Larsen NMO150/450/800 & Spring

Ghost4658 · #1 (**permalink**) 02-02-2008, 07:51 PM

I got a Larsen NMO150/450/800 antenna and I put a Larsen shock spring on it.
What I need to know is with a shock spring added do I need to trim the bottem of the whip on the antenna to keep it tune.
And if so how much would I need to cut off to keep it in tune?

fineshot1 · #2 (**permalink**) 02-03-2008, 12:02 AM

That info would normally be in the paper doc that comes with the antenna.

frootydawg · #3 (**permalink**) 02-03-2008, 05:02 AM

I did the same thing with mine and I didn't notice any difference in receive performance on those bands. If you used the antenna for transmit that would be a different story. When I added the spring it made the antenna resonant on the 2 meter and 440mhz ham bands.

Wyandotte · #4 (**permalink**) 02-03-2008, 09:02 AM

Quote:

Originally Posted by Ghost4658

I got a Larsen NMO150/450/800 antenna and I put a Larsen shock spring on it.
What I need to know is with a shock spring added do I need to trim the bottem of the whip on the antenna to keep it tune.
And if so how much would I need to cut off to keep it in tune?

How long is the antenna?

N1BHH · #5 (**permalink**) 02-03-2008, 09:52 AM

The Larsen NMO-150/450/800 is not recommended for transmitting. Adding a spring will cause the center frequencies to skew down. The antenna is 14 inches. I find no reason to put a spring on my antenna.

Ghost4658 · #6 (**permalink**) 02-03-2008, 07:43 PM

I use mine on a 1975 CJ5 jeep in the woods without a spring it would not last very long.
It is working just fine with the spring.

Universaldecoder · #7 (**permalink**) 02-03-2008, 07:51 PM

I have a spring on mine too. The spring itself is grounded, and therefore the spring becomes an extension of the cable's shielding. The actual signal coming from the antenna travels along another cable that runs inside the spring last I looked, and it's been awhile. Bend it and see. Also you can test it with a continuity meter - there should be no transfer of signal from the spring to the center contact.

The spring should not affect the tuning of the antenna as is. If you cut the antenna, then yes you will change the tuning.

Ghost4658 · #8 (**permalink**) 02-04-2008, 06:46 PM

We do not recommend putting a Spring on the NMO150/450/800 as it will degrade the performance. If you still want to put the Spring on this antenna you will need to measure the length of the spring and cut that much off the bottom portion of the whip.

Account Manager

Larsen Antennas

Universaldecoder · #9 (**permalink**) 02-04-2008, 08:36 PM

Quote:

Originally Posted by Ghost4658

We do not recommend putting a Spring on the NMO150/450/800 as it will degrade the performance. If you still want to put the Spring on this antenna you will need to measure the length of the spring and cut that much off the bottom portion of the whip.

Account Manager

Larsen Antennas

I was wrong! You are right. The spring is not insulated from the center.

Ghost4658 · #10 (**permalink**) 02-04-2008, 09:55 PM

I hope this helps you with your ant.
I cut mine today I will let you know if it works better or worse.

Universaldecoder · #11 (**permalink**) 02-04-2008, 10:06 PM

Retested it with a meter to make sure. Let me see if I'm right with this one - the reason you want to cut only the length of the antenna is because of the cable that runs inside the spring. It's the shortest route for the signal. If it did not exist, then you would have to cut the length of the spring's coil from the antenna.

Ghost4658 · #12 (**permalink**) 02-06-2008, 11:58 PM

The ant is working fine with the spring on 155Mhz & 800Mhz trunk I don't scan many 450Mhz so I don't know about that.
The wire in the center of the spring is to keep the spring from working like a load coal on the ant.

rfradioconsult · #13 (**permalink**) 02-07-2008, 10:25 AM

Quote:

Originally Posted by Universaldecoder

I have a spring on mine too. The spring itself is grounded, and therefore the spring becomes an extension of the cable's shielding. The actual signal coming from the antenna travels along another cable that runs inside the spring last I looked, and it's been awhile. Bend it and see. Also you can test it with a continuity meter - there should be no transfer of signal from the spring to the center contact.

The spring should not affect the tuning of the antenna as is. If you cut the antenna, then yes you will change the tuning.

The spring itself is not grounded but is part of the design of the radiator. The purpose is to keep the spring from acting as a loading coil and changing the electrical length of the antenna. Even the old spring found on low band quarter wavelength antennas contain this braid. If you had a customer with antenna issues this was one of the first things that was checked.

T