"William W. Plummer" wrote:

wrote in message ...


"Max T, CFI" wrote:

I disagree with the method described below for calculating a 5/8
wavelenth antenna.
As I recall, the formula you have below gives you the length in feet of
a half wave length antenna (not the full wavelength as you suggest).
So a 5/8 ths
wave antenna would be twice as long as you calculated. Also, if you
took a 2 meter
5/8's wave antenna, you'd need to lengthen it (not cut it down) to use
it as a 5/8 wave
antenna on 123 MHz.
You probably could take a 5/8 wave 2 meter ham antenna and cut it down
to a 1/4 wavelength
antenna for use on 123 MHz. The length for that would be somewhere
around 22.89 inches.
Max T, CFI (and ham radio operator)

EDR wrote in message
...
In article , CHANGE USERNAME TO
westes wrote:

How tall was that telescoping antenna?

I do not remember how long it extends to.
IIRC, the formula is 468/f(MHz) gives the wavelength. Multiply by 5/8
to get the antenna length in feet. For 123.0 MHz (middle of the band),
the length is 28.5 inches.

Does anyone sell an off the shelf 5/8 wave telescoping antenna with
a strong
magnetic base?

Just purchase a standard ham 2m mag mount antenna and cut a piece of
stainless steel rod that fits the mag mount to the above calculated
length.

I had mine cut for 128.0 MHz to optimize the VHF voice band. The nav
frequencies are less
important. Ham Radio Specialists in San Diego cut it, and had stock to do
5/8 wave length at
that frequency. The whip is much longer than 28.5 inches; somewhere
around 40 inches, or so.

The "5/8 wavelength" is an electrical, equivalent length. This can be
different than a physical length depending on loading coils (inductive) and
"top hat" (capacitive) loads are used. In general you can't cut premade
antennas.

What do I know? I trusted the guy at a ham radio store (which has a great
reputation) and the setup worked great.