Larsen NMO-27 destructive disassembly

[mmckenna]

Digging through my dad's old radio stuff, I found his Larsen NMO-27 from about 25 years ago. It's an older one, back when they were a grey body for the whip. This one got spray painted black at some point to blend in a bit better.

Anyway, I didn't need it, so I decided to open it up and share the contents.

Some work with a saw and some prying got it open. Not easy to do. They are built well.

Brass tab on the side connected to the outer threaded ring.


At the top where the whip cone screws on, there was a set screw hole on the inside. Loosening that up allowed the coil to come out. Coil is about 6 feet of enamel coated 12 gauge copper wire.




The base of the antenna where the center tab is to contact the NMO mount. small disc capacitor for matching. Every connection was soldered, except for the set screw connection at the whip connection point. The crack in the insulator was from me pulling it apart.

After 30 years of using these, this is the first time I've pulled on apart. Well built and this one still looked good after 25 years or so of use. No signs of water ever getting inside it. No corrosion. If the nut with the hack saw hadn't destroyed it, it would have still been a good antenna.

 

[K4EET]

A visible reason of why one should select this line of antennas when longevity is a requirement...

 

[AA4TX]

Can you read the value of the capacitor? Or provide a clear picture of the printed value?
Thanks!

 

[mmckenna]

Can you read the value of the capacitor? Or provide a clear picture of the printed value?
Thanks!

Let me see if I can find it. I know it was 1KV rated...
18pF, 10% tolerance.

 

[AA4TX]

Thanks!

 

[slowmover]

Thx!

 

[jeepsandradios]

I recently di the same with the all band antenna that ships with the 8500. Id show pictures but its hollow. Nothing at all inside.

 

[jeepsandradios]

Found my pics....

 

[N4KVE]

I too have one from 30+ years ago, & I’m still using it. But I wonder if the ones sold new today are built as “robustly”.

 

[AA4TX]

Found my pics....



View attachment 105127

All the "magic" must be in the black cylinder! /sarc
50 ohm resistor?

 

[mrweather]

I too have one from 30+ years ago, & I’m still using it. But I wonder if the ones sold new today are built as “robustly”.

My Larsen NMO27 is probably 15 years old and still works like a champ. I did have a NMO2/70 coil fail on my a few years ago.

 

[mmckenna]

Found my pics....

I know Motorola doesn't make those...

But why is it that Motorola seems to love selling antennas that suffer from water ingress? Is it some sort of marketing scheme to get agencies to buy more antennas? Went through that with those God awful 800MHz antennas they used back in the 90's. Never found any that had been deployed that didn't have signs of water intrusion.

 

[RFI-EMI-GUY]

Found my pics....



View attachment 105127

WTF? how does it work? Some sort of mageek? and whats with the huge stub?

 

[KN4EHX]

EMA antenna farm on my SUV. East Tennessee rain on a back road; I smacked a low hanging wet tree branch at about 40 mph with a Larsen NMO27. The sound was as loud as thunder and I’m pretty certain my heart skipped a few beats.

The antenna was a hand me down and likely lived a long life on something that lived in the sun for many years the base had obviously faded.

The base absolutely exploded and left a nice ring still attached to the NMO. Everything else is Larsen or Maxrad. This antenna smacked several other tree branches before the big wet Louisville Slugger. They are built like brick outhouses and the destroyed antenna base was promptly replaced with a new one that has made several low hanging branches reconsider their choices.

10/10 would recommend. I’d guess that antenna base is probably almost as old as me. Not much in this world of radio lasts that long.

I did replace my 5/8 VHF Larsen for the fact of smacking too many tree branches, but I do keep it in the SUV for the unusual occasions that I end up stationary and in a repeater fringe zone. Otherwise the V/UHF/7/800 all band antenna is much shorter and works in most cases for what I need. It still doesn’t replace dedicated band specific antennas. Life in the hills and valleys.

 

[jeepsandradios]

@mmckenna Thats why that antenna came off my truck. Went thru 4 of them until I switched to the Larsen all band. Every one of them would seep water inside the base. I go to the car wash weekly with the truck and always take off the antenna put rain caps on then dry and replace.

@RFI-EMI-GUY The stub I think your talking about goes to the base and was soldered to the bottom tab. Picture doesn't show well but the bottom hard plastic part is about 3/4-1" tall and screws into the aluminum portion of the antenna.

Correct MSI does not make the antenna. If I recall its made by PCTEL. You can look at the drawing here of the antenna and decipher from my pictures what goes where.

 

[AA4TX]

You can look at the drawing here of the antenna and decipher from my pictures what goes where.

Unless there is some sort of matching network in the hard plastic part, this is nothing more than a 1/4 wave VHF radiator, given the length of ~ 21".

 

[jeepsandradios]

Nothing at all. The medal "stub" hex shaped part was pressed into the metal tab and thats it.

You hit the nail on the head. Its basically a 1/4 wave VHF which doing sweeps sorta shows.

 

[KevinC]

So the big chunk of aluminum (?) is the radiator for everything above 174 I'm guessing? I may take apart my Browning or Laird to see if they are the same.

 

[RFI-EMI-GUY]

So the big chunk of aluminum (?) is the radiator for everything above 174 I'm guessing? I may take apart my Browning or Laird to see if they are the same.

I am thinking it i there to widen the bandwidth and a taper to match the impedance of the whip. I wonder how it really performs.

 

[KevinC]

I know Motorola doesn't make those...

But why is it that Motorola seems to love selling antennas that suffer from water ingress? Is it some sort of marketing scheme to get agencies to buy more antennas? Went through that with those God awful 800MHz antennas they used back in the 90's. Never found any that had been deployed that didn't have signs of water intrusion.

My favorite was the one GE used to sell (I "believe" they were made by dB?). I installed a ton of these mounts and replaced a ton after they got corroded.