Larsen NMOWB150 cutting chart

[westcoaster]

I have to say, the cutting chart Larsen supplied with their NMO WB antennas is somewhat lacking....

There are four supplied options. (1.5:1 VSWR)

139-145Mhz 40 1/2" (1029mm) 142Mhz (centre)
147-159Mhz 37 3/8" (948mm) 153Mhz (centre)
152-160Mhz 36 1/2" (927mm) 156Mhz (centre)
158-170Mhz 34 1/2" (876mm) 164Mhz (centre)

There is a HUGE gap of 11Mhz in the lower frequencies and a cutting difference of 3 1/8". I would like to target a centre frequency of 150Mhz.
I applied some simple linear math and arrived at a whip length of 38 3/16 for 150Mhz.

It would be nice if I could figure out the actual calculations for the whip length or even better if there was an updated cutting chart for the whips.

Thanks,

 

[mmckenna]

The cut chart will land you in the middle of that range, so if you want 150, cut for 38 3/16 will probably get you close enough. The specs for that antenna and cut chart will give you 1.5:1 swr within that range with the listed whip cut length.

I know some like to get it exact, but unlikely you'd notice anything wrong with 1.5:1 SWR.

If you really want to get it accurate, use the cut chart and then follow up with an antenna analyzer, like the NanoVNA. Each install is going to vary, so checking with the analyzer will tell you what is right for your specific installation. Cut charts are intended to fall into the 'close enough' range for the majority of installs.