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?
Why do you want a good impedance match?
Why don't you want to use a loop antenna?
At 5000m, atmospheric noise is very strong -- it would certainly
overwhelm any thermal noise that you'd receive if you did make a 1/2 wave
dipole (don't forget that your towers need to be at least 2500m tall to
get close to the ideal). Getting an appropriate impedance match is
mostly about maximizing your signal compared with your receiver's
internal noise; the strong atmospheric noise makes this less necessary.
This atmospheric noise also makes really efficient receiving antennas
rather unimportant. You want a good fraction of a wavelength for
_transmitting_, but it really doesn't make much difference for
_receiving_.
The two common receiving antennas that I know of at that sort of
frequency are tuned loops and capacitive whips.
A loop can be fairly small -- my understanding (which I've never tested,
YMMV) is that one square meter is plenty. Loops are nice because you can
tune them, so they give you some additional selectivity on your receiver
front end. You can impedance match the loop to your receiver, but most
of the impedance your receiver sees will come from the wire in the loop,
not the radiation resistance of the loop. Loops are also somewhat
directive, which helps to reduce the total static received, and if done
correctly (google "shielded loop") they can be arranged to reject sky
waves (I _think_ by polarization, but I'm not sure).
A capacitive whip is just a 1m long wire whip (like a coat hanger or
welding rod) feeding some high impedance amplifier like a JFET (or a
toob, if you want to be picturesque). Put the active element right at
the base of the wire for best signal. It's inherently wide band, and
hard to keep it from being so, so if you have some local interference
it'll kill your signal (my first try at these didn't work in my shop
because of a nearby electric fence transformer, but it worked fine at the
end-user's more-urban location).
--
http://www.wescottdesign.com