On Feb 18, 3:12*pm, Richard Clark wrote:
On Wed, 18 Feb 2009 12:06:09 -0600, Frnak McKenney

wrote:
What has puzzled me is that I have run across designs that use
(e.g.) a JFET isolation amplifier hooked to a whip or hunk-o-wire
with the statement (or implication) that this is done to ",atch the
antenna's impedance".

Hi Frank,

Matching provokes heated debates that in times past ran to 600+
postings - few knew what they were arguing (but enjoyed arguing
nonetheless) and little was offered.

A JFET at these frequencies does satisfy the naive requirements of
"matching," but that giving you a reception solution doesn't always
follow.

So I;ve been trying to figure out how to
calculate/estimate what it would be, without much success.

The Mohican schematic says quite planely Hi-Z input. *This is borne
out by the antenna connection feeding a tank circuit in the front end
where the input stage is fed from a low tap into a 470 Ohm resistor.
This would be your JFET feed Z, but you could choose any suitable
close value. *As for the input Z, the JFET input resistance is
perfectly capable of mismatching horribly high - although this is not
about optimal power transfer at these impedance levels. *What is at
risk, is the JFET input capacitance which could present a low Z at
some frequency. *Naturally, you select your JFET against this to
optimize. *It will be in some ratio to the antenna capacitance (if it
is bare, short wire) and that will establish the proportion of signal
that gets in by divider action.

I imagine that the inductance of a 6' extension cord (not plugged
in, just dangling from a planter hook grin!) is down in the
uH-or-less range, which would mean that most of the "tuning"
inductance would have to be supplied to achieve 60kHz. *I have this
image of a big (tens of mH) inductor in series with a moderate
capacitor and my (electrically) short wire;

That is one way, other ways work too and are electrically equivalent.
Loops help tune and match by a slightly more elaborate means, but
still fairly holds to simple requirements.

You don't need wire to build an inductor. *At these frequencies you
can use a capacitor in a Gyrator design.

all of the surrounding
EM sets the electroncs in the wire to dancing, but the series RC
blocks those which are wiggling "off-key" (e.g. *not dancing at the
"proper" rate of 60kHz).

And this responds to the filtering capacity (selection AND rejection).
This is called "Q" which also serves the yeoman's task of matching as
well (observe the input tank design for the conventional bands).

Well, there's no question that I have EM in the area. I hooked my
DVM -- set to ACV -- between the radiator and my 6' extension cord;
would you believe 8-10V??!! Not much current, though: feed it
through a 1k resistor and measure the voltage across it, suddenly
it's down in the mV range. grin!

Still a lot of power. *However, those are probably 60Hz fields because
DVMs rarely have the AC BW to go much above 1Khz.

The Mohican came with two 12V power "modules" which plug into the
back of the unit. *The AC power module has a transformer with a
12V-12V center-tapped secondary, which is good, but then they run
the line voltage out of the module and down into the receiver's
volume control's on/off switch. *The module's 12V power and 120V
switching connections are done through a 9-pin tube socket with
mating connector/cable; remember those? *grin!

Remembering isn't difficult. *I broke into electronics through
TV/Radio repair during high school. *If I could fix it, I got paid.

Anyway, power connections from that era brought "ground" notoriously
close to lethal if you plugged the radio into the wall wrong. *Some
used AC noise reduction circuit design that could almost guarantee
your chassis was floating at 70V if things went wrong.

I had an ET striker (Navy parlance for a student electronics tech) who
connected a TV antenna input to ground, and the insulation melted off
of the wire in a heartbeat. *This was in the day when we called TV
power line interlock replacements "suicide adapters."

73's
Richard Clark, KB7QHC

Its not uncommon to have a high impedance input into a preamp. This is
the one-size- fits-all
approach. While its not good engineering for the purist it works quite
well to make a casual user happy and may be the practical solution for
even the professional installation..
Ive had some experience limited working with VLF and it always seemed
the thing that made the difference between a good and bad VLF
antenna was the quality of the ground network

Jimmie