Here are some tips for those of you going mobile. This has worked well for
K4BAI and myself.
1) Most mobile signals are weak. If condx are good you might be loud in some
places but you
signal is probably still weaker that someone running a dipole. Don't get
discouraged. Think of
it this way : It's like you are a weak and rare DX station and everyone
is looking for you. When you
get spotted on the DX Cluster your rate will really take off.
Operating mostly CW will be you best bet for the best rate. Mobile
antennas are not very efficient
antennas. See the following from The DX Engineering webpage :
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Some Basics:
When an antenna is shortened from it's naturally resonant length at the
frequency of interest, the feedpoint becomes capacitive and it becomes
necessary to add offsetting inductance in the form of a loading coil. This
restores the resonance of the antenna. Unfortunately, the additional inductor
also has resistance that adds loss to the antenna. The resistance in the coil
eats up your power and you are not going to be as loud as you would be if the
coil loss were not present.
The more that you shorten the antenna, the larger the coil that you need to
use. The larger coil has more resistance and eats up more of the power. Mobile
antennas are usually pretty short, require large coils and hence have large
losses.
This loss resistance is also seen at the feedpoint and is partly responsible
for making it easy to match this shortened antenna to the 50-ohm feedline and
radio.
The impedance seen at the feedpoint is made up of the radiation resistance,
coil loss and ground loss. The radiation resistance of a full size vertical
antenna is about 35 ohms. As the antenna is shortened the radiation resistance
goes down quickly but the other losses are still there or even increase.
The table below shows the Real Radiation Resistance of your 6-foot mobile whip.
Radiation Resistance
of shortened Mobile Whips
Height (in.) Freq. (MHz) Radiation Resistance
(Ohms)
72 1.8 0.05
72 3.5 0.17
72 7.0 0.69
72 14 2.77
72 21 6.24
72 28 11.10
So, for instance, if you have a 72-inch, 20m-whip antenna and you are measuring
an SWR of 1:1 that means that you have a feedpoint impedance of 50-ohms. The
true radiation resistance of your antenna is somewhere near 3-ohms. That means
that you have a loss in your system of 47-ohms (50-ohms minus 3-ohms) and a
radiation efficiency of about 6 percent (3-ohms / 50-ohms). So, if you have a
100-watt transmitter in the car, 94-watts are being used to heat the air and
only 6-watts are being used to communicate! The other 94-watts are used up in
the loading coil and ground system.
If we were able to suddenly remove all the loss from the loading coil that we
were using then we would see the base impedance drop by the amount of the loss
in the coil.
How do we reduce the loss in the Loading Coil?
Basically, we try to use the one with least inductance and resistance that we
can still get to resonate the antenna.
One way to reduce the size of the required coil is to install what is known as
a capacity hat. A capacity hat can be thought of as an extension of the Marconi
antenna design that adds horizontal elements to the design of an otherwise
vertical antenna to add length. If the capacity hat is designed correctly the
currents in the horizontal sections of the hat offset one another and preserve
the vertical polarization of the wave radiated by the antenna.
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I have bought some DX Engineering Hot Rodz? for our Hustler whips to raise the
efficency. I am going to try them out this weekend.
2) Operate some SSB. You signal might be good if condx are good. If the rate
is not there just
try tuning around and call a few people and then go back to CW. SSB is
mostly for picking up
multipliers.
3) Make sure you operate both 40 and 20 meters during the day. There are guys
in the close in states
that can't hear you on 20 meters that want to work you. There are some
clubs like TCG that
sponsor plaques for the GQP. Thank them by working them on 40 meters. You
should be able to
work some east coast and midwest stations during the day on 40 meters as
well.
4) Check 15 meters every once in awhile. I remember one year K4BAI and I made a
couple hundred contacts on 15 meters in the GQP. This is the time of the year
we start getting some E-Skip.
5) Be SAFE. If you don't have a driver don't operate when driving. Look on a
map or something like Google Maps to find the shortest route between the
counties you plan to operate.
6) Have FUN. Everyone from out-of-state will be looking for you. The mobile
activity keeps up the
interest in the GQP. There is only a finite number of fixed stations for
them to work and you are
giving them a QSO and maybe a new multiplier from every county you operate
from.
73, Jeff KU8E
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