To All from PA0SE,

By chance I came across an interesting article in the Dutch weekly (!) magazine Radio Expres of 11 January 1934. It referred to an article in the Proceedings of the IRE of December 1933 by Nickle, Dome and Brown on a procedure to increase of the feed point impedance of a vertical antenna by means of top loading. A normal way is to use a series resonant circuit as in fig.1, consisting of a capacitive hat and a coil. The coil is in a awkward position for tuning. The alternative by Nickle, Dome and Brown is to use a short circuited stub slightly shorter than an quarter wave that at its top end produces the proper inductive reactance for series resonance with the capacitive hat. Indicated in fig. 2a. The system is tuned by moving the short circuit at the bottom up or down. The left leg of the stub is doubling as the radiator. An easier way of tuning is to make the stub too short and lengthening it with a coil(b) or a stub that is too long and shortening it by a capacitor (c). I modelled a quarter wave vertical for 80 m according to (c) using Antenna Optimizer by Brian Beezley, K6STI. When in resonance the feed point impedance was raised from 36 ohm without top load to 4925 ohm! The radiation diagram was identical to the one for the unloaded vertical within a few tenths of a dB.

It occurred to me that the system could perhaps also be used for a 136kHz vertical. Using system (b) in fig.2 the loading coil between top of the radiator and topload could be at ground level. The stub functions as a transmission line to transfer the inductive reactance of the coil to the top of the radiator. Of course a second coil could be inserted between bottom of the stub and ground for matching and feeding of the antenna.

What do you think?

73, Dick, PA0SE