[TowerTalk] Re: BC Band Selectivity
K7GCO@aol.com
K7GCO@aol.com
Tue, 9 May 2000 12:52:32 EDT
BC Band Selectivity Circuits That Work
There are those who feel very high Q is needed to get selectivity. Yes
and no. It's highly desirable and I explained some Hi-Q circuits for ham
band use previously and the use of directive ham antennas. There are ways
around it. I and a friend had on an almost identical problem of adjacent
interference of a strong local BC station 10 KHz lower, We wanted to hear
KKHI on 1550 KHz in San Francisco which played classical music 24 hr a day
and it would fade in an out. When out the local on 1540 KHz 20 miles away
was in. He made up a very simple single transistor preamp with regenerative
feedback that was just right so it amplified 1550 KHz only enough to
eliminate interference of the stronger adjacent signal on 1540 KHz. I have
it here some place and it could be tuned on 160M I'm sure
I then made up a passive circuit with a BC toroid bar and coil tuned with a
BC variable with direct external antenna connection to the toroid tank
circuit and a direct receiver connection to the "A" terminal. It's
selectivity was so good it did almost as well. Simplicity was adequate. I
was going to mount this in a tube and make it a no direct connection tank
circuit. Someone in the BC Band Listening Club already makes one that is
about 1" square and 8" long with a small variable Xc inside. I use it out
hunting to hear weak signals from Seattle for programs I want to hear or to
reduce interference.
In addition to the selectivity it had, there is a simple trick to reduce the
strength of a strong signal on the frequency or 10 KHz off. Merely orient
the toroid bar's null in the right direction. This gives the most benefit.
So with the ultimate in simplicity I had the desired selectivity without
super HI-Q circuits that some feel is needed. Innovate!
Bob Crane at 111 800 522 8863 sells a 12x2" tank coil with a variable Xc for
the BC band for about $65 that has a bit more gain but it is much larger.
Another of my long winter month projects is to do this. Take 4 of the BC
band toroids and Litz wire coils, series parallel them (same inductance) with
a BC variable across it for more capture area, lower losses and see what
happens. If that works I'll try 16 of them series parallel. I think I can
still tune them at 1900 KHz and it's Q hopefully will be higher. If not I
will add some inductance in series.
Feb 2000 QEX I think it was had a similar idea of taking several long round
toroids and series paralleling them in a tube and then winding a coil over
them. That should be more sensitive and equally as selective if heavy enough
wire is used. It's small and compact.
I've heard that Litz wire efficiency is less around 2 MHz. Litz wire is many
strands of about #40 enameled wire. For the same diameter solid wire, Litz
has more surface area therefore less loss in the BC band where it is used
extensively wound on these toroid bars. Apparently as the frequency
increases above the BC band the capacity coupling between the wires has the
affect of reducing the surface area. That being the case I will make a
progressively larger diameter Litz type individual wires to reduce the affect
and see if I can get less loss than the same diameter solid wire. I think
the average capacity between the larger diameter wires will be less and a
reduced loss happy medium stranded wire can be made although it will be
larger in diameter but hopefully less than solid wire of the same loss.
K0FF sent the material below for those who really want to do it right. It
has great great directivity, regenerative gain and selectivity just as I
recommend in my previous post for use in the Ham bands. I have recommended
to the designer he make one that covers 160M but hasn't so far. k7gco
>>In a message dated 08.05.00 17:22:19 Pacific Daylight Time, K0FF@ARRL.NET
writes:
<< here is one that uses regeneration! Geo>K0FF
here is one that uses regeneration!
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