On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney
wrote:

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")?

Antennas can be modeled by various NEC based programs. For example:
http://home.ict.nl/~arivoors/
http://www.eznec.com
http://www.nittany-scientific.com

Note that the common "atomic clock" gets its time from WWVB at 60KHz
(about 5000 meters) using a tiny loop antenna. Huge antennas are not
required for many application.
http://www.mas-oy.com/data/MAS_docu_AR.htm
http://www.leapsecond.com/pages/sony-wwvb/

Also, search Google for LOWFER.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff,

Thanks for the reply.

On Tue, 17 Feb 2009 08:36:23 -0800, Jeff Liebermann wrote:
On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney
wrote:

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")?

Antennas can be modeled by various NEC based programs. For example:
http://home.ict.nl/~arivoors/
http://www.eznec.com
http://www.nittany-scientific.com

Note that the common "atomic clock" gets its time from WWVB at 60KHz
(about 5000 meters) using a tiny loop antenna. Huge antennas are not
required for many application.
http://www.mas-oy.com/data/MAS_docu_AR.htm
http://www.leapsecond.com/pages/sony-wwvb/

That's what I keep reading; any caution you hear coming from me is
based on years spent in close association with Murphy. grin!

I'm a long-time devotee of the Divide'n'Conquer(tm) school of
analysis and troubleshooting. This only works well when one is
familiar with the appropriate problem-partitioning tools and has
experience using them; as it is, I'm trying to acquire the knowledge
that will let me know why things didn't work when they don't work as
expected. grin!

Also, search Google for LOWFER.

Thanks. I did see the phrase, but forgot to write it down; after
some hours searching the 'Web and reading it all seems to run
together. grin!


Frank
--
"When the government fears the people, there is liberty.
When the people fear the government, there is tyranny."
-- Thomas Jefferson
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

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

Tim,

Thanks for the response.

On Tue, 17 Feb 2009 11:02:42 -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--
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")?
--snip--

Why do you want a good impedance match?

Because I'm trying to snatch a signal that I have no experience with
"out of the aether", a signal that has to somehow excite an antenna,
feed into an upconverter, arrive at my receiver, and produce some
specific identifying evidence that I'm detecting the signal I hope
it will rather than (say) reporting that my neighbor is using his
electric razor. grin!

Each of these pieces (except perhaps the Mohican) are untested (by
me), and I don't have any tools that will help me easily distinguish
between (say) a bad upconverter, a poor antenna, or excessive noise.
Like many such situations, I'll know if I _succeed_, but if I don't
there won't be any clear indicators to help me figure out _which_
piece of the puzzle isn't fitting properly.

In short, anything that sounds like it might affect my results is of
interest to me.

Why don't you want to use a loop antenna?

It's a question of time and effort: it looks like it will take me
less of each to test the "wire" first. If it succeeds, I'm done; if
not, I can start experimenting with loops.

Which is, of course, a variant on one of my favorite puzzles: How
do you allocate your resources when you don't yet know what you're
doin... er, "under conditions of less-than-perfect information"?
grin!

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.

Yeah. All that, plus the funding. But mostly the funding. grin!

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_.

Which may well be true, but it seems puzzling. Why would it not be
important to deliver as much of the induced electron movement to a
receiver as possible?

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.

Talking about "radiation resistance" in a receiver antenna also
feels a bit odd.

... 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).

Yes. I'm reading up on loops so I have an alternative available in
caseXXXXXXX_when_ something goes wrong.

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

This is the direction I'm starting in.

... (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).

Gack! I don't suppose it used an old Model-T spark coil, or a
buzzer-and transformer equivalent? I remember in my youth (Dirt(tm)
_had_ been invented, but it was still considered cutting-edge
technology grin!) just how badly one could mess up AM BCB
reception with a string of those Christmas tree bulbs with built-in
bimetallic-switch flashers.


Frank
--
You'll never learn to do anything well until you're willing to
accept that you'll do it badly at first. --Anonymous
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

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

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

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)

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 100 uV/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. Balanis (Antenna
Theory Analysis & Design (1997), p.137) gives a formula for the
radiation resistance of a short dipole:

R = 80 * pi^2 * (W/L)^2 ohms

where W is the length of the antenna and L is the wavelength. The
value for a monopole is roughly half as much again.

Why do you request a non-loop antenna? A small circular loop antenna
also has a low radiation resistance but it can be increased by adding
turns. Balanis (p.209) gives a formula for the radiation resistance of
a small loop:

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

Joe,

Thanks for replying. I wan't entirely certain which of the four I
should respond to, but I'm assuming this was the vinfal version.

On Wed, 18 Feb 2009 06:11:29 -0800 (PST), J.A. Legris wrote:
On Feb 16, 11:22*pm, Frnak McKenney
wrote:
Back in December I posted a question about ways to receive LF/VLF
radio signals.
--snip--
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")?
--snip--
My other question has to do with how to interpret signal strength.
--snip--
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?

A field strength measured in 100 uV/meter is just that, but the
problem getting the energy out of the air and into a receiver.

Yes. It's not like I can just hang a bucket out the window and
bring it back full of electrons wiggling at just the right speed.
grin!

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. Balanis (Antenna
Theory Analysis & Design (1997), p.137) gives a formula for the
radiation resistance of a short dipole:

R = 80 * pi^2 * (W/L)^2 ohms

where W is the length of the antenna and L is the wavelength. The
value for a monopole is roughly half as much again.

Um... 1.5 * 80 * (%pi^2) * (1/5000)^2 is... 471 micro-Ohms? That 's
pretty low; why would anyone match that to a JFET input?

Why do you request a non-loop antenna?

I started there, ran into some questions, and wanted to clear up the
confusion in my own head before moving any further. It's not as
though I'm prejudiced against them; heck, some of my best friends
have radios with loop antennae. grin!

... A small circular loop antenna
also has a low radiation resistance but it can be increased by adding
turns. Balanis (p.209) gives a formula for the radiation resistance of
a small loop:

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

Oddly enough, I now have ten old transistor radios that I picked up
at FrostFest a few weekends back for $1 each. I was looking for
ferrite and wide-ratio tuning capacitors, as they seem to be in
scarce supply these days. I don't know where today's kids are
getting their crystal radio parts from these days; it certainly
isn't Radio Shack.


Frank
--
"A man should never be ashamed to own that he has been in the
wrong, which is but saying, in other words, that he is wiser
today than yesterday." -- Jonathan Swift
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

In article ,
Frnak McKenney wrote:

Big Snip.

Go find Radio-Electronics magazine for 1983, and read the five(?)
articles by Ralph Burhans about receiving VLF.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)