View Single Post
Old 02-20-2014, 01:05 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is offline
Hard Core 4+
Join Date: Jun 2006
Location: Larimer County
Posts: 8,440

Tessco has sold the whip on an NMO base for $34, that's the link I put in there. It does not come with any NMO, but screws onto my Larsen NMO mounts just fine. Be sure to use the thin rubber gasket they include, anything much thicker and the center contact may not reach the NMO base.

When I give a frequency, it's literally that frequency, not a shorthand for the band. IOW, 146MHz is 146MHz, not a substitute meaning the 2m band.

It takes some finesse and will have to be done carefully (I was taking 1/16" off at a time), but it looks like it can be done. It's also important to note that I have not done any testing while driving, this antenna is very flexible and my suspicion is that the SWR will swing fairly wildly as it moves around.

The take-away is you will have the SWR get beyond 2:1 somewhere in either the 2m or 70cm band and if you're not careful it will be in the FM phone portion in one of the bands. I selected the longer of the two values because I use 70cm quite a bit so wanted it to be low SWR on 440MHz through 450MHz. The ideal VHF length for my truck would be somewhere between these two cut lengths, I went too short by ~1/4" with 18-5/8". It would be fine on 2m, however it would be a very poor choice for UHF. It's 2.1:1 right in the center of my intended use on 70cm.

BTW, I will probably not need the shorter one if you're interested in it. How about $20? This would be definitely good for a 2m-only installation if someone else is following this thread.

I'm constructing a mathematical model for this, hopefully I get that working today or tomorrow. When I do I'll know more about how this antenna is operating.

Measured data with these conditions:
  • NMO-K on roof, approximately the middle left-to-right.
  • The mount is roughly 12" from the back and 36" from the windshield towards the rear of the cab
  • Cut to length RG-58 coax, runs under the headliner, down the B-pillar to under the passenger's seat
  • MFJ-269 analyzer
  • The value is OAL as measured from the tip of the whip to the bottom of the NMO base, the actual whip is about 1/16" shorter
  • Line is smoothed with a cubic spline

Name:  combined_measured.jpg
Views: 908
Size:  143.9 KB
'91 Pickup - Imelda
'08 Tacoma TRD - Donna

"The Only Thing That Is Constant Is Change." -- Heraclitus

Last edited by DaveInDenver; 02-20-2014 at 01:36 PM.
Reply With Quote