ON4UN describes a method of "over swamping" the base of the topless
verticals used in his four-square receive array to acheive wider
bandwidth. On 80M, he switches in additional series resistance 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 helps
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 BSEF (8-circle) and have a test
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 around 20 kHz.
73 Eric NO3M
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|