Dave,
Ur experience is far different from mine.
I use a HS antenna on a Dodge conversion van.
The antenna mount is made to insert in a 4"x4" hitch receiver. I had one
welded to the frame so it extends out of the left rear corner.
The feed point of the antenna is about 2.5 ft above the bumper and a very
good ground strap brings rf ground to the frame at that point.
Differences in loading and reported signal strength from the original
mounting (similar to your description) are very significant.
I do not know what instrumentation you are using --- but I would bet the
antenna does radiate from the lower section as would be predicted. The
frequency is important as well as the rf ground. Look at some of the
antenna books and you will see what the r.f. voltage/current distribution
should be.
Consider a mount as I have done (I bought the gem from HS) and I think you
will be happy you did.
Harv
K6EXO
----- Original Message -----
From: Dave Jordan <wa3gin@erols.com>
To: Towertalk <towertalk@contesting.com>
Sent: Saturday, June 05, 1999 1:08 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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