This project is a part of all the madness in "pre-conditioning" incoming radio signals - before
they get to your receiver. This phase is still very much in the R&D stages. Initial testing is
positive, but much more has to be done in terms of final component selection and circuit
functionality and flexibility.  

This is actually a "blanker" not a noise filter or noise reduction system. When noise is detected
on the feedline, the entire circuit is shunted to ground (earth), thus keeping the receiver's AGC
circuitry minimally affected.
Dynamic Noise Blanker
Dynamic Noise Blanking:
The blanker utilizes a pair of fairly high-speed diodes (silicon) across the incoming signal path. The basic principle  
is not unlike the traditional, standard Noise Blanker circuits found in many Shortwave receivers, but taken to a level
which meets the practical limits of modern receivers' tight AGC circuits, high-ratio IF filtering, improved dynamic
range and IP3, improved MDS (noise floor), operational stability, and overload recovery time.

External Signal Pre-conditioning: After some initial testing and circuit analysis, I have come to the very strong
conclusion that noise blanking (and noise management in general) should be approached, first as a precept of
"signal pre-conditioning." That is to say, prior to entering the receiver's circuitry. Noise, particularly impulse and
other transient signals, can, and usually do, exceed the dynamic range of sensitive front ends, shift solid state
junctions out of their linear operating range, overload AGC time constants and recovery times, and cause severe
ringing in high-Q IF filters. Add to all this the fact that the nature of the actual noise signal is enhanced (yes, it
becomes much worse!) as it progresses along the receiver's stages. Observing a transient on a scope shows that a
typical 3 to 5 uSec spike will be stretched over the (real) time base, many times over, as it moves through the
receiver's chain. This is due to group time delays, phase shift, and ringing. Of course, during this period, intelligable
signal information is lost and usually many operating parameters are shifted far beyond their designed operational
dynamics.

The Design Phase: I have added to the familiar "silicon switch" configuration, a three position, selectable biasing
"sensitivity" circuit (bias voltage divider) and a corresponding "threshold" (fine tune pot) control. This circuit allows
the clipping (blanking) diodes to be pre-biased toward conduction at any level from a few millivolts to well past full
conduction (> 0.6V). Additional "depth" and "width" (duration) control may be added, if their functionality and utility
appear to warrant it.

The depth control would provide a selectable amount of blanking, from 100% to (say) 10% and the width control
would integrate the turn-on control voltage to provide user selectable "hang time." I am still toying with those
function's validity and utility.

Prototype Testing: The biggest part of the R&D right now is (oddly enough) standardizing on a good broadband
noise source that would simulate lightning crashes, ignition impulse noise, and other transients. I have begun the
evaluation using a defective electronic ballast which produces very broadband hash across most of the LF, MF, and
HF spectrums. I'm getting S9 +30dB signals from (about) 30kHz to 15 MHz. It's alright for the initial testing, but I'll
need to refine my "noise generator" before finalizing the DNB. Next step is to put the ballast way out in the back yard
at the end of a 100 foot extension cord and let it transmit into one of my antennas - hope the neighbors have a good
sense of humor!
This is something like how the final project will look. The panel and chassis is the same as the "Type-5" preselector.
You can see those pages
here.
This is a refinement of the two Noise Blanker circuit (panel above). On the older one, No. 1 is active/passive with
adjustable setup parameters. No. 2 is a dynamic version where the clipping diodes are biased so that the threshold
can be adjusted for when the "hole puncher" circuitry kicks in. This improved version utilizes only one of the 4-group
modules shown above, so maybe the ANL grouping will go away.
The head end is a very simple protection circuit allowing for a variable amount of bleed and flash-over (see below).
These functions are really not (absolutely) necessary as "user adjustable" controls, but for maximizing the system for
use with various receivers and antennas, I elected to make then front panel controls rather than internal adjustments,
because I have had issues with front end protection in the past.
There are some very orthodox, time-honored, rudimentary ways to deal with the DC static electrical charge that
builds up on antennas, and the RF electrical, near-field, transients that become induced into the antenna system
(especially from nearby lightning strikes). Although this would apply to any antenna, especially those which are
outside a building, for end fed antennas in particular, this application is more that an option - it's an absolute
necessity - for listening convenience, equipment protection, and of course, personal safety. However, these features
are usually built into a receiver, are of fixed value, and pretty much transparent to the user.
Most of today's solid state radios have
some form of antenna input protection for
static transients and buildup. But not all.
Many portable receivers have had their
RF input transistor(s) fried from the simple
act of someone touching the portable
whip antenna. This seems to be more
common during the dry winter months
when humans tend to easily generate
static charges.
A basic "bleed" system in the
time-honored tradition (long before
transistors) was to shunt the antenna
terminals with a 500K or 1 meg resistor.
Here, we are just utilizing the basic
functions to their minimum and maximum
functional applications. A quick study or
the circuits will reveal their utility.
I have conducted many, many experiments and recorded my
observations over the past few decades with end fed, high-impedance
antennas (as well as other antenna types) and fully appreciate the
protection afforded by a few components, which cost a couple of
dollars, as compared to the inconvenient, if not expensive and
frustrating damage repair to a receiver's front end.
Even with my latest end fed (quasi-Marconi) antenna, which I use
almost exclusively for SWLing (and some 160 transmitting), there are
significant "bursts' of unwanted junk getting through to the shack. The
antenna is matched (and protected) with a very heavy duty 9:1 balun
(wired as an unun). This configuration provides a nice low impedance
path to ground  for static buildup on the wire. However, nearby bursts,
which are rich in RF component and harmonics do pass through the
balun, as RF energy, and on down the line to the radio. At one point,
before the balun was in place, I measured the number of times an NE2
neon tube flashed during a gentle, dry, quiet, winter snowfall. I coupled
the NE2 to a photocell, which triggered a transistor switch attached to
a digital counter. The counter registered 387 counts over the course
of 4-5 hours. It should be noted that the NE2 would flash over at about
85 volts. One night, just prior to a summer thunder storm, we
disconnected the long wire antenna down at the club. It is a 265
foot-long beauty. The PL-259 was removed from the switch box and
brought over to our ground bus which has a series of SO-239s all tied
to the shack's common, bonded ground system. As the connector
approached the ground panel there was a brilliant flash of blue/white
light which arced more than 1 inch - there was also a very loud, sharp
snap ... somewhere in the 130 dB SPL area!!! The club members
present that evening are now all believers in the power and
speed-of-buildup of static electricity on and antenna ... especially a hi
Z end fed!
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Above and below: Two possible panel layouts.
Left and Right closeups.
I'm still experimenting with the
Depth control. I may find that
using a cap-coupled adjustment
(to ground) will be more
effective than one that controls
RF and DC bias. I'll modify this
schematic as testing develops.

Also, I decided that a single
diode junction on each half is
sufficient and have abandoned
the 3-junction version that was
in the ANL project shown here
previously - it's just way too
much over kill.
Updated Oct 19, '08
NOTE:
The Dynamic Noise Blanker circuit shown on this page is strictly a starting place for
further prototype development. At this point, time constraints have limited our
ability to develop the circuit to its most efficient and effective form. One example is
the need for faster diodes, such as BAT-42 and a few other BAT-type junction
switches. Also of concern is junction leakage due to device capacitance. Chokes
must be at least 200 in Q-factor (Xl / R). Feel free to copy the original design and
experiment with it.