Whip Antenna - whipping my shell!

[Widget]

Hi - I've got a curious problem. The radio whip antenna on the front of my '72 safari gets some good gyrations going in certain winds and is smacking the heck out of the shell. Any idea of how to stiffen up the spring at the base? or do they make replacements.?

[AWCHIEF]

Tie it down when under way. That is what they used to do.

[CRH]

You can get replacement springs. Look at a truck stop.....Put a tennis ball on it like we did on 102" CB whips back in the day.

[Widget]

Hmmm. Tennis ball might work. I'll give that a try next weekend. Right now I have a piece of pipe insulation over it, but that looks pretty bad. Not sure if it could be tied down. This is the sliding extension antenna that's on the front drivers side. Whip antenna was probably a bad description because of the way it's whipping the shell.

I'll have to hit up a truck stop and see what I can find.

[AWCHIEF]

Yep, two different animals. Tying an extension ant. down would more than likely bend it.

[DGJackson]

What you are experiencing is vibration due to vortex shedding from a cylinder. This is part to the processes that resulted in failure of the Tacoma Narrows Bridge (Google that for a cool video).

Anyway - the antenna on my old jeep would vibrate and I learned what speeds would result in first and second overtones to get the various vibrational modes (harmonics). This was real handy when the speedometer broke because I knew second overtone was just below 65 MPH.

Well - in 1998 I bought a new jeep and Chrysler had wrapped a small wire around the length of the antenna - think of the stripe on a candy cane. This was all covered in black plastic - like heat shrink tubing. This antenna did not vibrate the way the metal only one on my old jeep did. While I'm sure there will be those that will want to debate different diameter, modulus of elasticity, and other various engineering parameters. I contend the spiral wire changed the flow direction in the boundary layer - effecting the vortex shedding along the length of the antenna. For all I know the antenna assembly may have been a plastic cylinder core and the spiral wire was the actual antenna. Regardless the spiral along the cylinder length stopped the vibration due to changes in airflow around the antenna.

My solution would be to spiral wrap a piece of wire (say #18 or #16) around the antenna and use heat shrink tubing to secure in place. I would have the wraps about an inch apart. Look at the antenna on several automobiles and you will get the idea. I'm not sure if this wire should be connected to the metal of the antenna or not - that's an electrical engineering question. If you used uninsulated wire - say solid grounding wire I don't think it would make a difference.

While I have a couple of degrees in Mechanical Engineering - I never get to drive the train. Let me know if this solution works - I'm real curious. The antenna on my old jeep was fascinating to watch.

[markdoane]

Originally Posted by DGJackson

Anyway - the antenna on my old jeep would vibrate and I learned what speeds would result in first and second overtones to get the various vibrational modes (harmonics). This was real handy when the speedometer broke because I knew second overtone was just below 65 MPH.

Interestingly, this only happens in one location, on von Karman vortex street.

[Widget]

I know exactly the spiral wrapped version you are speaking of. I'll have to think about how best to try this.

[slowmover]

Originally Posted by Widget

Hi - I've got a curious problem. The radio whip antenna on the front of my '72 safari gets some good gyrations going in certain winds and is smacking the heck out of the shell. Any idea of how to stiffen up the spring at the base? or do they make replacements.?

A set length antenna for AM/FM? On a single mount without a second to hold rigid? Literally on the front, and not at the front side of the cap?

I remember trailers equipped with 108" whips (and had one on a car; ex-police), and each had a clip to hold the tip at the vehicle.