Fig. 1212 - Top-loaded antenna. A parallel-tuned circuit, independently resonant at the operating frequency, is required for coupling to the transmitter when the top loading is adjusted to bring a current minimum at the lower end of the antenna.
Fig. 1213 - Capacity and inductance required at the top of a vertical antenna as a function of ground resistance, for a frequency of 1875 kc. These values are sufficiently close for the entire band, when the adjustment procedure described ib the text is followed.
Fig. 1214 - Capacitence of sphere, disc, and cylinder as a function of their diameters. The cylinder length is assumed equal to its diameter.
Fig. 1215 - Inductanc os coils of various diameters wound with No. 14 wire, 8 turns per inch.
Fig. 1216 - A skeleton disc for top loading suitible for 160 meter operation. This disk, constructed with a 4-foot diameter outer rim of quarter-inch copper tubing and wire "spokes" has a capacity of approximately 40 μμfd. Connection should be made to its center.
Top-Loaded Antennas
Instead of bending or folding up the antenna length required for resonance, it is possible to use a simple vertical wire with concentrated capacity and/or inductance at its top to simulate the effect of its missing length. This system is more critical as to frequency - that is, it is not quite as tolerant with respect to working over a band of frequencies - but us structurally advantageous since only one pole is required.
The top-loading apparatus may consist simply of a capacity or, better, a capacity and inductance suitably proportioned. The capacity used is not the usual type of condenser, which would be ineffective since the connection is one-sided, but consists of a metallic structure which exhibits the necessary capacity to space. Practically any sufficiently-large metallic structure can be used for the purpose, but simple geometric forms such as the sphere, cylinder, and the disk are referred because of the relative east with which their capacity may be calculated. The inductance may be the usual type of r.f. coil, with suitable protection from the weather.
The ratio for inductance to capacity depends, for a given frequency, principally upon the ground resistance. Fig. 1213 is a set of curves giving the values for 1875 kc., which is representative of the 1715- to 2000-kc. band. These curves are based on obtaining 75 per cent of the maximum possible increase in field strength over an antenna of the same height without top loading. An inductance coil of reasonably low-loss construction is assumed. The general rule is to use as large a capacity as the circumstances will permit, since an increase in capacity will cause an improvement in the field strength. It is particularly important to do this when, as is usually the case, the ground resistance is not known and cannot be measured.
The capacity of the geometric forms shown by the curves of Fig. 1214 as a function of their size. For the cylinder, the length is specified equal to the diameter. The sphere, disc and cylinder can be constructed from sheet metal, if such construction is feasible, but the capacity will be practically the same in each case if a "skeleton" type of construction, using screening or wire networks, is used. The disc is probably the easiest to make, and has less wind resistance than either of the other two shapes. A disc of the openwork type is shown in Fig 1216.
The bottom of the antenna is fed through a parallel-tuned circuit with one side grounded, as in Fig. 1212. The adjustment procedure is as follows: Starting with all of
L shorted out, adjust the tuning to give satisfactory transmitter input, using the method for parallel-tuned circuits described earlier in this booklet. Measure the field strength by means of a simple field strength meter (vacuum-tube voltmeter and antenna), or by using a receiver, equipped with an S-meter, some distance away. Comparative readings only are needed. Next, move the tap on
L to include a few turns, readjust the coupling parallel circuit tuning to maintain the same transmitter input, and note the new field strength. Continue this process until all of
L is in the circuit. Plot a curve or relative field strength in
L; the curve should rise at first as the turns are increase until a critical point is reached where there is a sudden drop in field strength. Finally, set
L a turn or two just below the maximum point.
