ON4UN describes a method of "over swamping" the base of the topless
verticals used in his four-square receive array. On 80M, he switches in
additional series resitance to arrive at a total feedpoint resistance of
300 ohm, then steps down to 75 ohm using a split winding 4:1 (2T:1T
ratio) transformer (also serves to mitigate common-mode). This
effectively increases operating bandwidth. See section 1.28 "The Mini
Receiving Four Square at ON4UN" in Chapter 7 / Receiving Antennas, Fifth
Edition.
I am doing some experiments with a 630M element currently using the same
concept (though it is top loaded). 1.2:1 bandwidth was originally a
mere 5 kHz. With the additional swamping resitance and step-down, it is
now at least around 20 kHz.
73 Eric NO3M
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