Homebrew mobile antenna questions

[N0YYW]

I've spent the better part of a year reading about and experimenting with mobile antennas.  I think I really aggravate my fellow hams since I'm using an FT-817 for my mobile right.  The benefit is that it has forced me to strive for the most efficient set that I can.

My current antenna is made up of the bottom 4' piece of a 10m Iron Horse hamstick clone, an ugly 4" air wound coil, and the 4' whip from the hamstick.  It's shunt-fed with a coil at the base (electrically, but not mechanically) to match Z.

I get on midcars reasonably well and have worked Europe and South America on 17m.

Questions:

1. The antenna is currently mounted to the bed of my Dodge Dakota just behind the driver.  I really should show a picture, but I'll describe it as best I can: a round pipe flange bolted to the bed, a 4" threaded steel pipe capped with a threaded PVC cap for insulation with a 3/8x24 bolt, a 4" spring, followed by the mast.  This puts my coil 2' above the roof of the cab, but a good portion of the mast is still below the sides of the truck.  Would lifing the mount point up away from the truck bed improve my radiation or would the ground losses increase and cancel any benefit of less obstruction to the mast?

2. Would replacing my 4' whip with a 102" whip and retuning the taps improve my efficiency very much?  This would be a coil 33% of the way up a 12.5' antenna.

3. Could I use my z11 Autotuner as a coupler if I mount it within an inch or 2 of the ballmount on my Jeep Wrangler and feed a 102" whip?  I realize it's no good at the end of a coax run, but how about right at the feedpoint?


Thanks for letting me post these long rambling questions and for all the advice I've read on here.  I've never posted before, but I've sure done a lot of reading and your collective input has been quite helpful.

-- Tom (n0yyw)

[K0BG]

You might want to visit my web site, and get some of your questions answered there first. Perhaps you'll gain a little insight with respect to what is needed. Look under antennas.

There are lots of aspects to operating mobile, and the most important one you didn't mention. That is minimizing ground losses.

You speak of length, but an 8 foot antenna is just barely adequate for 17 meters. It is relatively easy to run a 1/4 wave on 17 which doesn't need a loading coil. Nor will it need impedance matching as adding the ground losses and the input impedance will bring it very close to 50 ohms.

Your home work is cut out for you. Good luck.

Alan, KØBG
www.k0bg.com

[KF6IIU]

XQ2FOD's design:

http://ludens.cl/Electron/HFantenna/HFAntenna.html

Manfred is a pretty amazing Elmer and overall mad scientist. Check out the rest of his web site - there's some pretty cool stuff with excellent discussions about how it was designed the way it was.