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Theory of Operation
When an antenna is shortened from its natural resonant length, the feedpoint becomes capacitive. To offset
the additional capacitance, it is common to add inductance in the form of a loading coil to restore the
resonance. However, the additional inductor increases the resistance, which increases the loss of the antenna.
Hot Rodz solve this problem by using a capacity hat - a series of horizontal rods that counters the effects of
shortened vertical antennas.
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. The currents in Hot Rodz® horizontal elements offset one
another and preserve the vertical polarization of the related wave.
Antenna Length and Performance
The more the antenna is shortened, the larger the coil necessary to compensate. However, the larger coil
increases the resistance and consumes more power, resulting in greater loss. The loss resistance is also seen
at the feedpoint and is partly responsible for making it easy to match this shortened antenna. to the 50 Ω
feedline and radio. The base impedance will drop to about 16-22 Ω after the Hot Rodz® system is added.
We suggest using a mobile matcher, such as the
DXE-MM-1 -
Dual Impedance Transformer, to ensure your
system operates at optimum efficiency.
The impedance at the feedpoint consists of the radiation resistance, coil loss and ground loss, with the
radiation resistance of a full size vertical antenna equal to about 35 Ω. As the antenna is shortened, the
radiation resistance decreases, but the other losses remain or even increase. To reduce the loss in a loading
coil, we try to use one with the least inductance and resistance that still resonates the antenna.
Greater Efficiency at Half the Size
If the base impedance is reduced from 50 Ω total to 20 Ω total when the actual radiation resistance of the
antenna is 3 Ω, then you will have more than doubled the RF power coming out of your antenna by raising
its efficiency from 6% to 15%.
Half the Length on the 10-80 meter Bands
The Tarheel model 200A screwdriver antenna uses a 6-foot whip for the 10-80 meter bands (3.1 - 28.5
MHz). When combined with a Hot Rodz® system with three 12-inch rods, a 22-inch mast and a safety
spring, the total of the antenna is 6-feet 11-inches, less than half the size of the standard 12-foot Tarheel. This
makes an enormous difference in how often you have to get out of your vehicle to shorten the antenna.
The screwdriver antenna covers large potions of the HF spectrum with just three 24-inch rods, but antenna
efficiency increases with a greater number of rods. Using more (or longer) rods is like extending the whip on
a conventional antenna. The reason is that Tarheel and other screwdriver antennas are examples of the art of
engineering for the mobile environment. Due to design considerations for screwdrivers, the Q (a measure of
efficiency) of the coil is low - approximately 75-100 - and the smaller the coil, the better.
