To: <towertalk@contesting.com>
>Date: Thu, 07 May 1998 15:24:38 +0000
>From: Malcolm Clark <rudder1@ibm.net>
>
>Jiri Sanda wrote:
>
>[snip]
>
>> Another example is your car radio antenna. It is a round-ish
>> shape on the top to reduce the static buildup that would
>> otherwise make the radio unlistenable. If the top of the
>> antenna was spikey, wind generated static at 60 MPH would be
>> horrendous.
>>
>> Cheers, Steve K7LXC
>>
>>
>Not so sure about this one, since the ball on the car antennas
>is mainly for safety reasons. If you remove the ball - I did it
>once - the difference in noise while driving is negligible. Also
>you have a lot of various rubber car antennas with no ball
>functioning perfectly.
>
>[snip]
>
Actually, the ball is not there to "reduce static buildup". The
reason for it is also not related to "wind generated static".
the ball is there for two reasons. The first is eye safety. It
is much nicer to get poked in the eye with a 7 mm diameter sphere
than with 0.01 mm diameter cone tip. The second is to prevent
the formation of corona discharge (otherwise known as St. Elmo's
fire to sailors) from a field concentration point (sharp tip)
when the vehicle drives through a region that has a large
electric field gradient.
The rubber covered car antennas suppress the corona by preventing
the ionizable gas (air) from coming into physical contact with
the sharp end of a conductor. The ball suppresses the corona by
preventing the field from being concentrated by a sharp
conductive point.
The external overall field gradient is whatever the field
gradient is (typically several thousand volts per meter). But at
the end of a sharp point, the surface area is reduced so that the
field strength can be _effectively_ many times higher than that.
When the concentration raises the _local_ _effective_ field to a
level high enough to ionize air, a corona discharge forms.
Corona discharges are very noisy things.
>Just happened to be putting a new antenna on the car when this
>was posted, and happened to have a fine stainless steel wire
>brush lying around. I thought it might be interesting to try
>the new antenna with the brush (slightly larger than a
>toothbrush) installed in the manner of a static dissipater and
>report the results to the group.
>
>For testing, the radio was first set to AM between stations so
>that the only thing coming from the speaker was noise. The
>brush was taped on so that the antenna ball was jammed tightly
>into the bristles which were facing toward the rear. The car
>was then driven down a rural highway at 65 mph. I couldn't go
>any faster with the brush taped on because of the occasional
>wild gyrations, which I was afraid would damage my new antenna.
>
>I was unable to detect any difference in the noise level between
>several runs with the brush and without the brush. Just to be
>sure, I tried it a few more times with and without brush while
>listening to strong and weak AM and FM stations as well. There
>was still no apparent difference except for one anomalous run
>without the brush. In that case, listening to a weak AM
>station, noise would increase then drop sharply off, only to
>rise again and repeat the cycle. I was unable to duplicate the
>results on succeeding runs.
>
>I have to conclude the car antenna balls have no effect on
>static--at least here in hot and humid Florida. It would be
>interesting if someone could try the same experiment in a cold
>and dry location. Just be prepared for some strange looks if
>you too have an audience.
>
>73
>Malcolm KR4HP
Here is an experiment for you. Do it in the evening or at night
if possible. This will work fine even in the humid Florida air
(I was first licensed in Ft. Walton Beach).
1. Attach a small single conductor wire to the end of your car
antenna wire. Make sure that the end of this attachment
sticks up above the ball. Also make sure that this wire end
comes to a relatively sharp point. The added wire must make
electrical contact with your car antenna conductor.
2. Pull the feedline for your radio antenna out of the radio.
3. Clip a small neon light (NE-2, etc) from center conductor to
shield. Arange to position the neon light so that you can
observe it while you are driving. The neon light is both an
indicator and a safety device.
4. While driving around during relatively stormy weather, notice
that the neon light lights up occasionally. When this
happens during darkness, stop the car, get out and look at
the antenna extension wire end. Notice the strange blue glow
coming off the wire end. Don't get far from the car and
don't stay out long.
Normally, you won't be able to experience a strong field
gradient unless you are in a relatively open area (nothing
taller than your vehicle for a radius of several hundred
feet). You will see nothing when driving under or
immediately adjacent to power lines.
5. If you want to hear the accompanying noise, hold a portable
AM radio near the antenna. You could wind a couple turns of
the neon light interconnecting wire around the portable radio
for this. The noise isn't coming from the light. You can
short the lamp out and the noise will remain.
6. If you want to remove the wire end to see if the ball
suppresses the effect (It does stop the corona. But it
doesn't change the level or distribution of charge build
up.), ground the antenna wire to the vehicle first. Also, be
careful about becoming a path from the vehicle itself to
ground bypassing the insulating effect of the tires. These
potentials can be surprisingly high and the vehicle body
represents several hundred picofarads of storage capacity.
Thats enough to give you a pretty good pop.
73, Eric N7CL
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