"Uncle Peter" wrote in message
...

"Dave" wrote in message
...

"Uncle Peter" wrote in message
...

"Dave" wrote in message
...
Has anyone ever heard of anything like the experiment I am currently
concocting? I have taken an Amidon T50-2 toroid form and wrapped it
with 57 turns of #32 enamaled wire, giving it an inductance of 16 uH,
then wrapped over that layer with 150 turns of #36 wire. The first
layer (16 uH) I am using for tuning my home-built active antenna, and
am relying on resonance to send a signal through the outer layer, which
I *think* should act like a step-up transformer for that signal. This
layer is grounded on one end, and feeds into the first stage of
amplification on the other. And it does seem to work, only the toroid
seems to resonate at around 10 MHz with my tuning capacitor set to 114
pF, when it should (I thought) resonate at something lower, like around
3.7 MHz.

And like I said, the step-up transformer part does seem to work, but I
can't tune as low as I would like. Has anyone ever hear of anyone else
trying anything like this? If so, what is it called, and where can I
find info on the subject?

Many thanks for any help...

Dave

What are you trying to match? I suspect your secondary winding has
a lot of interwinding capacity that might be affecting it's operation.
For a ""step up", it might be easier, and more efficient, just to design
a pi or L matching network to do the transformation... Or, use
tapped windings on the coil used in the tuned circuit. Low impedance
at a lower tap point, impedance and voltage will increase on higher
taps..

Pete



Hello Pete,

I'm not actually trying to "match" anything, I am attempting to step up
the voltage of the incoming signal from my antenna (a 110' longwire use
for SWL) before passing it to my first stage of amplification. I think I
see what you mean however, this sounds like an impedance matching setup.
And I hadn't thought about the interwinding capacitance. Obviously, I am
still fairly new at this.

I saw a schematic in Joe Carr's Secrets of RF Circuit Design that used a
step-up transformer before the first stage of amplification, and that
gave me the idea for using my tuning inductors (toroids) with a second
winding that might act as a step-up transformer. And it does seem to
work, just not withing the frequency range I was expecting. My goal is
to pick up Voice of Korea here in Texas, at some level of
intelligability. And I am a lot closer to that than I was with the
plain-jane tuning setup. Today I picked up their 1500 UTC broadcast,
which is something I have never been able to do before. (Talking about
how happy Korean children are...) It is still far from perfect
however...

Thanks for your input. More food for thought... With time, maybe I'll
get what I am looking for.

Dave



For a broadband step up, you'd be better off using a ferrite core untuned
transformer or balun.

Are you using this between the coax and a coax leadin? Or, between the
antenna and RF antenna jack on the receiver?

Pete

Pete


I am doing this inside of the active antenna case, using the coils used for
tuning the tank circuit. It's really kind of an unusual setup, I gather. I
am trying to use the resonance of the coil, which is in parrallel with my
tuning capacitor, to set up a sympathetic signal in the second layer on
that same coil, and feed this sympathetic signal to my first stage of
amplification. That's the step-up transformer part. The whole thing just
resonates at a higher frequency than I anticipated. Thus my original
question: has anyone heard of this before, and what is it called/where can I
find more info on the subject. Fortunately it still allows me to pull in my
target shortwave station, but it's at the bottom of the tuning range rather
than the top.

Thanks for your help,

Dave

"Dave" wrote in message
...


I am doing this inside of the active antenna case, using the coils used
for tuning the tank circuit. It's really kind of an unusual setup, I
gather. I am trying to use the resonance of the coil, which is in
parrallel with my tuning capacitor, to set up a sympathetic signal in the
second layer on that same coil, and feed this sympathetic signal to my
first stage of amplification. That's the step-up transformer part. The
whole thing just resonates at a higher frequency than I anticipated. Thus
my original question: has anyone heard of this before, and what is it
called/where can I find more info on the subject. Fortunately it still
allows me to pull in my target shortwave station, but it's at the bottom
of the tuning range rather than the top.

Thanks for your help,

Dave


Hi Dave

Then in effect, you are trying to match a lower impedance to a higher one
since you are trying to increase the voltage gain.

Without knowing the active antenna you're attempting to use, it is kind
of hard to determine what advice to offer. I think you mentioned a 100 foot
long wire... That would have a much lower impedance over the HF range
than a 3 foot whip. If you just want voltage gain, use several turns on
the bottom of the coil for coupling the antenna to the tuned circuit.
The tuned circuit will provide the voltage gain. You will reach a point
where the active antenna will be prone to overload, even with the tuned
preselector input.

Pete

"Uncle Peter" ) writes:

Without knowing the active antenna you're attempting to use, it is kind
of hard to determine what advice to offer. I think you mentioned a 100 foot
long wire... That would have a much lower impedance over the HF range
than a 3 foot whip. If you just want voltage gain, use several turns on
the bottom of the coil for coupling the antenna to the tuned circuit.
The tuned circuit will provide the voltage gain. You will reach a point
where the active antenna will be prone to overload, even with the tuned
preselector input.

I always thought the concept of an "active antenna" was that you had a
short whip and barely loaded it down. The whip itself is connected
to a very high impedance point, and then the active components are
there to transform that into a lower impedance for the receiver.

You basically see the idea in old car radios, where the whip was
connected across the tuned circuit at the input. If you didn't use
the right cable to connect it to the antenna, and if you didn't use
the trimmer to adjust things to compensate for that cable, you'd
lose a lot of signal.

Then solid state devices came along, and that made it much easier
to do the transformation, since they took up less space and didn't
need filament voltage.

A preamp or even preselector is not the same thing as an "active
antenna".

Michael VE2BVW

"Uncle Peter" wrote in message
...

"Dave" wrote in message
...


I am doing this inside of the active antenna case, using the coils used
for tuning the tank circuit. It's really kind of an unusual setup, I
gather. I am trying to use the resonance of the coil, which is in
parrallel with my tuning capacitor, to set up a sympathetic signal in
the second layer on that same coil, and feed this sympathetic signal to
my first stage of amplification. That's the step-up transformer part.
The whole thing just resonates at a higher frequency than I anticipated.
Thus my original question: has anyone heard of this before, and what is
it called/where can I find more info on the subject. Fortunately it
still allows me to pull in my target shortwave station, but it's at the
bottom of the tuning range rather than the top.

Thanks for your help,

Dave


Hi Dave

Then in effect, you are trying to match a lower impedance to a higher one
since you are trying to increase the voltage gain.

Without knowing the active antenna you're attempting to use, it is kind
of hard to determine what advice to offer. I think you mentioned a 100
foot
long wire... That would have a much lower impedance over the HF range
than a 3 foot whip. If you just want voltage gain, use several turns on
the bottom of the coil for coupling the antenna to the tuned circuit.
The tuned circuit will provide the voltage gain. You will reach a point
where the active antenna will be prone to overload, even with the tuned
preselector input.

Pete



Hey Pete,

The active antenna is a home-built MFJ-1020A, which has a short whip and an
input for an external antenna. The amplifier has the gate of a JFET
(2N5486) as the input for the first stage of amplification. I can post a
schematic to ABSE if you like.

After reading your last post, I think I am beginning to understand what you
mean, about matching impedances. This subject has always been a tough one
for me, but it interests me, and now I have a reason to study and learn it.
I had no idea that was what I was trying to do.

So, it sounds like I am trying to match a low impedance signal source (the
longwire antenna) to a high impedance input (the amplifier.) Is that
roughly correct? If so, any idea on where I can find more info on impedance
matching?

Thanking you very much for your help...

Dave