HF MOBILE ANTENNAS
THAT work better than OTHERS
By Harvey Tetmeyer - K5LJM
EFFICIENT ANTENNAS
work better than
LOSSY ANTENNAS
When conditions are good, nobody knows.
When conditions get bad, everybody knows.
*
LONG ANTENNAS
work better than
SHORT ANTENNAS
Radiation resistance, the good stuff, goes up four times when you double the length of an antenna. The loss resistances usually go down. Go for the good!
*
HIGH-Q COILS
work better than
LOW-Q COILS
On the lower frequencies, coil resistance is usually the main loss. Improving coil Q greately reduces that loss.
*
CENTER LOADED WHIPS
work better than
BASE LOADED WHIPS
Moving the coil up increases the current in the whole whip.
*
TOP LOADED WHIPS
work better than
ANYTHING ELSE
Top loading brings the current up to the highest level possible. The disadvantage is the mechanical problems with a large capacity hat on top.
*
TUNED AND MATCHED ANTENNAS
work better than
UNTUNED/UNMATCHED ANTENNAS
Tuning and matching are both required for maximum antenna current. Tuning and matching can be done anywhere between the transmitter and antenna. The most efficient place is at the antenna, reducing other associated losses.
*
CAPACITY HATS SPACED ABOVE THE COIL
work better than
HATS NEAR THE COIL
Capacity hats near the coil reduce the Q of the coil and add coil loss.
*
GOOD CONNECTION
work better than
BAD CONNECTIONS
Poor connections will reduce power to the antenna and add noise to the receiver.
*
SMALL DIAMETER BASE SECTIONS
work better than
LARGE SECTIONS
Large diamter bottom sections have a higher capacity and shunt off current that would normally go to the whip.
*
AUTOMATIC TUNERS AT THE BASE
work better than
NO TUNER AT ALL
All-band, base mounted tuners are very flexible and will tune and match almost anything. The only ploblem is most of the power is lost in the tuner. High-Q loading and matching coils (or capacitors) are much more efficient than the small inductors in the tuners.
*
LARGE DIAMETER TOP SECTIONS
work better than
SMALL DIAMETER TOP SECTIONS
Larger diameter whips have more capacity than smaller whips and reduce coil inductance and thus coil loss.
*
HIGH MOUNTED ANTENNAS
work better than
LOW MOUNTED ANTENNAS
Don Johnson asks, "Would you rather have your dipole on the roof or under the
porch?" Who am I to argue with 40+ five years of mobiling? I have been at it that
long myself but still won't argue the point!!
*
ANTENNAS NEXT TO THE BODY
work better than
DUMMY LOADS
A ground plane antenna works best at right angles to the mounting surface. When it is mounted low and swings back away from the body, it is a V antenna directing the signal up. When it is next to the body, it is an unterminated open wire feedline. There is no antenna!
*
HORIZONTAL WHIPS
work better than
SHORT VERTICAL WHIPS
A number of motorhomers are using horizontal whips extending over their tow cars and are having excellent results.
*
GROUNDING TO THE SUPERSTRUCTURE
work better than
LONG GROUNDS TO THE FRAME ALONE
The best ground is the metal superstructure right at the base of the antenna. The superstructure is really the other half of the antenna and it is up in the clear. Additional grounding may give further improvements by increasing the size of the antenna.
*
CURRENT BALUNS OR CHOKES
work better than
RF ON THE FEEDLINE
A current balun or choke keeps the current off the feedline and in the antenna. Coax chokes can be made by coiling up approximately 10 feet of the feedline in a lariet fashion. About 10 turns is usually satisfactory. See the ARRL Antenna Handbook for details.
*
LARGE VEHICLES
work better than
SMALLER VEHICLES
The whip is, at best, only half the antenna. The larger the vehicle, the larger the other half of the antenna and ususlly the more efficient. In reality, a short whip is only the exciter. The vehicle is the antenna.
*
ANTENNAS ON THE CORNERS
work better than
ANTENNAS IN THE CENTER
Corner mounted antennas have a longer ground plane than center mounted antennas. On 75 meters, the radials need to be 60 something feet long.
*
UNTAPPED COILS
work better than
TAPPED COILS
Tapped coils ususally have a flying lead to short out part of the coil. Shorted turns lower Q and increase coil loss.
*
REAL HAMS
work better than
LIDS
The bottom line amounts to doing things right. Learn everything you can about
the hobby and your particular area of interest. Get the best equipment for what
you want to spend. Do the best installation you know how. Tune your system from
the power source to the tip of the antenna. Study propogation. Learn and apply
the best operating practices.
Then sit back and enjoy
a most rewarding hobby.
- . - . - - . - Ham Radio
Written and Contributed by:
Harvey Tetmeyer - K5LJM
For Harvey's Home Page
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