Classic Amateur Radio (G3TXQ) – Coax Common Mode Chokes for 2 M

From Steve G3TXQ’s posts at ===> http://forums.qrz.com/index.php?threads/making-a-balun-for-a-2-meter-j-pole.241824/

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“I measured 3 turns of RG58, solenoid wound with a 1.5” internal diameter, to be self resonant in the 2m band.  Anything radically different in number of turns or diameter had mediocre choking impedance.

73,
Steve G3TXQ”
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“(For LMR-400)  – if you make the (choke) diameter much larger then 1.5″ – 2″ you end up with fractions of a turn having a massive effect on the SRF.  I just wound 2 turns of RG213 on a 3″ diameter and the SRF was as low as 100MHz.

[Edit: also just measured 5 turns RG213 on a 4″ diameter – SRF=27MHz and negligible common-mode impedance at 145MHz]

If you have to use the larger coax, ferrite beads are probably the better option.

73,
Steve G3TXQ”
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“Slip 4 ferrite beads, type FB-31-1020, over the LMR400 immediately below the antenna feed-point. Beads available here – https://www.amidoncorp.com/items/43  – list price $2 each:

https://www.amidoncorp.com/items/43

Use heat-shrink tubing or plastic cable ties to keep them in place.

Steve G3TXQ ”
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– (thanks Steve for allowing me to re-post)

 

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