[TowerTalk] Mobile Antenna Mounting

[Dave Jordan]

Hi Folks,

I've got a screwdriver mobile antenna that has been leaning up against
the corner of my radio shack for the past few years.  I'm getting ready
to go
mobile with my Rodeo truck.  I've noticed that many ground mounted
verticals
and all the screwdriver antennas I've seen all have a mounting bracket
that
is a folded piece of two inch metal starting at the base of the antenna
horizontally, then it goes out about an inch, turns vertical, goes up
about 12",
then turns back to the horizonal with a plastic insulator grasping the
radiating
element.  I've always wondered if this metal support (which is grounded
to the car frame) has any significant negative effect on the performance
of the antenna.  I understand these mobile antennas are a compromise
antenna to begin with, as are some of the ground mounted verticals
(HYGAIN-DX-88).

I'd appreciate hearing comments from the list as to whether such a
mounting bracket has a significant adverse effect on either the
impedance or radiating ability of the mobile antenna.

Also, I have thought of mounting the screwdriver portion (COIL) of the
antenna horizontally on the roof rack, then run a flexible wire strap to
a 102" whip
mounted vertically in the middle of the roof rack.

I've made some field strenght measurements of a HB screwdriver which is
mounted on the back of  my motorhome and it looks like very little if
any RF radiates from the portion of the antenna beneath the coil.  The
coil has some radiation and the whip produces the highest voltages
measured. When I first
started with the RV ant. it was mounted on the rear bumper and performed

very poorly.  As I moved the ant. up the performance improved. However,
I couldn't notice any significant improvement once the coil portion of
the ant
had cleared the roof level which again causes me to wonder if the lower
portion of the mobile ant. radiates any significant RF.

All comments welcome -- thanks,
dave
WA3GIN


p.s. If anyone is not familiar with the screwdriver ant here are a
couple of
       web pages with pictures, "http://www.tjantenna@tjantenna.com/"
and
       "http://www.hsantennas.com/info/index.html"


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