NMOQ whistle

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NMOQ whistle

Postby arlojanis » Sun Jan 29, 2012 12:14 pm

A customer tells me his new Larsen NMOQ (no spring) mounted in center of roof on new 2012 Chevy heavy duty pick up truck whistles when driving. Antenna is cut for 159 mHz. Is there a cure or a different antenna that will not whistle?
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Re: NMOQ whistle

Postby Andy Corbin » Sun Jan 29, 2012 12:54 pm

arlojanis wrote:A customer tells me his new Larsen NMOQ (no spring) mounted in center of roof on new 2012 Chevy heavy duty pick up truck whistles when driving. Antenna is cut for 159 mHz. Is there a cure or a different antenna that will not whistle?


Is this a simple straight 18" whip or 3db gain? Does it have some sort of loading coil? If by chance it has an open coil you can try putting a small piece of heat shrink over the coil. If no open coil, try a different antenna. 1/4 wave whips for VHF are very inexpensive. Does the whistle go away if you remove the antenna? If not, obviously the whistle is elsewhere. Just some thoughts.

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Re: NMOQ whistle

Postby arlojanis » Sun Jan 29, 2012 1:02 pm

Simple 1/4 wave whip on small plastic base. Remove antenna and whistle is gone. Why whistle on this truck and not on hundreds of others?

Will what do you think?
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Re: NMOQ whistle

Postby Bill_G » Sun Jan 29, 2012 3:16 pm

It may have been mounted in a resonant part of the roof that amplifies the effect once the vehicle gets to speed. There's a car wash I won't go through with my current truck because the blowing air at the end sets off a bass note in the cab that blows out eardrums. Bend his antenna back to see if it changes the whistle, and try a whip without the corona ball on the end.
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Re: NMOQ whistle

Postby Jim202 » Sun Jan 29, 2012 3:17 pm

arlojanis wrote:Simple 1/4 wave whip on small plastic base. Remove antenna and whistle is gone. Why whistle on this truck and not on hundreds of others?

Will what do you think?



All the 1/4 wave whip antennas I have used don't have a plastic base. They all have just a metal ring to secure the antenna to the NMO base.

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Re: NMOQ whistle

Postby WCHija » Sun Jan 29, 2012 3:23 pm

put a rubber band on the antenna, in the middle. It will change the shape enough to make the whistle stop, without seriously affecting the performance.
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Re: NMOQ whistle

Postby WCHija » Sun Jan 29, 2012 3:26 pm

put a rubber band on the antenna, in the middle. It will change the shape enough to make the whistle stop, without seriously affecting the performance.
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Re: NMOQ whistle

Postby resqguy911 » Sun Jan 29, 2012 4:19 pm

Jim202 wrote:All the 1/4 wave whip antennas I have used don't have a plastic base. They all have just a metal ring to secure the antenna to the NMO base.

Jim


Most Larsen antennas have plastic bases Image Why not try another whip with the metal base like Jim mentioned from Comtelco or PCTel (or /\/\) and see if it goes away?
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Re: NMOQ whistle

Postby mmckenna » Sun Jan 29, 2012 4:57 pm

Same antenna on a crew cab Ford F150 does the exact same thing. It never really bothered me before. At least it lets me know the antenna is still there.

What some auto broadcast radio antennas have is a spiral wrapped up the antenna and covered. Likely you could do the same thing with something non-conductive, maybe like fishing line and heat shrink. Then again, maybe just a piece of heat shrink placed on the antenna at just the right place would break up the air flow, similar to the rubber band, but more permanent.

Let me know what fixes it. While it doesn't bother me now, it'll probably drive me nuts now you've mentioned it.
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Re: NMOQ whistle

Postby Andy Corbin » Sun Jan 29, 2012 5:16 pm

arlojanis wrote:Simple 1/4 wave whip on small plastic base. Remove antenna and whistle is gone. Why whistle on this truck and not on hundreds of others?

Will what do you think?


I tried to reply to this earlier but apparently it didn't "send".
This might be a "perfect storm" of where the antenna is mounted in "just the right place" where it is at resonance with the aerodynamics of the vehicle. A few inches in either direction may make a difference. But then you have an additional hole in the roof. You might try a different 1/4 wave antenna such as one of the "motorola" styles with the "silver dollar" base instead of the higher profile Larsen base. You might try a piece of heat shrink over the the whip or try bending the antenna back a little bit. If that doesn't work, you could try one of the butt ugly "oil can" antennas. Sti-Co makes some highly flexible antennas that may bend more in the wind. Be aware Sti-Co is mightily proud of their antennas and charge accordingly.
Basically, anything to change the antenna's aerodynamic profile when the vehicle is in motion. Also, I am pretty certain there are NMO VHF high band cellular look alike antennas that may have enough difference in form factor to eliminate the problem......or perhaps at least give you a different musical note. :) I have a Larsen 144/440 low profile (about 18") antenna on a permanent mount on the roof of my car. Low profile, black and does not draw attention and good performance with basically unity gain.

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Re: NMOQ whistle

Postby rydzewski » Mon Jan 30, 2012 2:29 pm

I have the Laird Low-profile black VHF "puck" on my truck... works great for me. I used to have a quarterwave on my truck and the noise caused me to go with the low profile's.


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Re: NMOQ whistle

Postby FireCpt809 » Mon Jan 30, 2012 3:14 pm

The "Low profile" have a very narrow bandwith. Every customer I have ever sold one to brought it back and replaced it with a 1/4 wave
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Re: NMOQ whistle

Postby rydzewski » Mon Jan 30, 2012 3:36 pm

I have been told that people were having issues with the low profile type of antenna's. I live in a city where coverage is not a problem so I'm guessing that is why I have not had any issues.
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Re: NMOQ whistle

Postby tuckerm » Tue Jan 31, 2012 2:03 pm

Mine also whistles. Doesn't bother me.
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Re: NMOQ whistle

Postby jhooten » Tue Jan 31, 2012 3:24 pm

Driving along hearing the tune the antenna is playing. Then a loud PING and the whistle is gone. Next stop look at the antenna and it has sprouted 4 wings. Yep, dragon fly impact.
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Re: NMOQ whistle

Postby mmckenna » Tue Jan 31, 2012 3:29 pm

So obvious solution to the problem is to find yourself a dead (or not) dragonfly and tie wrap him to your antenna. Problem solved.
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Re: NMOQ whistle

Postby KE7JFF » Thu Feb 09, 2012 7:51 pm

I had this problem once and I solved it by putting one of those "Jack in the Box" antenna balls on the top of the antenna. I'm not kidding.
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Re: NMOQ whistle

Postby motorola_otaku » Fri Feb 10, 2012 9:03 am

That makes sense. Anything you do to change the weight or size of the antenna is going to affect its resonance.

One thing that will work is switching to the slender-whip design like these.. they're too flexible to pick up a "buzz."
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