The function of the umbrella-wires is to add capacitance to the top of the antenna, improving the current distribution on the vertical mast radiator to increase the radiation resistance and radiated power.
32.
It can be seen that, since the radiation resistance is usually very low, the major design problem is to keep the other resistances in the antenna-ground system low to obtain the highest efficiency.
33.
These have a radiation resistance which is much greater, closer to the characteristic impedances of available transmission lines, and normally much larger than the resistance of the conductors, so that their efficiency approaches 100 %.
34.
Due to their low radiation resistance they are inefficient, radiating only 10 % to 50 % of the transmitter power at most . with the rest of the power dissipated in the antenna / ground system resistances.
35.
If the feed point is distance " x " from one end of a half wave ( " ? " / 2 ) dipole, the radiation resistance relative to the feedpoint will be given by the following equation.
36.
This is fortunate, since antennas at lower frequencies which are not rather large ( a good fraction of a wavelength in size ) are inevitably inefficient ( due to the small radiation resistance R r of small antennas ).
37.
Small antennas are characterised by low radiation resistance and relatively high reactance, so that a tuning component must be added in series with the antenna to cancel its reactance and assist matching to the circuit to which it is connected.
38.
The cause of these losses is related to the relatively low radiation resistance of an electrically short radiator with respect to the r-f resistance of the ground system and matching networks, all of which are in series with the antenna current.
39.
Often the ground plane rods are sloped downward toward the ground, which improves the radiation pattern and increases the normal 36.8 ohm radiation resistance closer to 50 ohms to provide a better impedance match with standard 50 ohm coaxial cable feedline.
40.
"But another stress _ dehydration _ afflicts Deinococcus radiodurans and also causes massive DNA fragmentation, " and so the " radiation resistance of the bacterium may be a serendipitous result of its ability to repair its DNA after severe dehydration ."
