[TowerTalk] Deluxe BC tuner/filter DE K0FF

[K0FF]

Well let me describe this beast, and maybe I'll get the circuit and some
pictures scanned later (by someone else, I have no scanner).

Even though most of this refers to crystal radios, the only difference in
the circuitry is that you actual hook a modern radio to the detector tap.

I am an antique book collector, especially early radio books. In some of the
1920's radio experimenters books, they really got into crystal radios. The
NBS (national Bureau of Standards) even gave a figure of merit to all the
different types hookups being used then. Using this as well as modern info
from the Crystal Radio Society, I set about building each and everyone of
the circuits to see for myself. Keeping in mind that I can see the red
lights on each of the six (6!) towers used on the 5 kW station KHMO (1070
kc) from my shack window...
What I wound up with is a crystal radio/tuner/preselector breadboard that
allows easy experimentation of the various hookups.
It consist of a wooden board about 14 inches wide and 6 inches deep. On it
are mounted three 3-section BC variables from the junkbox, each fitted with
a knob and having their sections brought out to Vector spring clips. On the
back side of the board are mounted 3 coils, two of which are wound on 3-1/4
inch OD PVC plumbing pipe. These are 50T of solenoid wound 28Ga magnet
wire,and tapped every 5 turns. The third coil is 50T of the same wire, wound
on a PVS pipe cap which is 2-3/4 inch in diameter, no taps. This coil is for
the series tuned trap.

There are dozens of configurations available, and I must have tried them
all.

What seems t work best for me is a double parallel tuned circuit.
inductively coupled, tuned above and below the offending station, and a
series tuned circuit to ground, tuned exactly to the offending station. This
makes a very successful crystal radio, and can drive a 2K earphone into the
pain level on a strong station. It is also powerful enough to drive an S
meter directly, and that make nulling out the strong station very easy.

To use as a preselector for an modern AM radio, just play with the last tap
position, and hook it to the antenna terminals.
If you want to use it to notch out AM interference to your 160 or 80M
receiver, change the parallel tuned circuits to tune the desired band, and
leave the trap the way it is. Feed the output the the RX input of your
transceiver via a preamp, and enjoy noise free reception. (I have a 575'
Beverage looking directly at KHMO and have worked DXCC in one season on 160
with it- from the Midwest)

Here's the setup then:
two coils of 50T ea. wound on opposite ends of an 8 inch long piece of PVC
tubing which is 3 -1/4 inch OD, and mounted 2" above the board on threaded
standoffs. Make the windings over a piece of waxed paper, and you can slide
them back and forth, to set the coupling later.The left hand coil need not
be tapped but has a 4T coupling loop wound over the high end. The antenna
goes to the top end and the bottom end is grounded. Also is grounded the
bottom end of the main coil and the rotor of the 365 tuning cap which is
place across the whole of the main coil.

The right hand coil is not in any way attached to the left hand one, except
by mutual coupling. It is resonated by another variable across it's whole
winding. Wound over the BOTTOM, or cold end is a 4T coupling loop which goes
to the next stage.

The bottom of this loop goes to ground, and the top of it goes to the
antenna terminal of the radio. It also goes to the series tuned trap, via a
slide switch (this is so you can judge it's performance). The coil of the
series trap MUST BE AT RIGHT ANGLES to the main coil. Mine is mounted on a
single standoff so that I can rotate it slightly for the exact rejection.

That's it. Even though It take two hands to tune, it's worth the effort..

To use, dip the offending station with the trap and peak the wanted stations
with the other two caps. It may be possible to tune both the parallel
circuits buy using a single cap with 2 sections, but I have not tried that,
because I think there is more flexibility with 3 knobs.

Geo>K0FF


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