11

168

WIRELESS TELEGRAPHY

The portion of the total decrement due to the earth currents may at
times be as large as the radiation decrement. Even in spite of the greatest
precautions in grounding, in the attempt to keep this portion of the
decrement at a minimum the results will depend ultimately upon the
nature of the soil. Such results as can be obtained at sea are probably
never attained over poorly conducting ground.153

Induced currents come mainly into question in guys, stays, iron masts
and similar rnetal parts on board ships, and in the towers supporting the
antennae and their guys in land stations. Experience has shown that
these currents, which always mean a waste of energy, may harm the
radiation considerably and be generally detrimental. A method of coun-
teracting the bad effect of these currents is to insert insulating links in the
conductors affected, or, in any case, insulating them from ground. This
was very well provided for in the old Nauen antenna [Art. 92c]; the only
conducting parts in which currents could be induced were the three guys
holding the tower and these were well insulated from the tower at their
upper ends and from the ground below.159

It is well known that the brush or leakage discharge, which at night is
visible over a large part of the antenna, has a very bad effect upon the

FIG. 207.

decrement; it is therefore important to avoid sharp points and edges in
the aerial. As increased surface (larger radius of curvature) for the
conductors tends to reduce the brush discharge, it has been proposed to
surround the antenna wires by metal piping or tubing joined conductively
to the wires (Pig. 207), or else to use metal bands or strips, preferably
having rounded edges, wound around rope, as the aerial conductors.
The use of well-insulated high-tension cable instead of bare wire is
perhaps even more effective. Specially designed insulators1™ to prevent
brush discharge are frequently used at the ends of the wires.

Circuit losses* may of course occur in any oscillator such as, e.g., a
condenser circuit. They have not been previously discussed for the rea-
son that they are easily prevented in all other forms of oscillators and
hence are of no importance when ordinary precautions are taken. With
antennae as used in radio-telegraphy, however, the prevention of circuit
losses, in view of the high potentials involved and the severe weather
effects156 is a much more difficult matter.

100. Determination of the Decrement.—a. Any of the methods
already given may be applied to find the total decrement of the natural
oscillations; a quenched gap circuit offers a suitable means for excitation.

* lliis is intended to include losses due to spark discharges (to ground, etc.).

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