I have mounted my Sierra screwdriver using a stainless steel ball-mount to
the side of my Explorer at the driver's side rear. The base of the antenna
is "hot", with the car body the ground side of the antenna. The coil is at
roof level, with the whip above.
I would expect that the folded mount has some method of isolating the
antenna base from ground, as the ball mount does. This is the way the
antenna was designed to work, and I can't imagine the antenna resonating
successfully with the base grounded.
I have heard of people permanently mounting the whip on the roof of a camper
(isolated from ground, again, connected to the coil), with the coil mounted
horizontally inside the camper, and the antenna still performed well (though
I doubt it did as well as it does with the coil connected directly to the
whip). My own experience is that the screwdriver performs well as I have it
connected. I have made solid 5/7 - 5/9 contacts with Japan while mobile,
from my home on the East Coast. My MARS group accused me of using a linear
while operating mobile during MARS nets, so I'd say it does well. I operate
with a Kenwood running 100 watts.
73,
Jim N1RUI
----- Original Message -----
From: Dave Jordan <wa3gin@erols.com>
To: Towertalk <towertalk@contesting.com>
Sent: Saturday, June 05, 1999 4:08 PM
Subject: [TowerTalk] Mobile Antenna Mounting
>
> 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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>
>
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