4
$\begingroup$

My 20 meter inverted vee dipole is fed with 56 feet of 50-ohm coax. The SWR measures 1.2 at the center of the band. According to http://arrl.org/grounding a balanced antenna doesn't need a ground, so my transceiver is not grounded. I've read that an inverted vee lowers the 73 ohm dipole impedance closer to the 50-ohm coax impedance but connecting a coax unbalances the antenna, so I wanted also to test using a current choke balun. I studied How to determine number of turns for a 1:1 balun?.

Within that post, I found How to detect common-mode currents or “RF in the shack”?. I constructed a snap-on ferrite detector with analog meter. enter image description here

Without any balun, the CM meter clipped on the coax near the output of the transceiver just barely deflected when transmitting. That was encouraging but I wanted to compare it using a choke balun.

I made a choke balun on a ferrite rod 3" x 1/2" (scavenged from a Hy-Gain 1:1 balun) by wrapping a bifilar winding of 12 turns of #16 magnet wire, each winding in series with a dipole element. I inserted this balun at the antenna feed point. The SWR measured about the same as without the balun, but this time the CM meter showed about 10 times more current than without the balun.

Does that mean that the antenna doesn't need a choke balun, or the balun is not designed right, or both? Even if a wrong number of turns (I planned to test it empirically as Phil Frost suggested), I don't understand how it apparently increased the common mode current. Could my testing be bogus because the transceiver is not grounded? (I don't have a ground rod installed yet to connect). I haven't had a chance to transmit to someone but the antenna receives distant signals well so it seems correct in that respect.

Improve this question
$\endgroup$
6
  • $\begingroup$ Perhaps it is resonant. In any case, do yourself a favor and build a foolproof, wideband common-mode choke using coax in lieu of wire (that's what I always do) based on this. $\endgroup$
    – Mike Waters
    Aug 23, 2019 at 19:35
  • 1
    $\begingroup$ @Mike Thanks, I've been through that document a few times before and it is great, but with so much related information I have a hard time finding one particular part that answers my question(s). From my results above, I'm inclined to just skip using a balun but I'm trying to understand what it's doing to my feed line. $\endgroup$
    – Peter Buxton
    Aug 23, 2019 at 20:42
  • $\begingroup$ If nothing else, you changed the length of the feedline by adding the balun you made, which may have moved a high-current point of the standing wave onto your clip-on meter. $\endgroup$
    – hobbs - KC2G
    Aug 23, 2019 at 21:15
  • $\begingroup$ Peter, I had the same problem. But after printing it out and carefully studying what this genius was saying, a light bulb came on. Perhaps we all can help you sort out what is applicable to you. Actually, my balun was made from his previous edition. $\endgroup$
    – Mike Waters
    Aug 23, 2019 at 23:35
  • $\begingroup$ Even easier... go here: audiosystemsgroup.com/2018Cookbook.pdf and find page 7. 12 turns around a 2.4" OD #31 toroid core. $\endgroup$
    – Chris K8NVH
    Aug 24, 2019 at 1:51

2 Answers 2

Reset to default
4
$\begingroup$

I'm answering my own question after seemingly solving the problem, thanks to the helpful comments I received. Evidently, the ferrite rod core that I salvaged from a Hy-Gain balun was the culprit. I couldn't find any information on what kind of ferrite it is but it was manufactured about 30 years ago. I don't know much about baluns yet but earlier questions I had about that same stock Hy-Gain balun prompted me to post Voltage vs. Current Balun for Dipole.

So the solution is, I wound a new balun per info in http://audiosystemsgroup.com/2018Cookbook.pdf. Although the document says to use a 2.4" #31 torroid core, I had a 1.4" #43 core so I wound it with #20 wire just to test if it would work. enter image description here Now there is no increase in common mode current as was measured before, and the SWR is only 0.1 higher than without the balun, so I think this qualifies as solved. Hopefully this post can somehow help others with similar questions.

Improve this answer
$\endgroup$
4
  • 1
    $\begingroup$ Nice. #43 seems similar to #31. An estimate of the choke impedance can come from manufacturer data. The cookbook designs ~5k Ohm chokes. The Fair-Rite catalog lists 2.4" OD #31 (0.5" depth) as Z=40 Ohms @ f=10MHz and 1.4" OD #43 (same depth) as Z=28 Ohms. So your choke impedance should be somewhere around 3800 Ohms, which is quite good. $\endgroup$
    – Chris K8NVH
    Aug 24, 2019 at 17:58
  • $\begingroup$ +1! You should seriously consider marking your own answer as accepted. $\endgroup$
    – Mike Waters
    Aug 24, 2019 at 18:16
  • $\begingroup$ @Chris - I appreciate your calculations. $\endgroup$
    – Peter Buxton
    Aug 24, 2019 at 18:23
  • $\begingroup$ @Mike - Will do, and will study document as you suggested. $\endgroup$
    – Peter Buxton
    Aug 24, 2019 at 18:23
1
$\begingroup$

You might want to try a choke balun on the coax itself, as the outside of the shield of the coax can couple to the antenna even if the currents on your wound wire pair or internal to the coax are balanced.

Improve this answer
$\endgroup$
1
  • $\begingroup$ The simple CM meter that I made (photo added to original question) proved that the CM current increased significantly with the faulty choke. Without a choke there was no meter indication--or even with the new torroid choke. I might find more problems about the antenna when I actually transmit to someone. Your good suggestion will be useful to try then. $\endgroup$
    – Peter Buxton
    Aug 24, 2019 at 18:59

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .