When it comes to controlling common mode currents with a half wavelength antenna, its not rocket science, although it does take a little more work than some other antenna lengths.
I made a few models to show what I am talking about, first the baseline model with no choke.
As you can see, I chose a length of feed line intended to show a worst case scenario, the currents on the feed line are nearly as strong as the currents on the actual antenna.
On the w8ji page linked above, he said:
And this is true, as I will show you here. The blue mark on the following model is an 8000 ohm choke, one of the higher impedance chokes you can make/use these days...
As we can see, it made little difference, so w8ji's statement above is accurate. The question is why did it make such a little difference? The answer is the choke is a current device, putting it in a location that is almost purely voltage, like the area near the feed point of a half wavelength antenna, is an error in and of itself.
My concern with w8ji's statement on said page is that is as far as he went. He never considered other possibilities.
So what if we place a choke at another location on the feed line? Can we get better results? The answer is yes. The key is to put the choke further away from the feed point, in this case I put a 200 ohm choke about 1/4 wavelength down from the feed point. I want to point out that you would need 40 times the impedance of this choke to match the impedance in the choke above.
Putting the choke in this location I have tested this model with other feed line lengths, even mounting the antenna higher, and it made no real difference at all. The lengths that did show some currents below the choke had only enough to tell that they were present. A higher impedance choke, which would actually be used by people, will minimize those currents even further.
Conclusion, when it comes to chokes, location, location, location. A far superior choke in a bad location will be noticeably less effective than a poor but functional choke in a better location.
Hope this helps.
The DB
