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Cut Larsen 2/70 NMO to GMRS

Tuning Antenna

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#1 WRJN263

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Posted 14 October 2020 - 10:04 PM

I am a new GMRS licensee. I am making an ammo box repeater & have a question about the Larsen NMO 2/70. It was given to me by a friend whose Dad was a longtime ham.

I know it is a dual band 2 meter/70 cm antenna with the coil in the middle. I’d like to cut it down to GMRS only, but my antenna analyzer doesn’t do UHF. Has anyone done this, and what are the whip lengths? I plan to use the 462.725/467.725 pair, so the 462 frequency would need the lowest SWR. Each of the two elements are currently around 15 inches each.

#2 BoxCar

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Posted 15 October 2020 - 07:22 AM

Save yourself the grief of trying to tune a dual band antenna and purchase a GMRS antenna. https://www.amazon.c...02764409&sr=8-5 will attach to your existing NMO base.


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#3 kidphc

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Posted 15 October 2020 - 07:24 AM

Instead of cutting. Contact larsen to see of you can get a po-450 whip. Should be about $10.

Po450 is their gmrs 5/8th model tuned for 450-470. The bases and parts on larsens models are mostly interchangeable.

If you are super impatient then find some steel stock rod from the hardware store about the same thickness. You'll be targeting a length around 1.25 range then adjust from there.

The 2/70 is a collinear design. So tuning them is a no no. You usually screw it up more then anything for the 70cm service you basically have two stacked antennas with the coil being the separator. If you start trimming you may end up with phasing issues or even worse erratic and weird vswr numbers and curves..


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#4 SteveC7010

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Posted 15 October 2020 - 07:29 AM

Save yourself the grief of trying to tune a dual band antenna and purchase a GMRS antenna. https://www.amazon.c...02764409&sr=8-5 will attach to your existing NMO base.


Agree. Trimming and tuning a ham spike to GMRS requires the right test gear. Save the 2/70. You may want it someday if you get your ham ticket. Or sell it to offset the cost of a proper GMRS antenna.

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#5 kidphc

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Posted 15 October 2020 - 07:38 AM

Frankly, for an ammo can repeater. I probably would do a roll up jpole with some coax.

 

Get some fishing sinkers and 50lb test (or heavy) mono filament. Tie the sinker to the line, and huck it over a tree branch. Even better if you have a sling shot and are decent shot. Then tie the roll up jpole, attach the coax to the line and pull upwards. Till you can't get it higher or run out of coax.  Really starting to like RG400 over LMR400 for some of the runs I am doing. But can be pretty lossy in 100 lengths. Plus you really don't need the Teflon isolator in GMRS. Nut OMG how flexible RG400 is over LMR400.

 

But with that setup you can get the antenna up higher. Giving you better LOS and range. If you decide to add solar it will also give you some more isolation from the antenna, as long as RFI isn't making it in by the radio and power lines.






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