On Feb 16, 11:22*pm, 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?
Frank McKenney
A field strength measured in Volts/meter is just that, but the problem
getting the energy out of the air and into a receiver.
A short linear antenna has a very low radiation resistance ( 1 ohm)
which is a poor match to a practical transmission line, whose
characteristic impedance is typically 1000's of times larger. The
radiation resistance of an antenna is the component of its complex
impedance that is associated with the power captured. Poor impedance
matching is equivalent to low energy efficiency, in this case very
low.
One solution is to use a small circular loop antenna whose low
radiation resistance can be increased by adding turns. Balanis
(Antenna Theory Analysis & Design (1997), p.209) gives a formula for
the radiation resistance of a loop smaller than 1/25 wavelength:
R = 20 * pi^2 * (C/L)^4 * N^2 ohms
where C is the circumference of the loop, L is the wavelength and N is
the number of turns.
Better still is to use a ferrite loop antenna. You may be able to get
one out of an old AM radio and adapt it to your receiver. The
resulting formula is identical to the above, multiplied by the
relative permeability of the core, u (SQUARED !), so you can use a
very small-diameter loop and/or fewer turns, getting improved
selectivity and sensitivity (i.e. high Q) in a tuned circuit:
R = 20 * pi^2 * (C/L)^4 * N^2 * u^2 ohms
--
Joe