"K7ITM" wrote in message
oups.com...
On Jun 1, 5:35 pm, "Dave" wrote:
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

The reactance of the coil at 3.7MHz is about 375 ohms. Your long
antenna will likely put an impedance across that tank that's on the
same order, or maybe lower, impedance. That will pretty much kill any
resonant peak you got from the coil and capacitor.

You should start, perhaps, by realizing that what you are looking for
is generally not the largest voltage from the antenna, assuming you
can add whatever amplification you want, but rather the best signal to
noise ratio or the lowest distortion. In fact, atmospheric noise is
so large at these HF frequencies that it doesn't take much of an
antenna to pick up more of that atmospheric noise than the receiver
will contribute, assuming a decent receiver. Making the signal larger
just increases the distortion and the chances that strong signals will
cause distortion that covers up the weak ones.

There IS an advantage to having sharply resonant circuits in front of
any gain stages in the receiver, because you can use them to get rid
of unwanted signals, and preferentially select the signals you want to
listen to. But to do that, you need to think more carefully about the
design of the resonant circuit.

You may in fact find that it works a lot better if you couple your
antenna to the tuned circuit (your toroid core and capacitor) rather
loosely, like with very few turns--perhaps a single turn-- through the
core for the antenna connection. Even back in the very early days of
radio, the advantages of a sharp resonance achieved by loose coupling
to a resonant circuit were understood: they used something called a
"loose coupler" that let them adjust the coupling from the antenna to
the tuned tank/detector. For best selectivity, couple lightly, just
enough to get a usable signal. That means coupling the receiver input
lightly to the tuned circuit, too.

Cheers,
Tom


Thanks, Tom,

I ran a single turn of wire from my antenna input through the toriod, and it
does give me much better tuning ability. And WWV at least comes in like
it's in my back pocket. The 1300 UTC broadcast of Voice of Korea was lost
in the noise, but I'm guessing that could just be atmospheric conditions.

Thank you for the input. Any other ideas on how I could sharpen the
resonant peak of the tank circuit?

Dave