Coax Loop Balun for 6M Moxon

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Re: Coax Loop Balun

Postby VK4WDM » Tue Dec 23, 2014 8:25 am

I must confess I am getting very confused with this discussion. It happens quite easily at my age :shock:

I always use an air-cored (if air can be cored!) choke immediately below the feed point on all my antenna to stop common mode currents on the feed line. My rule of thumb is 10 turns 90mm dia for 80-10m, 5 turns at 50mm dia for six meters. I make no attempt to make the choke resonant. If I suspect that common-mode currents are still causing problems (unstable SWR etc) I will put a couple of ferrite beads on the lower end of the feed line as well. For 2m and above I just clip on a ferrite bead on the line at the feed point. I very rarely have any matching problems and the antennas appear to work as designed.

Am I being too simplistic? What would the real gain (pun intended :D ) to use a more complicated approach?

73

Wayne VK4WDM
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Re: Coax Loop Balun

Postby GM3SEK » Wed Dec 24, 2014 5:30 am

VK4WDM wrote:I must confess I am getting very confused with this discussion. It happens quite easily at my age :shock:

I always use an air-cored (if air can be cored!) choke immediately below the feed point on all my antenna to stop common mode currents on the feed line. My rule of thumb is 10 turns 90mm dia for 80-10m, 5 turns at 50mm dia for six meters. I make no attempt to make the choke resonant. If I suspect that common-mode currents are still causing problems (unstable SWR etc) I will put a couple of ferrite beads on the lower end of the feed line as well. For 2m and above I just clip on a ferrite bead on the line at the feed point. I very rarely have any matching problems and the antennas appear to work as designed.

Am I being too simplistic? What would the real gain (pun intended :D ) to use a more complicated approach?

Just knowing what common-mode currents are puts you well ahead in the game. And if the chokes you are using solve the problems that you have, then you must be getting things right :wink:

And if one day you need more, you also know there are some heavier-duty solutions around.

All sounds good to me.

73 frok Ian GM3SEK
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Re: Coax Loop Balun

Postby VK4WDM » Wed Dec 24, 2014 7:04 am

Hi Ian

I must confess that for my Butternut HF vertical I do use a heavier-duty commercial choke, so it is "horses for courses" as you say. I think it very important to try something simple first and then move to the more complex rather than dive into a complex situation first up and get out of one's depth - especially for the newer hams.

73, and have a great Christmas and New Year!

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Re: Coax Loop Balun

Postby VK6ZFG » Wed Dec 24, 2014 10:49 am

Hi Wayne

You are addressing the important part - using an balun.

The discussion has been on how good is one balun design vs another. In other words, the quest for the ultimate in balun design.

What has not been touched on in all has been what is an acceptable level of isolation. Here I would imagine having at least 15dB of isolation should suffice in most instances. However I stand to be corrected on this as this statement as it is not based on any research on my part.
73s
Igor
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Re: Coax Loop Balun

Postby GM3SEK » Mon Dec 29, 2014 1:05 am

VK6ZFG wrote:The discussion has been on how good is one balun design vs another. In other words, the quest for the ultimate in balun design.

What has not been touched on in all has been what is an acceptable level of isolation. Here I would imagine having at least 15dB of isolation should suffice in most instances. However I stand to be corrected on this as this statement as it is not based on any research on my part.


It's a good question but there isn't any universal answer, because problems with common-mode current can vary so much. How good a choke is needed will depend on how much of a problem there is... which is extremely difficult to quantify.

What Igor measures as "isolation" is specific to his particular type of test equipment, which measures the isolation or loss when the choke is inserted between two 50-ohm test ports. However, it's always better to translate those results into an equivalent value of impedance, because impedance tells you more about how the choke will actually perform.

Experience tells us that the old "500-ohm" wet finger estimate is often pretty useless. 500 ohms isn't enough impedance to suppress common-mode currents reliably in every application, and may also lead to the ferrite cores overheating with high power. That is why most writers today are suggesting at least 5000 ohms and preferably higher, to make sure that the choke will be able to tackle even the worst common-mode problems. But chokes like that tend to use large ferrite cores, and also special types of ferrite which can be expensive. We don't want to overspend on big ferrite cores for problems that don't actually need it.

So what do we do? Do we start with cheaper but less effective chokes and work our way up? Or do we always go for the 'big ferrite' to nail the problem first time, even though we might be overspending? It probably depends. If it's entirely my own problem then I might experiment a bit; but if it involves other people, then I usually go straight for the 'big hammer'.

That's why I haven't answered your question, Igor. :shifty:


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