[Icom] Mobile antenna considerations

Fri, 28 Feb 2003 06:42:32 -0800

Over the years I have use multiple antenna types.

1. Most efficient antenna is a monobander with a large diameter, well spaced, large
wire coil. It was very efficient, very narrow frequency response, and difficult to
tune.

2. Next I tried a webster band spanner. It covered 80 to 10 meters. To change bands
you had to get out of the car. The coil was not particularly high Q.

3. Hustler with coils for several bands. I found this antenna to perform fairly
well. This system was easier to adjust to a specific frequency. I found the coils
to get quite warm using a swan 350 (roughly 170 Watts output).

4. BB-3 screwdriver antenna. I have found this antenna to be very effective and
easy to tune on 75 and 40 meters. Finding the resonant frequency on 14 MHz and
above has proven difficult, because the antenna moves too fast across the
frequency. I also find it takes a long time to change frequencies, especially
moving to and from 75 meters and other bands.

I have never used an automatic antenna tuner such as Icom's and SGC's. I like the
idea of fast automatic tuning of the antenna. However, these systems are base
loading a mobile antennas. Long ago studies were made with actual measurements on
placement of loading coils on a mobile antenna. The measurements confirmed the
theory that the higher on the antenna a properly designed high Q loading coil, the
more efficient the antenna.

Today the compromise is to center loading because of the structural considerations
of top loading.

Being a little bit (very) lazy, I like the screwdriver antenna for relative
efficiency, and I don't have to get out to tune, especially in inclement weather (I
live near Tacoma and Seattle).

If you use a screwdriver antenna, there are several helpful ideas.

I have seen pointer arms with scales attached to them that are visible through rear
vision mirrors. These are used to approximate the center of each band, on on 75
meters possibly a couple of positions in the band.

Automatic control of the screwdriver antenna is possible. I believe it was K6OYD
brought out a device that counted the number of turns the antenna motor made and
had 16 user preset able points you could set the antenna too. Another form that was
available, (I don't remember the company) used a frequency discriminator or ratio
detector to fine tune the antenna. AS I remember this device only worked a few KHz
each side of resonance.

My personal IDEAL screwdriver antenna would use both the motor turns counter to
approximate the desired frequency on the antenna, and the ratio detector to bring
the antenna into exact tune.

Just my thoughts.

           Rolly

Al Waschka wrote:

> 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/