On 11/06/2010 5:55 AM, 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

Looks to me like what you're seeing is a helical whip. They were used
most often on 27 Mhz CB radios. They often had a base loading coil at
the lower end. This would look like a solid piece of cylindrical pipe at
the base of the antenna.
The spiral of wire is the "active" element of the antenna. The whip
itself is usually made of fibreglass much akin to that which a fishing
rod is made from and has a support function. The assembly allows a
degree of flexibility without damage to the whip, the coil of wire, or
the mounting point, a feature necessary in mobile applications.
By spiralling the active element, you simulate the desired length of the
active element. In the 11 meter CB band, a quarter wave is about 9 feet.
Therefore, a 9 foot quarter wave aerial would be resonant in the 11
meter band thus having the best transmission and reception ability. A 9
foot aerial isn't practical in most automotive applications as they are
easily damaged or may damage other objects. The compromise then is to
helical wind the element. In most 11 meter applications, the helical
winding isn't sufficient so some extra matching is required. On some
helical whips, this takes the form of the windings being wound more
finely near the tip of the aerial (top loading). In other applications,
the resonance is achieved by a "loading coil" attached to the base of
the whip (bottom loading). It is said that top loading provides the best
results but I always preferred base loading coils for my applications.
I came across the last relic of my CB era the other day when I was
cleaning out my garage. It is an old magnetic base whip. It wasn't one
of the helical wound whips. Instead, it had a base loading coil and the
whip itself was about 5 feet of spring steel. It proved very useful in
the 70s when I was involved in desert rallying. Just attach it to any
vehicle, no drilling or mounting brackets required, and it used the
entire vehicle roof area as a "ground plane". Being magnetic, it was
somewhat less useful on some of the early Toyota Landcruisers with the
fibreglass rooves.

So, in summary, yes, it really does help reception as it provides a
better impedance match between vehicle and radio.

Krypsis