In article ,
Fred Wilson wrote:
I have several mobile antennas that I don't know the frequency of. Some CB, others 2 meter or 440, or dual. How can I find what the frequency is?

Simplest initial approach is to look for a label with a part number,
and then search the Internet. I'll assume that you've already tried
this approach, and found the labels (or Internet information) absent.

Simplest technical approach is to borrow a wide-frequency-range
"antenna analyzer" such as an MFJ-269 or one of its competitors.
Mount the mobile antenna properly (some require a solid ground
connection, others are ground-agnostic, so mounting them on a
connector that's well-grounded to the vehicle body is the safest bet),
connect the analyzer to the feed coax, "sweep" the analyzer through
the frequency ranges in question, and look for frequency ranges where
the SWR drops to (e.g.) 2:1 or less.

You can perform a similar analysis with a grid dip meter (somewhat
rare these days), a high-end scalar or vector network analyzer ($$),
an RF signal generator hooked to a return loss bridge and RF level
meter, or any of numerous other lashups.