Preselector circuit and function
evolution has brought us to this.
These six modules (described in
RED)are designed to be mounted
in an equipment rack. Also, any
one may be used alone or in
combination with any other
Preselector Control: is pretty
self-explanatory. It allows the
entire system to be bypassed for
comparison purposes or when not
needed. Signal Mode grounds all
RF activity and thus provides an
RF-Mute. The Antenna module
provides various level and
functions of static protection
utilizing, bleed resistors, Neon
bulb discharge, MOV supression
and other forms of Transorbs.
The Preamp has an attenuator,
an input level control to preclude
overload/intermod, and an actual
gain control. The Transmatch is
either a "T" or "Pi" filter which can
tame any "wild" antenna
impedance -- more for the
experimenter than the active
The heart of the system is two modules: The Preselector, and the optional "null" filter which is great for bandstop
functions. In practice, these two units should be of the 12 Ohm or 6 Ohm variety (see previous schematics) since the
overlapping skirts of "pass" and "notch/null" should not occur much above the 10 to 20 dB range, IE: when using
both, you should be able to achieve significant peaking without the nulling affecting the peak as you "slide" them
together in use. Obviously, the sharper the filters are, the closer in frequency, to one another, they may operate
effectively. In actual usage, many times it will only be necessary to utilize the "lengthen" or "shorten" functions in
order to resonate the antenna system ... and bandpass not necessary.
What is still missing here is a control in the middle of these two units, marked "Bandwidth." It would be a simple
matter to take advantage of the internal impedance of the circuit and provide a continuously variable bandwidth
control to the "peak" and "notch" functions. I have tried this, but I still haven't convinced myself of the utility or
practicalltiy of that function.
Preselectors: A Modular Approach