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antenna tuning ringsPictured here is the CB radio antenna that I installed on my car recently. I haven’t done much to tune it yet, but it’s working well.

The antenna has these two metal rings that the instructions said could be used to tune the antenna by moving them up or down. I believe the only reason there are two of them is so that they can “jam” each other in place and not move. They are not electrically connected to the antenna or my car in any way, they are completely isolated with plastic

My question is, how does moving these rings change the tuning of the antenna? Can someone share an intuitive understanding of how this works? I’m not necessarily interested in tuning this antenna further, just curious of the theory behind this method. The packaging called this something like a “ring tunable antenna” but I’m not finding much info from Google.

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    \$\begingroup\$ A cross-section and materials analysis of the pieces would be helpful. That's... probably not too practical though. \$\endgroup\$
    – Tim Williams
    Commented Sep 25 at 18:10
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    \$\begingroup\$ Does this help? en.wikipedia.org/wiki/Choke_ring_antenna \$\endgroup\$
    – Amit M
    Commented Sep 25 at 18:24
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    \$\begingroup\$ @AmitM: A choke ring on a microwave antenna is a very different thing from what the OP has. \$\endgroup\$
    – Dave Tweed
    Commented Sep 25 at 18:58

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A short (in terms of wavelength) antenna like that1 has a loading coil at its base, inside the threaded plastic part.

Varying the position of one or more shorting rings near the coil — a closed metal loop forms a "shorted turn" — will change the effective inductance of that coil, and vary the matching of the antenna to the transceiver.

1A full-size quarter wave whip for CB is 108" (9 feet) long.

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