Antenna Cutting

[busher12]

I have a tram 5/8 wave NMO mag mount that came with a cutting chart. I am wondering where to measure from to cut the antenna. Top of Antenna to bottom of Antenna , or top of Antenna to top of base? The one I have goes into the base about an inch or so, thanks.

[KG4INW]

Just the metal rod itself, out of the base. That extra inch will be counted in the overall measurement. You then use either a watt meter or antenna analyzer (once you've reassembled it) and make sure the reflected power is acceptably low.

[k8mcn]

From my experience there is often enough adjustment with them by just moving the antenna whip up or down and closing the set screw... remember,once you cut it off its gone
As you go up the band, antennas get shorter...on a mobile mount you will drive your self nuts going for a 1:1 match,get it close,but don't sweat the exactness of it for local repeater work,just make your rig happy with a safe SWR.
Also remember that the SWR is affected by open doors on the vehicle,the location of the antenna on the vehicle etc.. my Hamstick for 20 meters is a 1.75:1 with the truck driver side door shut----with the door open it'll jump sky high..........

[busher12]

Originally Posted by KG4INW

Just the metal rod itself, out of the base. That extra inch will be counted in the overall measurement. You then use either a watt meter or antenna analyzer (once you've reassembled it) and make sure the reflected power is acceptably low.

thanks for the help, to make sure i have read you correctly if it says to cut at say 45 inches, it should measure 45 inches sticking out of the base correct? thanks again.

[jhooten]

Take the rod out of the base, measure 45" from the top of the ball, cut from the bottom, fat end, so the ball stays in place and the rod maintains maximum flexibility.

[LtDoc]

And since those 'cutting charts' can never be all that accurate, it's been my experience that making two 'cuts' is often better than just making one. That first cut being for a frequency lower than actually wanted, or let it be a little longer than the chart says. Check to see how it reacts, and if necessary make another 'small' cut, or two, or three, whatever. Make sense?
- 'Doc

[KG4INW]

Yep, what jhooten and LtDoc said!