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. Based on the suggestions made by a number of people here I
decided to use my existing Heathkit Mohican receiver and add this
upconverter kit from Jackson Harbor:
http://jacksonharbor.home.att.net/lfconv.htm
The kit arrived and was half assembled before I turned on the Mohican,
its first power-up in some years; the horrible squeal that erupted from
the speaker put a bit of a damper on things. It now appears that
replacing the two output transistors (Germanium, no less!) with
NTE102As from Mouser will fix that, so I'm thinking about an antenna
that might be a little more snesitive to LF signals than the Mohican's
built-in whip.
Along those lines, I have a couple of (what I hope are) simple questions
that I'm hoping someone can help me get started with.
First, the need for impedance matching between an antenna and a
receiver. My understanding is that a resonant halfwave dipole will have
an impedance around 73 Ohms; unfortunately, unless I can obtain research
funding from the just-passed Congressional Economic Stimulus bill I'm
going to have trouble paying for 2.5km of copper wire, some towers, a
crateload or two of porcelain insulators,and the land to build it on.
(Hey, I promise to dump it back into the economy ASAP. Really!
grin!)
So any non-loop antenna I can construct will necessarily be a "short
wire" or "electrically small" antenna (two useful search terms). But how
does one go about calculating the impedance of a coat hanger or an
extension cord ("short piece of wire")?
I've done Google seaarches and read what seemed like the relevant
sections of the 2004 ARRL Radio Handbook and their Antenna Book;
unfortunately, most authors restrict their discussion to quarter- wave
or longer antennae. Any starting points, hints, or references on
impedance calculations for less-than-1/10-wavelength antennas will be
appreciated.
My other question has to do with how to interpret signal strength. The
first "standard reference" transmitter I'll be attempting to receive
will be WWVB out of Fort Collins, Colorado (60kHz/5000m). Per the NIST
documentation at:
NIST Special Publication 250-67: NIST Time and Frequency Radio
Stations: WWV, WWVH, and WWVB
http://ts.nist.gov/MeasurementServices/Calibrations/
Upload/SP250-67.pdf
figure 4.5 seems to say that I could reasonably expect to see a signal
of at least 100uV/m. Does this mean that I should expect to see 100uV
from any one-meter hunk of wire strung out horizontally in the optimum
direction? Or is there something more subtle going on I need to be aware
of?
This may be a duplicate answer: I _know_ I wrote one, but it seems to
have fallen into the bit-bucket.
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.
(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.)
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?
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.
--
http://www.wescottdesign.com