Larsen Mobile Tri-Band Question

[Dewey]

Just ordered the Larsen Tri-Band for my ride. Since it is going into the roof of an SUV, I also ordered the optional spring. Would it be a good idea to cut the approximate length of the spring out of the lower element (if doable)? In the DC area, I monitor all bands with preference going up with band. Maryland State is about the only thing I'm interested in on VHF-Lo. Virginia State, the Feds and a few cities are of interest on VHF-Hi. My primary target is UHF which is DC PD and PG County (MD), and the next and very close preference is 800.

As ALWAYS, thanks to all,
Dewey

 

[Tweekerbob]

The spring does not affect performance; it is there as a safety feature so you don't whack the crap out of your antenna while passing under low overhangs or other objects.

Cutting the element itself will affect performance on all bands. The two elements along with the center coil are part of a factory tuned antenna system. Cutting the elements or messing with the coil without knowing exactly what you are doing will adversely affect performance on all bands.

 

[mrweather]

I've never used my Larsen tri-band for low-band listening. However, given its short stature I'd suspect it won't work very well.

I found it worked well on VHF and 800 and okay on UHF. YMMV.

 

[AK9R]

The spring DOES affect performance. The Larsen spring is internally shorted from top to bottom. All of the Larsen antenna whip cut charts show two different lengths, with and without spring.

As Tweekerbob states, the top and bottom elements of the Larsen, or any, multi-band antenna along with the center coil are all precisely designed to work together. However, when you add the spring to the bottom of the antenna, you are, in effect, lengthening the lower element by the length of the spring. You then have to cut off the length of the spring to compensate.

Dewey, your antenna should come with installation instructions printed on a piece of paper that's folded up in the package. I don't have the installation instructions for the Larsen 150/450/800 antenna handy, so you'll have to read yours carefully to see if it mentions cutting the antenna if you are using the spring. My gut tells me that you'll have to cut the lower element of the antenna.

P.S. Larsen whips are made of stainless steel. You will have a tough time cutting them with wire cutters. If you have a Dremel tool with an abrasive cut-off blade, that will cut the whip pretty quickly. If not, take a small file and file a nick in the element where you want the cut to be. Then, with two pairs of pliers positioned just on either side of the nick, you should be able to snap the element.

 

[fineshot1]

Just ordered the Larsen Tri-Band for my ride. Since it is going into the roof of an SUV, I also ordered the optional spring. Would it be a good idea to cut the approximate length of the spring out of the lower element (if doable)? In the DC area, I monitor all bands with preference going up with band. Maryland State is about the only thing I'm interested in on VHF-Lo. Virginia State, the Feds and a few cities are of interest on VHF-Hi. My primary target is UHF which is DC PD and PG County (MD), and the next and very close preference is 800.

As ALWAYS, thanks to all,
Dewey

The cutting chart and install directions that come with the antenna will tell you what to do.

What model number is the antenna? Perhaps you can use google to find an online download
for the install directions to preview the answers for your questions.

 

[Dewey]

The spring DOES affect performance. The Larsen spring is internally shorted from top to bottom. All of the Larsen antenna whip cut charts show two different lengths, with and without spring.

As Tweekerbob states, the top and bottom elements of the Larsen, or any, multi-band antenna along with the center coil are all precisely designed to work together. However, when you add the spring to the bottom of the antenna, you are, in effect, lengthening the lower element by the length of the spring. You then have to cut off the length of the spring to compensate.

Dewey, your antenna should come with installation instructions printed on a piece of paper that's folded up in the package. I don't have the installation instructions for the Larsen 150/450/800 antenna handy, so you'll have to read yours carefully to see if it mentions cutting the antenna if you are using the spring. My gut tells me that you'll have to cut the lower element of the antenna.

P.S. Larsen whips are made of stainless steel. You will have a tough time cutting them with wire cutters. If you have a Dremel tool with an abrasive cut-off blade, that will cut the whip pretty quickly. If not, take a small file and file a nick in the element where you want the cut to be. Then, with two pairs of pliers positioned just on either side of the nick, you should be able to snap the element.

Thanks much W9RXR! I figured since the spring added length to the lower element, it would affect the phased balance... which is what made me ask the question. BTW, I'm used to cutting antenna elements using the "nick" method since I've never had to cut one of the thicker elements like the one on the bottom of RS's magnetic two coil scanner antenna.

Tweekerbob, I'm sure you were trying to help, but I just couldn't figure how adding a spring (element length) could not affect intended specifications since it would result in a longer lower element.

The cutting chart and install directions that come with the antenna will tell you what to do.

What model number is the antenna? Perhaps you can use google to find an online download
for the install directions to preview the answers for your questions.

Thanks for the help. Since I just ordered the antenna, I was unaware that it came with a cutting chart. Actually, it is a surprise to me that multi-band receive antennas come with cutting charts. But then I really wouldn't know since I normally just go with commercial antennas for my most desired band (Antenna Specialists are my favorite), then hope for good performance on the rest. I don't know model number for the Larsen Tri-Band since everywhere I've seen it mentioned only calls it a Tri-Band, that is unless the band specs "NMO-150/450/800" is the model number.

Again, thanks to ALL...
Dewey

 

[fineshot1]

Thanks for the help. Since I just ordered the antenna, I was unaware that it came with a cutting chart. Actually, it is a surprise to me that multi-band receive antennas come with cutting charts. But then I really wouldn't know since I normally just go with commercial antennas for my most desired band (Antenna Specialists are my favorite), then hope for good performance on the rest. I don't know model number for the Larsen Tri-Band since everywhere I've seen it mentioned only calls it a Tri-Band, that is unless the band specs "NMO-150/450/800" is the model number.

Again, thanks to ALL...
Dewey

You may be correct about there may not be a cutting chart included. I have noticed over the last few years with multi-band antenna's there have been none that I could find, but if there is a spring part option there usually is one included.

 

[W4KRR]

Assuming the OP is talking about the Larsen NMO 150/450/800, I have purchased two of them, and neither came with a cutting chart, as they aren't designed to be tuned.

And just for the record, what is the model or part number for the "optional spring"? I was unaware that you could get a spring that was for use with the NMO 150/450/800. If there is, I would like one myself!

 

[Dewey]

Assuming the OP is talking about the Larsen NMO 150/450/800, I have purchased two of them, and neither came with a cutting chart, as they aren't designed to be tuned.

And just for the record, what is the model or part number for the "optional spring"? I was unaware that you could get a spring that was for use with the NMO 150/450/800. If there is, I would like one myself!

The credit goes to someone else here who pointed the springs out to me some time ago. While I don't see part numbers, chrome $9.00 and black $12.75 springs can be found here: http://www.cheapham.com/ant-mob.html. They are near the bottom of the "Larsen" section.

Dewey

 

[Dewey]

Well, my package arrived today...

FIRST, kudos to CheapHam.com by Hometek (Cheapham.com)!!! Within two hours of placing my online order at 8:11 pm on a Saturday Night (3/7), they had emailed me back stating that all of my parts were not in stock. I was then given the option to cancel my order, or wait for the Friday 3/13 date when they expected their next delivery. On Friday 3/13 @ 12;51, I got an email stating that my order had shipped USPS with a tracking number. I came home today to my package!

NOTHING like that unnamed company in Brooklyn, NY, who charged me $12.95 shipping for a $17.80 NMO mount. After waiting past their estimated 10 business days to ship, I contacted them to ask the status, only to be told that the part was backordered, and expected to ship in ANOTHER 2 to 3 weeks. Remember that $12.95 shipping charge... well that order was promptly cancelled.

Now the reason for reviving this thread. Sure enough, the Larsen does not come with a cutting chart for the spring option. The spring is 2.25 inches long. Would it do more harm than potential good if I cut 2.25 inches out of the lower element?

As always, thanks to all,
Dewey

 

[AK9R]

Well, you could test it with the spring prior to cutting to see how it performs. Check known VHF, UHF, and 800MHz signal sources and possibly compare it to other antennas. If the performance is not what you expect, cut the lower element to account for the length of the spring and test again.

My gut feeling is that you'll need to cut the antenna, but without Larsen's express instructions to do so, it's a crapshoot.