Standing Waves in Car Antenna... 1

[potteryshard]

Warning: Completely useless question...

Driving home last night at highway speeds in drizzle, I noticed that the factory antenna was vibrating in a standing wave pattern... I've had a couple of other vehicles whose antennas did the same thing: oscillate in light rain or heavy fog, otherwise stable at all speeds.

What phnomenon might be triggering this switch in behavior?

Because the antenna oscillates vigorously at several hundred Hertz it seems likely that it would fling off any bulk water making a mass per unit length frequency change an improbable culprit.

Vortex shedding also seems to be an unlikely cause; the difference in vortex activity between the dry and saturated air states wouldn't seem to be large enough to account for the increased activity.

Most likely, it would seem to be some sort of combination phnomenon; maybe the fluid film on the antenna surface actually is periodically shedding mass which in turn triggers a vortex shedding action.

Just curious...

 

[MacGyverS2000]

And here I thought this was going to be a sparky question... turns out it's actually a, aero/physics question.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[BrianE22]

Possibly it is a change in damping. Maybe the wet surface somehow drops the damping - opposite to what I'd expect though.

 

[BrianE22]

...or, the wet air produces a larger force on the antenna as it slices through the air. Or possibly hitting the droplets of water causes "impact" loading that would tend to excite the natural frequencies.

 

[DRWeig]

Geez, Potteryshard, my brain was on the edge of the frying pan and you nudged it right in there. Sizzle!

I observed the same phenomenon a few years back, could not come up with any explanation that stood up to scrutiny.

Good on ya,

Goober Dave

 

[msquared48]

The answer is obviously that you were tuned in to a rock station and not your regular classical. You have a very sensitive antenna.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[swall]

I had this happen to me one time and it started to annoy/bother me to the point where I was thinking about diverting from my route to find a hardware store to by some lead fishing weights for dampening. I finally settled on 3 or 4 pennies held on with duct tape.

 

[potteryshard]

After posting, a semi-plausible thought occurred...

Although the oscillating motion of the antenna would seem to throw off any bulk water, there will of course be a water film on the surface. (The film will be quasi-static, an average consisting of surface wetting added to by incoming droplets and minus those droplets breaking away at the trailing edge...) The airflow past the antenna will distort that film somewhat; it would seem to thin on the high pressure leading side and thicken on the trailing side of the antenna structure.

This in turn would seem to suggest that the envelope of the water film in conjunction with the circular cross section of the whip would create a crude airfoil shape. Not much of an airfoil, but then again, not much lift is needed to create harmonic motion if that force is timed to the motion.

I would tend to think that the mass of the thickened trailing film would inertially lag the acceleration of the antenna because the center of mass of the film is aft of the center of pressure created by the restoring force of the antenna elasticity. (Think of the antenna structure pushing against the inside of the water film envelope.)

This suggests that in effect the combined trailing edge thickening and the lagging mass of the fluid rotates the angle of attack of the airfoil shape so that whatever lift it creates is in phase with and additive to the acceleration; in short, just what is needed to augment harmonic motion.

Please note that it was necessary to develop this thought only because the radio in all cases has been off, thus invalidating the rock theory.

 

[DRWeig]

Nicely thought out, Potteryshard. It sure seems plausible to me.

Now turn your radio back on! Music soothes the pains of the day.

Good on ya,

Goober Dave

 

[swall]

My 2008 Chevy truck has what appears to be a spiral wrap of wire along the length of the antenna mast.The wire appears to be bonded to the mast via a black coating (unless it is also brazed, soldered or adhesive bonded). Perhaps the spiral serves to break up the lift postulated by potteryshard. I don't think I have noticed any standing waves in the two years I have had the truck.

 

[msquared48]

The antenna of our old Plymouth Fury III had a standing wave above 50 mph to around 65 or so. Never noted that it was weather related.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[rmw]

Any chance it was right near freezing when you observed this? While traveling out through the badlands of Oregon one misty day where it was right at or just above the freezing point, but not cold enough to ice up the roads or vehicle, ice would build up on the lee side of the antenna and that would set up a vortex shedding reaction that would make the antenna whip until I thought it would snap off.

When it would get to a point that I couldn't stand it, I would stop and get something wooden and whack the antenna and break off the ice and go back to driving with no whipping motion detectable.

As the ice would build up again, I would have to repeat the process. It took me several stops to make my way across eastern Oregon. I didn't want to take a chance of breaking the antenna off with as many miles in front of me as I had (heading to the deep south.)

I worked with vortex shedding metering equipment for air and flue gas ducts, so I had no problem understanding what was going on. What was interesting was the degree to which the shape of the teardrop profile due to the build up on the back side of the antenna affected the amount of motion.

That same antenna made hundreds of thousands of miles dry and simply wet without any tendency to whip. Add some ice, however.....

rmw

rmw