Sleve Balun and/or Ferrite Choke on CoCo?

FlightAware ADS-B Flight Tracking
1

Hello antenna hobbyists,

Like probably every tenth ADS-B beginner, I tried out a few self-made antennas and ended up with the 7+1 element CoCo as the best solution. Of all the designs tested by @abcd567, I use the one with the λ/4 * k tip, then short circuit, followed by λ/4 * 1 whip. I used German Technisat 3011, 5.6mm cable with well documented data sheet, k=0,85. Quite good performance: has a raw signal strength at same level of my 1.5mm² made Franklin - while Franklin is amplified by an 10€ inline LNA.

At the bottom end I connected an λ/4 upward sleeve made of heat shrink tubing + aluminum household foil + outer heat shrink tubing to the shield.

Now the questions remains:

  • Do I need a ferrite against common mode current?
  • And if so, where is the ideal placement for it?
  • Does a rule also apply on the cable shield, where standing waves have a “current belly”?
  • And should the ferrite go exactly there? Or better elsewhere?
  • And: Is the feeding point in CoCo equal to the “current belly” or to the “voltage belly”?

I can’t transform the CoCo myself to dipole or GP concept, because in my understanding CoCo with 7 elements is a loop antenna made of 7 dipoles in a series connection with 7 phase shifters. So I couldn’t imagine the feeding point in this world.

Thanks to all experienced HAM and antenna experts.

(My shame, I learned all this stuff years before, but it was the time when i8088 started hunting the Z80… long ago.)

2 Likes
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I’d like to push up my question once more.

Reading Jeroen Steeman’s antenna calculator for CoCo, the schematic is showing some ferrite. But he remains a little unclear as to whether he actually used a ring ferrite, a series of folded ferrites, or what else.

As I read at several HAM pages, the usual cheap folding ferrites are not well enough for VHF or UHF, winding some loops through a ring ferrite would be better. I have no experience how many µH I would need, if an air coil would be sufficient and which end of the coil should be placed exactly at λ/4 (or λ/2?) from feeding point.

Could you, dear reader, share your experience here?