On Jun 11, 8:32*am, "D. Peter Maus" wrote:
On 6/11/10 10:10 , bpnjensen wrote:





On Jun 10, 5:00 pm, "D. Peter *wrote:
On 6/10/10 14:55 , Twitchell wrote:

I've seen some car radio whip antennas that look like they have a wire wrapped
around them. *The wraps are not close but lazily spiral down the length of the
antenna.

Does this help reception? *Is the coil attached somehow to the car or just the
antenna itself?

twitch

* *Chrysler started doing this a number of years ago on their fixed
length car antennae. Ford and GM do it, now also.

* *It appears as though there's a wire, under the black polymer
coating, wound around the metal post end to end. If you look very
closely, however, you see that the 'wire' ends just before the small
ball at the top and just before the hex fitting at the bottom. It
appears to be nothing more than a superficial contrivance.

* *In fact, it's there to break up the airflow around the antenna at
speed. Fixed length antenna made of the usual single piece steel rod
tends, under some conditions at speed to vibrate wildly both
loosening in it's fitting, and making unnecessary noises in the cab.
They also have an unfortunate tendency to whistle.

* *Chrysler started with a simple sheath with different aerodynamic
properties, made of plastic, slipped over the rod to eliminate these
properties. They made quite an issue of them in TV and billboard ads
in the 90's.

* *Ford suppliers, on the other hand, made the rod with this same
spiral twist on it's stainless steel antennae. Expensive, but
retaining the brightwork appearance of their traditional fixed
length antennae. And if you can find one, you'll see that the spiral
is actually manufactured into the surface of the rod, with a sharp
cut on one side of the spiral and a gentle slope blending into the
rod on the other. But this was expensive, and Ford, too, have gone
with the black polymer coating with what appears to be a wire embedded..

* *Since these antennae are all provided by an handful of suppliers,
all the car manufacturers using and fender mounted fixed length
antenna have gone with the black polymer coated spring steel rod,
with what appears to be a wire embedded.

* *This is for aerodynamics. Not for any electrical property.

Thanks for this Peter. *I always thought that concept as an RF
resonator was suspect! :-D

* *It's not a bad thought. There are a number of antennae that are
made with a helical coil around a form. RF System's MTA is made this
way.

* *But we're talking many more turns, around a broader form, and a
tighter gathering of turns along the length to be practical as an
antenna.- Hide quoted text -

- Show quoted text -

Indeed, something where the coil would become an actual inductor and
serve to provide additional electrical length where physical length is
restricted. Along with HF broadband antennas, mobile CB and ham
loaded antennas are typical examples of what you are talking about, I
assume - can't see much reason for this with an FM antenna, the lambda
being so short.