TopBand: Re: Two wire Bevs

[Frank Donovan]

Walt,
The wires in a two-wire Beverage operate in two distinct modes: 

  - Antenna mode, operating in common mode with the two wires
carrying received signals in identical (ideal case) amplitudes and phases
to EACH end of the Beverage.

  - Transmission line mode, operating in differential mode carrying
signals received at the far end termination in common mode back to the
head end in transmission line mode.

The technical challenge is to:

  - At the termination, completely convert signals arriving in "antenna
mode" (ie common mode) into a differential mode reflection for
transmission back to the head end.  To preserve the front-to-back ratio of
the Beverage, the common mode reflection must be at least 20 dB down from
the differential mode reflection.  The reflection transformer must be
appropriately designed to match BOTH the characteristic impedance of the
two wires operating in common mode against ground (typically 400-600 ohms)
AND the characteristic impedance of the two wire transmission line (for 12
inch spacing about 800 ohms). You can use any convenient transmission line
wire size and spacing as long as the reflection transformer is
properly designed for the impedance of the transmission line you actually
use.

  - At the head end, completely separate the common mode and differential
mode signals with enough fidelity to preserve 20 dB or more of
front-to-back ratio.  If the reflection transformer is properly
implemented, the common mode signals will be only the received signals
arriving from the direction of the far end (ie the termination end) of the
two-wire Beverage and the differential mode signals will be only those
that arrived at the reflection transformer from the direction of the head
end.

73!
Frank
W3LPL
donovanf@sgate.com

 On Tue, 26 Nov 1996, Walter Miller
wrote:
> How closely spaced can the wires on a two wire Beverage be without
> compromising performance?  I note that the published designs generally use
> 12 inch spacing.  
> 73, Walt AJ6T


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