Larsen Low band cutting

[737mech]

Ok, I got my Larsen today here's the cut chart. I'm asking what would you do? Trying to scan for stuff 40-50 on the Nellis ranges here in Nevada. Military low band, I'm planning on getting near the ranges around Mercury or Beatty. Maybe 30-50 would be a better cut? Need help please ?

 

[mmckenna]

Which coil did you purchase? There are several models that cover from 27MHz up to 54MHz.

 

[737mech]

Coil

No coil just the NMO mount

 

[mmckenna]

Order the NMO-40 coil and a whip. Cut it for around 45MHz if your interest is the 40-50MHz area.

 

[737mech]

Coil

Ok I see what your saying, is this required for RX only? I just watched a youtube on loading coils. So shorter with coil could be same as longer without.

 

[mmckenna]

Yes, you'll need the coil. They don't sell a full 1/4 wave NMO antenna without a coil.

The Larsen antennas are base loaded 1/4 waves. The coil allows the whip to be a bit shorter.

Since you already have the NMO mount, that's the way to go.

Other option would have been a simple 1/4 wave whip, but they use the 3/8x24 mounts common to CB antennas. They can also be harder to find as they are not quite as popular as the base loaded antennas. Resist the urge to buy one of the NMO to 3/8x24 adapters and use the 3/8x24 1/4 wave whip. That will put a lot of stress on the NMO mount.

The closer the antenna is to 1/4 wave, the better. The base loading impacts performance a bit, but probably not enough for you to notice. The NMO-40 with the whip will be a good choice.

 

[Golay]

Sorry, but I got a little confused OP. First you say "I got my Larsen today", then you say "No coil, just the mount". So are you saying you just bought an NMO mount made by Larsen?

 

[Ubbe]

Look at it this way, if you had a loading coil big enough to reduce the whips lenght to nothing, would you receive anything? No, the loading coil are used to reduce the lenght of the element to a more practical lenght and get a good swr for a transmitter but it will receive less.

Is it for car use or do you have the nmo mount at home? If you have it home you could go full 1/4 wave lenght without any loading coil. 45Mhz are 160cm measured from groundplane to top of element, including the mount.

/Ubbe

 

[prcguy]

The Larson and similar VHF lo- band antennas are very narrow band. If you use them 5Mhz away from where they are tuned you will notice a big drop in reception. Use them 10Mhz away and they can be 20dB down in reception.

If you want full band VHF lo coverage with fairly consistent results you will need a broad band military vehicle antenna, but they are big and bulky. They will also be a bit worse than a full 1/4 wave whip tuned for a specific frequency.

Edit: Look at post 14 in this thread where I compared a Shakespeare broad band military 30 to 512Mhz antenna to an Austin Spectra and A/S MON series. At resonance the Austin and A/S were 6 or 8dB worse than the military whip and it went downhill fast from there. At 50MHz the military whip was about 45dB better than the Austin that was tuned in the low 30MHz range. That's a huge difference.

The green reference line on the chart is all frequencies received by the military whip and the red and blue lines are the same signals received on the Austin and A/S.

https://forums.radioreference.com/scanner-receiver-antennas/264330-austin-spectra.html
prcguy

 

[737mech]

NMOWB40C

Ok cool great info. I do have a browning nmo coil I could use. (1 7/8 " plus the 1" whip holder with set screws)
The antenna is a Larsen NMOWB40C and this thing is 66" from the bottom of the black NMO Base (I guess that's a small coil?) to the top of the ball. I was hoping to just go out on the quad or in the truck and set up remotely with my scanner and find some Military Low Band 30-50? Maybe I should expect it in 40-50? That's the whole plan for this setup. I have other scanners for VHF and UHF. So having a look at the cut chart if I cut to 57 5/16" I should be ok from 42-48. I would still be able to catch up to 50 correct? I was confused when the antenna arrived at 66" long. That's why I asked you guys the experts. Thanks again for the help!!

 

[prcguy]

Most base loaded VHF low antennas are very narrow and maybe ok for +/- 1MHz from wherever they are tuned, that's it. If you set one up for 45MHz it will probably be at least 10dB down at 50MHz and maybe 30dB down at 35Mhz.

The Larsen NMOWB40C you mention does cover 40 to 50Mhz without tuning but below 40MHz it will proably fall off quickly in performance. You might be better off with a version tuned a little lower in frequency like the Laird CWB37 that covers 37 to 46Mhz without tuning.

For any more band width than that sign up for a big military whip or understand you will be missing lots of signals.
prcguy

Ok cool great info. I do have a browning nmo coil I could use. (1 7/8 " plus the 1" whip holder with set screws)
The antenna is a Larsen NMOWB40C and this thing is 66" from the bottom of the black NMO Base (I guess that's a small coil?) to the top of the ball. I was hoping to just go out on the quad or in the truck and set up remotely with my scanner and find some Military Low Band 30-50? Maybe I should expect it in 40-50? That's the whole plan for this setup. I have other scanners for VHF and UHF. So having a look at the cut chart if I cut to 57 5/16" I should be ok from 42-48. I would still be able to catch up to 50 correct? I was confused when the antenna arrived at 66" long. That's why I asked you guys the experts. Thanks again for the help!!