Umbrella Antenna

Author: Edmund A. Laport

Figure 1.8 illustrates an antenna of the umbrella type made up of three diamond antennas supported by seven towers and mechanically arranged so that each section can be raised and lowered separately. In turn, each diamond can be divided at the center for sleet melting if required, and therefore the down lead from each diamond has two conductors as shown at 1-2, 3-4, and 5-6, each pair connected in parallel near ground. The cross triatic of each diamond has a low-tension insulator (represented by a dot) at the middle to divide the antenna for sleet-melting purposes.

Let us assume that power is to be introduced at the down lead 3-4. The coupling apparatus will be located under this point. Only the multiple-tuning inductors are located under the other two points. This requires that the flat-tops of the three sections be electrically connected at some point, such as at the center. This could be done by base insulating the central tower for the whole antenna potential and placing the cable winches on the tower above these insulators. Keeping in mind the sleet-melting circuit (if used), three-phase Y connections could be made with the neutral connected to this central tower and the three phases connected to points 1, 3, and 5.

FIG. 1.8. An arrangement of three diamond sections to form a flat-top for a very low frequency antenna.

There is another arrangement that should not be overlooked. The towers I, III, and V could be adequately base-insulated and employed as the down-lead conductors and the tuning points located near their bases. This would eliminate the insulators from the end points of the diamonds, which would be thus connected directly to the three insulated towers. The aerials would be insulated from the other four towers. The desired mechanical flexibility of three separate flat-top sections could be realized very well, but the sleet-melting circuits would be somewhat different. Balanced three-phase power could be applied directly at the base of the three insulated towers. This would be an excellent arrangement for the sleet-melting circuits since, with the wire configuration shown, there would be an optimum equalization of the sleet-melting currents in the wires of the flat-tops.

The procedure of multiple tuning a circularly symmetrical antenna

system, after computing the approximate values, is to adjust all tuning inductances to identical values until there is a condition of zero reactance between ground and the inductance in the power lead. The system will then be adjusted for equality of currents in all down leads.