Rod,
Unfortunately, with antennas as in many other things in life, there is
usually no free lunch! As Misek points out in "The Beverage Antenna
Handbook," the end termination of a two wire Beverage may take many forms,
BUT it must be properly designed to accept the common mode signals
received by the Beverage antenna and convert them to transmission line
mode signals reflected back to the head end of the two wire Beverage. The
simple open circuit/short circuit termination is a simple but COMPROMISE
termination that CANNOT fully meet the requirements for
properly terminating a two wire Beverage!
The open circuit/short circuit termination is perhaps the simplest
termination for a two wire Beverage; however as Misek points out, this
simple termination requires an unachievable 4:1 ratio between the
transmission-line-mode impedance and the antenna-mode impedance of the
two-wire line. A compromise 2:1 impedance ratio can be achieved with 12
inch spacing between the wires. Wire spacings less than 12 inches produce
impedance ratios approaching 1:1 which significantly degrade Beverage
performance because of severe mismatch between the antenna and
transmission line modes produced by the simple open circuit/short circuit
termination. Spacings much wider than 12 inches produce very inefficient
transmission line operation because the individual wires of the two wire
line interact severely with the nearby earth.
Harold H Beverage, the inventor of the Beverage, understood this "no free
lunch" principle of antenna design, and his famous two-wire Beverage
antenna design used an optimum reflection transformer termination! The
reflection transformer termination can be easily designed to perfectly
match the antenna mode and transmission line modes of the two-wire
Beverage. For a discussion of reflection transformer design see Victor
Misek's "The Beverage Antenna Handbook" volume 2 page 55.
73!
Frank
W3LPL
donovanf@sgate.com
Roger Graves wrote:
> Your discussion of the 2 wire Beverage accords well with that in the
> Vic Wisek book except that (at least as I recall) your emphasis on a
> reflection transformer. Wisek's 2 wire Bev's just have one wire
> grounded, one non-terminated to create the far end mismatch.
> Could you give me a reference for how to design far end transformer?
> Thanks & 73,
> Rod, VE7FPT
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