To spring or not to spring a Larson NMO antenna?

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To spring or not to spring a Larson NMO antenna?

Post by Jonathan KC8RYW »

Planning a mobile install.

Since it's a hatchback with a sunroof, I'll be doing two L-bracket mounts near the hood with NMO's.

I'm planning on using Larson 5/8 wave VHF antennas.

Larson has optional springs you can buy.

What are the pros or cons to using springs?

Thanks, guys!
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Re: To spring or not to spring a Larson NMO antenna?

Post by MSS-Dave »

Pro: Might prevent breakage of coil IF you hit something low. If you are putting antennas on L brackets on the hood, if you hit something low enough to worry about the antennas, you will have more of a problem with your roof or your face as it hits whatever is that low.... :lol:

Con: Could cause noise or bad SWR if the braid inside the spring corrodes.

I personally have NEVER used springs on any VHF or UHF antennas. Never broke anything either, probably lucky though. Had 5/8 Larsen VHF and long ASP colinear UHF on top of a full size van for years. Just be aware of your surroundings and you should be golden.
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Re: To spring or not to spring a Larson NMO antenna?

Post by dbartkowiak »

We here in Western NY use low band for fire and public works. We quite often mount antennas with 42 inch whips on top of fire trucks and snow plows. On these high profile vehicles we always use a spring to help absorb the shock of tree limbs and overhead doors. Even with the springs, they still get damaged and sometimes even pulled out of the roof. I do believe the spring helps save the antenna in most cases. It does require periodic inspection to make sure the braid inside is not broken and the spring has not become an inductor.

Having said that, any antenna we install at trunk or hood level does not get a spring. As MSS-Dave points out, if you hit anything that low, your antenna is the least of your worries. It is not worth the price of the spring or the extra maintenance. As I recall the Larsen spring is a straight spring as opposed to a Spectrum spring that is tapered. It is more flexible and at highway speeds will flex more than just a whip. This could affect SWR as the antenna becomes closer to the body metal. We have noticed this at low band frequencies (46 MHZ). I suspect not so much at higher frequencies.

By the way, another tip from the land of salty roads. If you use an NMO mount on your right angle brackets, seal the base of the mount with silicone sealer. I had two antennas installed and one day one of the two radios started performing very poorly. When I went to connect my test cable to the suspect radio, I found the antenna connector crusted with salt. The salt water from the road got into the cable and migrated down the cable to the radio and then dripped out the connector. When I inspected the other antenna cable, I found the braid inside the cable was black up to about a foot from the radio. I suppose if you live in Arizona, you would probably never have this problem.
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Re: To spring or not to spring a Larson NMO antenna?

Post by Solo »

My rule of thumb is that springs are only used to prevent or attenuate damage from trees, roofs, etc.

If your mounting is at the hood level, anything which the car will fit under is high enough that a spring probably wouldn't flex anyhow.
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Re: To spring or not to spring a Larson NMO antenna?

Post by RadioSouth »

If the sunroof is the only thing keeping you from drilling you might want to pull the headliner to check clearance, it might fit near the back of the roof.
Different cars but I had over a foot of clearance on a Nissan Maxima that I installed an antenna on.
If you do a lot of highway driving you'll notice the spring makes the antenna bend more, performance has to take a hit when your antenna is bent 45 degrees.
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Re: To spring or not to spring a Larson NMO antenna?

Post by SlimBob »

I recommend putting a spring on the antenna if you're too lazy to get out and take it off before driving through the parking garage.
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Re: To spring or not to spring a Larson NMO antenna?

Post by nukedude »

Unless it is going on the roof of a fire truck or high profile vehicle, the spring is not needed. If you can drive through parking garages and not have the antennas hit something, then the spring is not needed. Where I work, we have hundreds of 1/4 whips for VHF and UHF in use. The very few that have springs are corroding as they tend to hold onto water more than the coat hanger whips. Simpler is better if the spring is not required.
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Re: To spring or not to spring a Larson NMO antenna?

Post by Jim202 »

If your having damage done to the roof of a vehicle with the long whips hitting something, your not using a Larson NMO coil. Many years back I was taking care of a large fleet of cement trucks that were using ASP 3/4 inch mounts. These will do damage to the roof and tear them open like a tin can if the coil hits anything. Won't happen with the Larson coils. The brass ring on the inside of the coil will come out or the plastic will break long before any roof damage is done.

Cut the body work on repairing the roof of the cement trucks to ZERO after using the Larson coils and long whips.

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