/C and low on point B.



A second test for the  voltage balun is to connect the resistors in parallel and touch bot output terminals, SWR should be very low.

Results:

The 1:4 voltage balun closed with 2 resistors of 100 Ohm in series showed a SWR  of 1:1,3 to 1:1,6 just like shown in DH1TW's diagram.









First test with clip to A/B/C:

1,85 MHz A:8,4 - B:1,3 - C:5,7
14.2 MHz A:1,8 - B:1,3 - C:1,8
28,5 MHz A:1,9 - B:1,6 - C:1,8

Second test with clip and parallel R

1,85 MHz 1,6/1,6
14,2 MHz 10,3/7,5
28,5 MHz 10,3/10,3

So what does this tell me? Well, I guess the balun is working! Is it good enough for my use? The second test with the parallel R shows quite a high SWR on higher bands! Only a VNA will show correctly I think. I really think I'll order a nanoVNA in the near future.

Now, those that are experienced building these kind of baluns will tell me that you need a #43 kind of ferrite. And yes they are probabely right. But not between a antenna tuner and a vertical that is. G8JNJ has described some tests he did using a 1:4 ruthroff design balun between autotuner and antenna. According to him such a balun will give unpredictable results due to self resonances. Best is to use a balun on #61 material. Other possebilties are a 1:1 current balun or no balun at all on the output of the tuner. With other words I have to experiment...

Last post about common mode chokes has resulted in many comments. As expected because everyone knows best or can tell something about CM chokes. Though there are not many that actually have (good) results with it in practice. John MW1CFN gave me another diagram/example of a guanella current balun with 2 times a #61 ferrite ring. This is not a choke but a transformer and balancer, it forces equal currents in both sides of the antenna but isn't a simple 1:1 balun doing the same?

Now, I've done a bit of reading about baluns and chokes lately. You can find much about it on the internet these days. A real eye opener was the page of DJ0IP.

I like his view on things as written on the first page of his site:

BALUNS BALUNS BALUNS
- IMO, no other area of technology is so vastly misunderstood as Baluns (and RF Chokes). The web is full of all kinds of misleading and/or wrong information on this topic.  
- I am still learning and sharing what I have learned.  A major part of my learning process is building and testing baluns in real antennas, NOT modeling or theorizing as most other "would be" experts seem to enjoy doing!

Slowly I come to the conclusion that a single choke/balun will not cover the entire HF band like I want to have for my multiband vertical which works from 1-30 MHz! I do not know the impedance of the antenna on any frequency at all and so a 1:4 balun will not be safe for me to use. Best is to use a 1:1 current balun or actually multiple baluns with different specs to cover the entire HF.

5 comments:

  1. Hallo Bas, ik heb ervaring met verschillende CM chokes en ze kunnen prima werken om de CM stroom sterk te reduceren (merk je in de praktijk en kun je meten). De enige beperking daarbij is de bandbreedte - die afhangt van design en materiaal.

    Bij mijn thuisopstelling met endfed is de CM stroom zonder choke zodanig groot dat de buren gaan klagen. De choke lost dit op.

    Succes met knutselen!

    73, Lars

    ReplyDelete
    Replies
    1. Hallo Lars, dit ging dan niet specifiek of CM chokes. Heb ik eerder over geschreven, zie paar postings terug. Heb jij nog een ander methode om metingen te verrichten of een CM choke werkt? De bandbreedte is inderdaad een dingetje. Er zullen 2 chokes gemaakt moeten worden die achter elkaar gezet en dan de HF bestrijken.

      De endfed staat bekend om het stralen van de coax dus daar moet zeker een choke bij gebruikt worden lijkt me.

      73, Bas

      Delete
  2. Nee helaas geen andere methode zonder VNA. Toevallig post-e RF Man vandaag nog een leuke youtube video over unun en balun chokes. Niet wereldschokkend (als in: alles je wel bekend) maar hij legt het wel mooi uit: https://youtu.be/MEbe3nLjMcs

    ReplyDelete
  3. Wise words, for sure! Anything that carries my name in relation to baluns is very much simply to be understood as 'options' and 'what worked for me'. I have no technical understanding of baluns, and it is indeed an entire area of knowledge to itself! But it is interesting to try and learn, all the same! Perhaps the best quote I found online is this one:

    "The name choke refers to the electrical component, whereas the name 1:1 current balun refers to the job it is doing.

    There's more than one way to construct a balun.

    There are purposes for a choke that are not baluns."

    ReplyDelete

Thanks for your comment. Bedankt voor je reactie. 73, Bas