I have built a homebrew MFJ-1020A (tunable active antenna) which works
reasonable well, but am not satisfied. I am wanting to incorporate a Q
multiplier in it for the purpose of narrowing bandwidth of the received
signal, as well as increasing sensitivity so I can pick up weak stations
better (like Voice of Korea, which for me is right on the edge of
intelligability). The circuit I am working with uses an Armstrong
Oscillator set to *almost* begin oscillation, with its inductor being a loop
in parrallel with the loop antenna I have hooked up to the 1020A, as well
another coil of wire which feeds the preamplifier in the 1020A. My loop
antenna has four turns of about eight inches diameter, and I have enough
wire for the preamp pickup to have 25 or 26 coils, but am not sure how to
construct the single coil used for the Armstrong Oscillator. Should I have
a single coil parrallel with all the rest (both tuner and preamp pickup) or
should I wrap wire around the other two sets of coils, toroid-style, and
make that the coil for the Armstrong Oscillator? And should I use very thick
wire (like 10 guage multi-strand) or something thinner (like 24 guage single
strand)?

I really have no idea which approach would be more effective for the
Armstrong Oscillator, and would be grateful for any input the group has.
Just for the record, the Q-multiplier I am trying to incorporate is taken
from Joe Carr's Practical Antenna Handbook, and is also in his book called
Antenna Toolkit.

Again, any help would be appreciated. I really am (obviously) an RF newbie.

Many thanks.

Dave

"Dave" wrote in message
...
I have built a homebrew MFJ-1020A (tunable active antenna) which works
reasonable well, but am not satisfied. I am wanting to incorporate a Q
multiplier in it for the purpose of narrowing bandwidth of the received
signal, as well as increasing sensitivity so I can pick up weak stations
better (like Voice of Korea, which for me is right on the edge of
intelligability). The circuit I am working with uses an Armstrong
Oscillator set to *almost* begin oscillation, with its inductor being a
loop in parrallel with the loop antenna I have hooked up to the 1020A, as
well another coil of wire which feeds the preamplifier in the 1020A. My
loop antenna has four turns of about eight inches diameter, and I have
enough wire for the preamp pickup to have 25 or 26 coils, but am not sure
how to construct the single coil used for the Armstrong Oscillator. Should
I have a single coil parrallel with all the rest (both tuner and preamp
pickup) or should I wrap wire around the other two sets of coils,
toroid-style, and make that the coil for the Armstrong Oscillator? And
should I use very thick wire (like 10 guage multi-strand) or something
thinner (like 24 guage single strand)?

I really have no idea which approach would be more effective for the
Armstrong Oscillator, and would be grateful for any input the group has.
Just for the record, the Q-multiplier I am trying to incorporate is taken
from Joe Carr's Practical Antenna Handbook, and is also in his book called
Antenna Toolkit.

Again, any help would be appreciated. I really am (obviously) an RF
newbie.

Many thanks.

Dave



Update: I tried the 26 turns of #10 wire for the input to the preamp, and a
tuning loop of four turns of coax with the shield and center conductor
connected, as per Joe Carr's shielded loop example, and the whole thing held
together by winding #24 wire around the loop toroid style, with that as the
Armstrong feedback loop. Waste of time. Couldn't tune the antenna to pick
up anything. Put it on the test bench and determined that two turns of
shielded coax was needed, in a smaller diameter loop. Then used one turn of
#10 wire for the feedback loop and another for the pickup for the input to
the preamp. It does work, but nothing like I expected or was hoping for. I
can tune the antenna to 10 MHz with my 10-400 pF tuning cap, but nothing
very far to either side of that. And the Armstrong oscillator does it's
job, but just barely. Am thinking I need more amplification for the
Q-multiplier circuit. Currently using an NTE-451, with a gain of something
like 100 or 120 if I remember correctly, and am going to try an NTE-199,
with a gain of 400. May even put two of those in, in a Darlington
configuration. I don't know. Making it up as I go along.

Also realized that I no longer actually have an MFJ-1020A, as I have
replaced the tuning circuit from that schematic with a variable capacitor to
tune the antenna and the Armstrong oscillator. The only thing left from the
1020A is the amplification portion of that circuit. Now trying to tune
everything by tuning the antenna.

Would really appreciate any input anyone has, but will understand if all of
this is too vague. Hope no one minds my posting. It helps me think of what
I want to try next.

Thanks for reading. Sorry if I'm just wasting bandwidth.

Dave

"Dave" wrote in message
...

"Dave" wrote in message
...
I have built a homebrew MFJ-1020A (tunable active antenna) which works
reasonable well, but am not satisfied. I am wanting to incorporate a Q
multiplier in it for the purpose of narrowing bandwidth of the received
signal, as well as increasing sensitivity so I can pick up weak stations
better (like Voice of Korea, which for me is right on the edge of
intelligability). The circuit I am working with uses an Armstrong
Oscillator set to *almost* begin oscillation, with its inductor being a
loop in parrallel with the loop antenna I have hooked up to the 1020A, as
well another coil of wire which feeds the preamplifier in the 1020A. My
loop antenna has four turns of about eight inches diameter, and I have
enough wire for the preamp pickup to have 25 or 26 coils, but am not sure
how to construct the single coil used for the Armstrong Oscillator.
Should I have a single coil parrallel with all the rest (both tuner and
preamp pickup) or should I wrap wire around the other two sets of coils,
toroid-style, and make that the coil for the Armstrong Oscillator? And
should I use very thick wire (like 10 guage multi-strand) or something
thinner (like 24 guage single strand)?

I really have no idea which approach would be more effective for the
Armstrong Oscillator, and would be grateful for any input the group has.
Just for the record, the Q-multiplier I am trying to incorporate is taken
from Joe Carr's Practical Antenna Handbook, and is also in his book
called Antenna Toolkit.

Again, any help would be appreciated. I really am (obviously) an RF
newbie.

Many thanks.

Dave



Update: I tried the 26 turns of #10 wire for the input to the preamp, and
a tuning loop of four turns of coax with the shield and center conductor
connected, as per Joe Carr's shielded loop example, and the whole thing
held together by winding #24 wire around the loop toroid style, with that
as the Armstrong feedback loop. Waste of time. Couldn't tune the antenna
to pick up anything. Put it on the test bench and determined that two
turns of shielded coax was needed, in a smaller diameter loop. Then used
one turn of #10 wire for the feedback loop and another for the pickup for
the input to the preamp. It does work, but nothing like I expected or was
hoping for. I can tune the antenna to 10 MHz with my 10-400 pF tuning
cap, but nothing very far to either side of that. And the Armstrong
oscillator does it's job, but just barely. Am thinking I need more
amplification for the Q-multiplier circuit. Currently using an NTE-451,
with a gain of something like 100 or 120 if I remember correctly, and am
going to try an NTE-199, with a gain of 400. May even put two of those
in, in a Darlington configuration. I don't know. Making it up as I go
along.

Also realized that I no longer actually have an MFJ-1020A, as I have
replaced the tuning circuit from that schematic with a variable capacitor
to tune the antenna and the Armstrong oscillator. The only thing left
from the 1020A is the amplification portion of that circuit. Now trying
to tune everything by tuning the antenna.

Would really appreciate any input anyone has, but will understand if all
of this is too vague. Hope no one minds my posting. It helps me think of
what I want to try next.

Thanks for reading. Sorry if I'm just wasting bandwidth.

Dave



Forgot to mention, I am using a 1K linear taper switched potentiometer to
control the feedback in the Armstrong oscillator. Suspect I need a 10K pot.
As soon as I switch it on, it peaks out, and then drops off as I turn it up.
On the other hand, it does work, even if not to the level I had hoped for.
Wanted to include this for those who wondered about the construction of the
whole antenna/feedback loop/input to the preamp loop package and how
feedback was controlled.

Again, sorry if I am just wasting bandwidth. Appreciate your patience.

Dave