[Icom] Mobile antenna considerations

Sat, 01 Mar 2003 14:02:00 -0500

George -

I tried two installations, both of which should have worked better than they
did.  The first car was a 1981 Oldsmobile Cutlass station wagon.  The tailgate
was metal and parallelled the whip a few inches away for a distance of about
2'.  The whip was mounted on the bumper.  In the first attempt on this car, the
tuner was mounted in the spare tire well and had a run of about 18" of wire to
the antenna.  I suspected that coupling to the body from the wire was a problem,
so I mounted the coupler under the car just in front of the bumper and had a
short length of wire directly to the whip.  Neither installation worked very
well.  I also had the setup on an Aerostar, again with a whip mounted on a plate
mounted to the bumper.  The Aerostar had a composite lift gate and shouldn't
have been much of a problem.  This time I started with the tuner on the outside
to be sure to get as short a run as possible.  Still no joy.  In both cases,
ground was s short length of coax braid to the car frame.

I did a reasonably rigorous test.  I had a friend a couple of miles away with
two HP variable attenuators in his feedline.  I would transmit on one antenna
and he would set the attenuators for S9.  Then I changed antennas and he did the
same thing.  The hustlers always won, by as much as 15-20 dB on the lower bands.

To be sure, if the antenna were mounted on the roof it would have worked better,
but so would the hustlers.  I keep coming back to the fact that the inductors in
the tuner are so small.  They are probably ferrite core, and capacitively
coupled to the ground plane in the tuner.  When tuning on 75, for example, the
inductors would get HOT and the output power would fall off after a few
seconds.  That sounds like resistive loss to me.  I never tried a 16' whip
because that was an impracticality on the car.  But the specs said the tuner was
OK to 80 meters with an 8' whip.

I plan to try the AH2 on my sailboat.  Maybe with a 16' helically wound
fiberglass whip, maybe with the backstay.

73,

Al

W4LT George Tonsmeire wrote:

> Al
> I suspect that your problem may be that the whip is too close to the body of
> the vehicle and too low - I know it is very difficult to have the whip mount
> on the roof , but the proximity to the body detunes the antenna. The whip
> worked well for me once I got it up and away from the body of the car.
> Another problem may be the location of the tuner itself - it is best when it
> is at the base of the antenna and having a radiating wire running through or
> near the body adversely affects the output - And the tuner does work much
> better on the higher bands just because the whip is a much greater portion
> of a wave length and has a higher resistance than the lower bands. I would
> be interest in how you had the tuner and antenna mounted -
> 73
> George
> W4LT
> ----- Original Message -----
> From: "Al Waschka" <[email protected]>
> To: <[email protected]>
> Sent: Thursday, February 27, 2003 7:32 PM
> Subject: Re: [Icom] Mobile antenna considerations
>
> > I have used  (1) Icom AH-2 with a whip and (2) Hustlers.  The Hustlers win
> > hands down.  The Icom tuner with an 8' whip is virtually useless below 20m
> and
> > it ain't great on 20.  The AH-2 works !GREAT! with a 50' long wire and
> would
> > probably work well on a sailboat backstay, but simply does not work with a
> > whip.  I did some comparative groundwave tests and the AH-2/whip combo was
> > 15-20 dB below the Hustler on 40 and 75. There are really two problems.
> The
> > Hustlers (and the sticks) are center-loaded antennas.  It takes less
> inductance
> > to resonate an antenna if the loading is at the center. The efficiency of
> an
> > antenna is the ratio of the radiation resistance to the loss resistance (Q
> > effects plus ground resistance).   "Q" is the ratio of resistive loss to
> > inductance (and you get it by the cubic inch.) The more inductance, the
> more
> > resistance for a given Q so the base-loaded whip takes more inductance and
> (for
> > a given Q) has more losses than a center-loaded whip.  For a given
> > installation, the ground losses are the same.  So you want to minimize the
> > required inductance and you want to maximize the Q.  If you look at the
> inside
> > of the tuner, you can see why the losses are high.  The inductors are
> lumped
> > circuit potted elements about 1/4" x 1/4" x 1".  The Q of these inductors
> is
> > very low, so the losses are high.
> >
> > You can make the same comparison between the Screwdriver and the Hustler
> or the
> > Hamsticks.  Look at the diameter of the loading inductors.  Remember what
> I
> > said about Q coming by the cubic inch.  I have never owned a
> screwdriver-type
> > antenna but I can easily understand why they beat Hustlers and Hamsticks
> in
> > head-to-head shootouts.
> >
> > That being said, I only have experience with the AH-2 which is the tuner
> for
> > the IC-735, so I can't speak for the later (706-compatible) tuners but the
> > basic mobile tuner was derived from the marine tuner which was designed to
> tune
> > a 50' or so backstay on a sailboat, not an 8' whip.
> >
> > If you can afford it, go for the screwdriver.  If not, get the Hustler or
> the
> > "sticks".
> >
> > 73,
> >
> > Al K5TAN
> >
> >
> > Jack Hartley wrote:
> >
> > > I'm getting ready to mount my 706MKIIG in my pick-up.
> > > In looking at antennas and have a question:
> > >
> > > There are 3 considerations-
> > > A: 'sticks'  for each band.  Which I'm using now and this system, while
> not
> > > convenient, works very well
> > > B: Screwdriver antenna
> > > C: Whip & tuner
> > >
> > > The questions is asked to those of you that have had at least two of the
> > > configurations and like one or the other.
> > > Does either of these 3 choices have a proven performance advantage?  Has
> > > anyone done a comparison?
> > >
> > > Price wise the screwdriver types can run up into the $500+ range once
> you
> > > include the heavy duty mounts required.
> > > ICOM's combination, which includes a very expensive whip is about the
> same
> > > cost.
> > >
> > > Jack Hartley
> > > K4WSB
> > > ARRL - QCWA - OOTC
> > > http://www.ultimatehat.com
> > > 10% discount to fellow Hams
> > > Put your call on the order
> > >
> > > ----
> > > Your Moderator: Dick Flanagan W6OLD, [email protected]
> > > Icom Users Net: Sundays, 1700Z, 14.315 MHz
> > > Icom FAQ: http://www.qsl.net/icom/
> >
> > ----
> > Your Moderator: Dick Flanagan W6OLD, [email protected]
> > Icom Users Net: Sundays, 1700Z, 14.315 MHz
> > Icom FAQ: http://www.qsl.net/icom/
> >
>
> ----
> Your Moderator: Dick Flanagan W6OLD, [email protected]
> Icom Users Net: Sundays, 1700Z, 14.315 MHz
> Icom FAQ: http://www.qsl.net/icom/