TT:
Below is the QST article that Al NW2M was kind enough to permit me to
forward to the denizens of this reflector.
73 de
Gene Smar AD3F
Subject: Fwd: QST Submission - Wind Noises Generated by Mobile Antennas, NW2M
Hi Gene. Here is my submission for QST for you to share. NP. 73, Al, NW2M
PS: It can even be seen from space!!! 73- Al
<https://eoimages.gsfc.nasa.gov/images/imagerecords/0/987/misr_guadalupe.jpg>
Title:
Wind Noises Generated by Mobile Antennas, by: Al - NW2M
A known Fluid Dynamics problem: Trailing airflow vortices causing mobile
antennas to "sing".
Abstract:
Many hams experience the "whistling antenna" effects after installing a new
antenna on their car. At highway speeds, this can be quite annoying. The
problem is not related to the antenna's length, sharp points, nor the acoustic
resonance of the antenna itself, but rather, the trailing airflow wave pattern
(vortex) which then uses the antenna as an acoustic pathway to the roof of the
vehicle and into the passenger compartment. There is a solution.
Text:
The electrical performance of a thru-roof mounted antenna is absolutely
fantastic. On the 2M, 440, and 6M amateur bands, there is no better ground
plane available. Yes, I did the unthinkable deed of drilling a hole in the
metal roof of my new car for a NMO mount to achieve this ultimate performance.
I then installed a traditional 19" tall quarter-wave antenna for 2 meters.
This antenna has very low wind resistance and exhibits a broad SWR curve across
the 2M band. Life is good- until 70 MPH....
At highway speeds, the wind produced a strong 1,000 Hz tone heard inside the
vehicle. At lower speeds, the tone would randomly toggle between 1,000 Hz and
2,000 Hz. This was very annoying to say the least. The entire roof became a
sound box for this cacophony of musical tones. Hearing this harmonic
relationship, my first response was to change the mechanical resonance of the
antenna at the 1/3rd mark by attaching a split-shot lead fishing weight. I
wanted to add mechanical mass at one of the nodal maximums. No such luck.
Clearly something else was going on here.
Enter Karman1: In fluid dynamics, a "Kármán Vortex Street" is a repeating
pattern of swirling vortices2 caused by a process known as vortex shedding
across a blunt object, and is responsible for such phenomena as the "singing"
of suspended telephone or power lines, guy wires, and the vibration of a car
antenna at certain speeds.
Based upon my findings, I needed a quick trip to the hobby shop for a 19" piece
of 1/16" square plastic tubing. I intended to perturb the airflow across the
antenna (blunt object) by using sharp edges (square tubing). I simply slid the
small square tubing (from the bottom up) over the entire length of the antenna
while keeping the top static ball as a physical stop. No change was observed
to the SWR. The result- It worked perfectly! I no longer hear the acoustic
effects of trailing vortices carried inside the car. Ahhh, silence at highway
speeds- and beyond!
Conclusion:
This is a widely known problem with many articles and solutions discussed. So
the next time you see spiral windings around a smoke stack or the spirals
around your AM/FM antenna on your car, you will know that the engineers are
purposefully addressing the negative effects of this fluid dynamics phenomenon.
References:
1 https://en.wikipedia.org/wiki/Karman_vortex_street
2
https://upload.wikimedia.org/wikipedia/commons/b/b4/Vortex-street-animation.gif
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