Hi, Tim. grin!

On Wed, 18 Feb 2009 02:03:44 -0600, Tim Wescott wrote:
On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney wrote:

Back in December I posted a question about ways to receive LF/VLF radio
signals.
--snip--
First, the need for impedance matching between an antenna and a
receiver.
--snip--
This may be a duplicate answer: I _know_ I wrote one, but it seems
to have fallen into the bit-bucket.

Received and replied to, but your rephrasing is also appreciated.

In short:

For receiving you don't need to couple well enough to the ether to
overwhelm the receiver's noise with the Faintest Possible Signal. You
only need to overwhelm the receiver's noise with atmospheric noise.
Given the amount of atmospheric noise at 60kHz, that ain't hard.

When you get to the point where you hook up the antenna to the rig and
you heard static over the noise of the receiver, you know your antenna is
good enough.

Well, I'm getting static with the built-in whip. On the other hand,
I haven't hooked up the downconverter yet.

(Transmitting is a different story, but try transmitting at 60kHz and
after the FCC gets done with you antenna size will be the least of your
worries.)

Yeah, but the picture of all the local "Atomic Clocks" changing at
once _is_ rather appealing. grin!

Whazza matta widda loop? They work fine, they provide some welcome
selectivity (well, at 60kHz one may provide _too much_ selectivity),
they're easy to construct, they're reputed to reject sky waves -- what
more could you want?

Laziness? A Scot's instinct to thrift? grin!

If you don't want to use a loop, the last time I did anything at MF a
short (1m) whip going to a JFET source follower was considered the bee's
knees to solve this sort of problem. The whip will pick up atmospheric
noise just as well as it'll pick up the intended signal, the JFET will
impedance match from that low-capacity whip to your receiver input
(I assume, I don't know what the nominal input impedance of your
rig is), and all will be well.

Well, from my point of view you just justified the effort you put
into this second post; you got me digging into the Mohinca's manual
which lists sensitivity (10uV) and Selectivity ("3 kc wide at 6 db
down"[sic]), but no specific antenna impedance. According to the
_schematic_, the two screw-lug connections on the rear of the
chassis are "HI-Z" and "GND" (apparently the Heathkit designers
thought of a "short wire" antenna as high impedance as well). The
HI-Z line runs through a "12uuF"[sic] to the whip, and then both are
connected through a 22pF capacitor to the Main Tuning and Antenna
Trim capacitors.

Fortunately the LF upconverter comes (IIRC) with 1MHz and 4MHz
crystals, so I won't be trying to force a 60kHz signal past those
itty-bitty little capacitors. grin!


Frank
--
Ninety-Ninety Rule of Project Scheduling:
The first ninety percent of the task takes ninety percent of the
time, and the last ten percent takes the other ninety percent.
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)