Antenna trimming question

Rick Rock · Post by **Rick Rock** » Sun Mar 05, 2006 11:54 pm

I have a /\/\ VHF NMO gain antenna that I need to trim for 151mhz.- does anyone have the trim chart for these? I can't find the one that came with the antenna...

kb0nly · Post by **kb0nly** » Mon Mar 06, 2006 12:20 am

1/4w or?

I swear i saw a similar post on here a while back with a cutting chart. Or maybe it was somewhere else i come across it... Damn my memory!

These days i use an antenna analyzer i recently acquired, MFJ 259. It would have also been nice to have the 269 which covers UHF as well, but most of the stuff i deal with is VHF anyway.

kb0nly · Post by **kb0nly** » Mon Mar 06, 2006 12:22 am

Found it in my favorites... Good place to start and should get you in the ball park. If you want to be really picky cut it a bit longer and then fine tune it with an SWR meter or Bird 43 set for reflected and tune for minimum reflected power.

http://www.shultzie.com/downloads/antcharts.htm

Rick Rock · Post by **Rick Rock** » Mon Mar 06, 2006 12:23 am

No, the 3db gain antenna.

kb0nly · Post by **kb0nly** » Mon Mar 06, 2006 12:25 am

A 3db gain antenna should be a 5/8w, there is a cutting chart for that there.

I wouldn't think that the Motorola version is anything special.

wa2zdy · Post by **wa2zdy** » Tue Mar 07, 2006 2:24 pm

Yes, you really need to use an SWR bridge. The chart is just a starting point, and as stated above, start long and trim down 1/4 inch at a time. Once it's too short, you're screwed.

The reason you can't go by just the cutting chart is that chart wasn't made up with YOUR installation in mind. Your antenna might be closer to the edge of the ground plane than the manufacturer's test range, or you might have it on the trunk, have another antenna near-by, etc.

Good luck.

kb0nly · Post by **kb0nly** » Tue Mar 07, 2006 2:41 pm

I think most would be surprised just how far off they are using the cutting chart alone. I spent some time checking antennas on installs done by a local shop, they were cut exactly to chart length, and weren't that bad, but i can do better than 1.5:1 with a little time and patience.