elfa wrote:
In article , --exray-- says...

elfa wrote:


Actually....I have no idea how long the missing segment (or segments) are but
I'm thinking that with 24 inches still there, that's probably enough for FM.

Amazingly, I have yet to find a website that explains FM whip antennas and the
best way to use them.

A quarter-wave length on the FM band is about 30", +/- depending on the
actual frequency. I'd aim for that.

-Bill



Thanks Bill....that's the first piece of information I've ever heard on FM whip
antennas.

elfa

Then maybe if I expound others will chime in with their info.
Generally you can consider a FM whip as a quarter-wave vertical or
'ground plane' type antenna. You can go out and measure the one on your
car to get a good idea of the consistency of this length.
A quarter-wave vertical gives a low input impedance, theoretically
around 30 ohms but in practice a bit higher.
A key part of the quarter-wave vertical system is the ground plane. In
a portable radio its not so easy to establish a good ground plane but
thats not a big issue. Portable sets are typically most interested in
local signals and you can adjust the whip around if the radiation angle
or length ain't book spec.
If you were doing the same for an outdoor fixed antenna you'd want to
focus on the ground plane to obtain uniform low angle omni-directional
coverage.
FM broadcast stations these days tend to use circular polarization more
often than not. That tends to minimize the differences between a
horizontal receive antenna (like a dipole) and a vertical antenna (like
a whip). Some old timers still transmit with one or the other, or a
combination of the two, but circular tends to provide better coverage
given that FM rcv antennas may range from a Walkman headphone cord to a
new Lexus driving up a steep hill.
I'm still a bit miffed about the TO instructions for 18"...maybe their
input is high impedance and the 18" configuration would certainly send
the impedance to the high end of the scale???.
The classic formula for a quarter-wave vertical/whip is 264 divided by
Megahertz, the result is in feet. 98 MHz would give 2.69 feet or 32.3
inches. The diameter etc have a bit of effect on the formula but its
close enough for hacking around with.
So there. Now you have some ammunition to play with.

-Bill