[TowerTalk] (no subject)
Sun, 7 May 2000 18:18:03 EDT
In a message dated 01.05.00 11:47:04 Pacific Daylight Time,
> I was wondering what options would be available to filter out one adjacent
> AM broadcast radio station from another. In this case reception from one
> about 60 miles away is desired, but a second one on a close spacing is
> causing some overload of the receiver. Frequencies are both around 1200
> KHz. Thanks for any ideas.
The problem you have and the solution depends on how the
frequency difference and strength difference between the stations.
73, Tom W8JI email@example.com
It also depends on the selectivity of any RF tank circuit that might
exist before the mixer if it has one like with tube receivers. Modern (?)
receivers have untuned mixers and selectivity creation from here on has to be
very sophisticated. The horse (adjacent signals) is let in the barn (mixer)
in the modern receiver and all kinds signal by products occur even when a
strong signal is not near the desired receive frequency. Why doesn't some
smart receiver mfg add the tuned circuit like tube receivers used? Hey some
Mfg., please steal this idea. I think this would be a cheaper solution than
the complex circuitry used now in all the various MULTIPLE IF circuits and
all the fancy filters. I use tuned tank circuits outside present receivers
all the time and it solves many overload problems. It greatly reduces the
amount of signals and noise pounding the mixer input. Directive antennas
help even more. Fixed antennas with variable phasing adds another level of
There was an article in CQ of a passive tuned circuit prior to a receiver
that I really like. It was 2 tank circuits back to back on each band with a
ganged variable Xc across both tanks for peaking in the band. It was a link
input ciucuit 50 ohm input with a 10 pf coupling XC off the Hi-Z end coupling
to the Hi-Z end on a 2nd tank and then coupled out the 50 ohm link to the
receiver. The high impedance ratio step up and then the step down creates
selectivity before it gets into the receiver. Changing the 10 PF Xc +/- also
changes the selectivity. A bandswitch and an out or bypass switch is also
used. When the bands really open up, the so called modern receivers are
going to be heavily overloaded. There are only a few that can handle big
signals now on the ham bands. I have a SX-88 and a Pierson KP-81 all ready
for the selectivity wars coming up.
5 years ago I had a 7 element beam on 10M and the band really opened up on 10
M one day like old times. I got 4 reports of 60/9 on the East coast and many
stations were calling in and complaining about my audio. Would you believe I
gave them audio burns off their audio gain control and RF burns off the RF
gain control? I over loaded their so called modern receivers heavily. Even
locally there are receivers I never overload.
In a 4/41 QST there is a Hi-Q pre-amp for 56 Mc receivers by a LeRoy May as I
remember after all this time of 59 years. I have the article around here
someplace. It uses 2-41/2' lengths of 4" copper tubing with a copper tube in
the middle making a 1/4 wave coaxial tank in the input and output of the very
highest Q possible in a very clever circuit that combines the highest gain
and selectivity. Above 10M, tank circuit loss becomes a factor. Just as
soon as I saw what design idea was presented and realized it's potential, I
zoomed to the last page to see what the claims were. It claimed that it gave
7 S-Units of gain, 2 of noise and 5 of signal gain. If true WOW! Inaddition
the potential selectivity of this circuit, it has to be the highest ever made
and prior to getting into a receivers of the day that had tuned input tank
circuits in them also. I call it "tunnel reception!"
I've heard that during the F layer openings on 6M years ago in Seattle, you
could work 100 JA's in an hour with 10W and many receivers had big overload
problems. This prior selectivity is going to be needed again on all
frequencies. One of my long winter month projects is to build the 4"
diameter tube version with silver plated tubing. I may make some even larger
diameter tubes forming them by using copper sheeting that I will have silver
plated also to increase the Q even more. They were side by side in the
construction. I may have to make them back to back to keep the leads short.
Or taper the tube to a smaller diameter at one end and to the side at the
grid and plate connections. If it works on 6M like claimed it will work on
the lower frequencies and who knows what the signal improvement rewards
await. The copper tubes can get bit long on 10M and below but if I had to
I'd have it outside or in the attic with a remote peaking and gain control.
I should get on this project pretty soon. Of the hundreds of projects I've
done, this one holds the record of the one that took the longest to
start--about 59 years. I didn't get on 6M until about 5 years ago and the
adjacent strong signal Post jogged my memory about this concept.
I have the Millen R9'er RF Pre-amp from many years ago that had plug in coils
for 20-6M. They were small diameter wire coils with a tuning slug and 2
variables for resonating in the box. This was very very Lo-Q tank circuit.
It had a gain control by controlling the screen voltage with a wire wound
pot. I'm going to use these 4" copper tubes as the plug in tank circuits for
the R9'er on 6M and use 1 KW tank coils for 160-40M. I don't think this has
been done before as preamps were seldom made for these frequencies and never
with coils this big. I'll report on the results. If you find a Millen R9'er
in a flea market with plug in coils--grab it. I got an extra one for $10
RME made a tunable RF preamp form 500 KHz to 40MHz that I have and will see
what I can do for that also. If you see one of these grab it also.
Now there is another way to get preamp gain and selectivity. Years ago
regenerative receivers created gain and selectivity by using a small variable
Xc from the plate to grid to create in phase feedback which was progressively
amplified until it took off.
Then I saw this concept used in the pocket BC radios in the 455 KHz IF. This
gave the desired and ample gain and selectivity in ONE IF stage only. A
great design. I then decided that I'd like to try that in RF preamps on
160-6M and made big plans to modify the R9'er and other RF preamps I have.
Gain controls they already had would be needed--instead of attenuators.
I then found in QST about 30 years ago a transistorized RF preamp using this
very idea in a 20-10M circuit--with variable feedback WOW! RF preamps all
used different tank circuits on each band to maintain the highest Q. This
circuit used one coil and variable Xc for 20-10M as the gain was so high and
this compromise was small and simplified the circuit.
So my goal is to combine the very low loss Hi-Q tank circuits with adjustable
in phase feedback and gain controls so as to not overload the input circuit
to the main receiver. This has got to be a killer amp to drag the weak ones
out of the mud. The original 4/41 QST circuit claimed far more signal gain
than noise gain than existing preamps do now and if true that could uncover a
concept that would be very useful on all bands.
All kinds of possibilties. Don't let the signal horse in the barn (mixer) is
the first K7GCO design priority of good design. Stay tuned. K7GCO.
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