I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.

I spent some time simulating antennas in spice and was able to get a bit
of a feel for the circuit, but I'm not convinced it would work the way I
want. Just before I set the project aside I was told I needed to model
the radiation resistance. That has the potential of wrecking the Q of
the circuit. I am counting on the high Q to boost the output voltage.
If the radiation resistance is at all appreciable I would lose the high
Q and need to start over.

Anyone have an idea of how to estimate the radiation resistance of a
tuned, shielded loop antenna?

The other factor I don't understand how to factor in is the distributed
capacitance of the coax. Is that a significant influence on an antenna
or is it in the noise compared to the tuning capacitor. The coax is
RG-6-Solid Coax Cable. The loop is made up from 50 feet of this. The
specs are 16.2 pf/foot and 6.5 mOhms/foot in the center conductor, or
would the resistance be a round trip measurement of both inner conductor
and shield? I assume the shield has a much lower resistance than the
inner conductor but I don't know that for sure.

--

Rick

rickman wrote in :

I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.


MSF time signals? Just a thought... If you're interfacing an analog signal to
digital, one trick I used (for audio but it ought to help here too) is a
CA3140 with a bit of positive feedback through a few Mohms for hysteresis to
clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or
6V, could be sensitive to take a lot of strain off your antenna. Whether this
alone gives you enough gain I don't know, but it is cheap to try.

On 10/28/2014 5:24 PM, Lostgallifreyan wrote:
rickman wrote in :

I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked the idea of a large shielded loop made of coax tuned with a
capacitor. My goal is to get as large a signal as possible from the
antenna and matching circuit to allow the use of a receiver with very
low sensitivity... in fact an all digital receiver.


MSF time signals? Just a thought... If you're interfacing an analog signal to
digital, one trick I used (for audio but it ought to help here too) is a
CA3140 with a bit of positive feedback through a few Mohms for hysteresis to
clean the signal a bit. The resulting Schmitt trigger, powered by about 5 or
6V, could be sensitive to take a lot of strain off your antenna. Whether this
alone gives you enough gain I don't know, but it is cheap to try.

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive, the question is whether I need mV
level signals or maybe just uV signals which might not require an amp.
By using positive feedback the threshold would be shifting and the
amount of level shift would set the floor for the signal level from the
antenna I think.

--

Rick

rickman wrote in :

MSF time signals? Just a thought... If you're interfacing an analog
signal to digital, one trick I used (for audio but it ought to help
here too) is a CA3140 with a bit of positive feedback through a few
Mohms for hysteresis to clean the signal a bit. The resulting Schmitt
trigger, powered by about 5 or 6V, could be sensitive to take a lot of
strain off your antenna. Whether this alone gives you enough gain I
don't know, but it is cheap to try.

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive

Well, try it. If it works then inputs are better these days. Or at least,
more sensitive to small changes. As far as I know, digital inputs are usually
specified with a wide dead band for levels, amounting to HUGE hysteresis and
a need for a lot of gain first sp you already ned an op-amp stage no matter
what unless your digital inputs have hair triggers at exactly the threshold
you wanr.

The thing about the CA3140 is that with just three passive parts: M-ohmage of
positive feedback, input series capacitance, and input ground resistor after
the cap, you can empirically set some very nice signal preconditioning as
well as raw gain, all on a very convenient single rail supply at 5V.

On 10/29/2014 6:53 AM, Lostgallifreyan wrote:
rickman wrote in :

MSF time signals? Just a thought... If you're interfacing an analog
signal to digital, one trick I used (for audio but it ought to help
here too) is a CA3140 with a bit of positive feedback through a few
Mohms for hysteresis to clean the signal a bit. The resulting Schmitt
trigger, powered by about 5 or 6V, could be sensitive to take a lot of
strain off your antenna. Whether this alone gives you enough gain I
don't know, but it is cheap to try.

Thanks for the suggestion. I'm not sure this would be any better than
feeding it directly into my digital input. That is a differential input
and I expect to use feedback to overcome the residual input offset. So
the input will be pretty sensitive

Well, try it.

Yes, easier said than done. The receiver isn't built yet, I am
currently looking at the antenna design again and wish to improve my
simulation by adding the radiation resistance. If the antenna will only
put out microvolts even after tuning I will need to figure out how to
add the amp without having to double or quadruple the power budget.


If it works then inputs are better these days. Or at least,
more sensitive to small changes. As far as I know, digital inputs are usually
specified with a wide dead band for levels, amounting to HUGE hysteresis and
a need for a lot of gain first sp you already ned an op-amp stage no matter
what unless your digital inputs have hair triggers at exactly the threshold
you wanr.

This is a differential input which is not far from an analog input.
Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that. But there is always some because of
the parasitic capacitance between the input and output of the buffer.


The thing about the CA3140 is that with just three passive parts: M-ohmage of
positive feedback, input series capacitance, and input ground resistor after
the cap, you can empirically set some very nice signal preconditioning as
well as raw gain, all on a very convenient single rail supply at 5V.

This design won't have a 5 volt rail. Most of the design will run on
1.2~1.8 volts with some I/O at 3.3 volts to drive an LCD. It's very low
power, remember?

--

Rick

rickman wrote in :

Actually even single ended digital inputs don't have much hysteresis
unless they are designed for that.

Well, as a proportion if they only go high above soem fairly close approach
to V+, then low when close to 0V, then the dead band could be wide, the aim
was to eliminate false states so they ARE usually designed for it. I take
your point on very low volt systems, if the actual difference is small even
though proportionally it may not be.

Anyway, now I know that the supply is so small, your suggestion of discrete
transistors is almost certainly the way to go, unless there is enough similar
demand out there to have cause an off-shelf part to be made.

Normally I'd just look at how others are solving similar problems, so I guess
the question I can ask is: what is the signficant difference in this case
that prevents the nearest off-shelf answer from working?

rickman wrote in :

By using positive feedback the threshold would be shifting and the
amount of level shift would set the floor for the signal level from the
antenna I think.


Yes, basically like a noise gate. The op-amp trick is nice though, it gives
you fine control of it.

"rickman" wrote in message
...
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and liked
the idea of a large shielded loop made of coax tuned with a capacitor. My
goal is to get as large a signal as possible from the antenna and matching
circuit to allow the use of a receiver with very low sensitivity... in fact
an all digital receiver.

I spent some time simulating antennas in spice and was able to get a bit
of a feel for the circuit, but I'm not convinced it would work the way I
want. Just before I set the project aside I was told I needed to model
the radiation resistance. That has the potential of wrecking the Q of the
circuit. I am counting on the high Q to boost the output voltage. If the
radiation resistance is at all appreciable I would lose the high Q and
need to start over.


I don't think I would try and reinvent that type of antenna. There are
several designs on the web that use a loop about 3 feet in diameter and
several turns of wire inside the shield. In most cases a low noise preamp
is needed, but that shold be simpleand inexpensive to build.

Go to this page and go toward the bottom for some loop antenna ideas.
http://www.w4dex.com/lf.htm

I have known Dexter for around 40 years.




---
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On 10/28/2014 6:14 PM, Ralph Mowery wrote:
"rickman" wrote in message
...
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and liked
the idea of a large shielded loop made of coax tuned with a capacitor. My
goal is to get as large a signal as possible from the antenna and matching
circuit to allow the use of a receiver with very low sensitivity... in fact
an all digital receiver.

I spent some time simulating antennas in spice and was able to get a bit
of a feel for the circuit, but I'm not convinced it would work the way I
want. Just before I set the project aside I was told I needed to model
the radiation resistance. That has the potential of wrecking the Q of the
circuit. I am counting on the high Q to boost the output voltage. If the
radiation resistance is at all appreciable I would lose the high Q and
need to start over.


I don't think I would try and reinvent that type of antenna. There are
several designs on the web that use a loop about 3 feet in diameter and
several turns of wire inside the shield. In most cases a low noise preamp
is needed, but that shold be simpleand inexpensive to build.

Go to this page and go toward the bottom for some loop antenna ideas.
http://www.w4dex.com/lf.htm

I have known Dexter for around 40 years.

I am not sure what you mean by "reinvent" that type of antenna. Every
antenna can be optimized for a given design. My requirements are very
unique. I need as much voltage from the antenna as possible. My
receiver input impedance can be very high (~1 Mohm) which is very
different from a typical receiver.

I have already gone down the road of looking extensively at loop antenna
designs. I have not found a significant difference other than the ease
of construction. That is one reason why I chose to use coax rather than
wire within a shield like pipe or a bicycle rim (as I found in one
project).

My current design is 100 feet (the 50 feet I said originally was due to
my poor recollection) wound on a 2 foot diameter spoke arrangement of
wood which turned out pretty well for a first pass. I have yet to
characterize the antenna which may be the easier path than trying to
construct a good model from theory and the known details.

Several people have suggested that a preamp will be required. That may
be possible. But this is not an analog receiver and don't need a lot of
SNR for it to work. The time code signal is modulated at 1 bps using
both phase and amplitude modulation and pulse width bit encoding. I
will need a resolution of no worse than 100 milliseconds to decode the
bits. So I figure a bandwidth of 10 Hz should be plenty enough. This
means I can vastly over sample the signal and get lots of gain digitally.

So the tricky part is to overcome the poor analog characteristics of the
differential digital input. I only need it to turn the input signal
into a one or a zero, but it needs to be sensitive to a very small
signal. With the various imperfections of input offset, hysteresis,
etc., I will be lucky if it works with very low voltage signals at all.
I could rig up a test circuit and see just what signal levels are
needed.

The other part is that the purpose of this design is to receive the
signal digitally on as low a power level as possible. The entire power
budget is a couple hundred microwatts. I have yet to find an amplifier
that will fit this power budget. Oddly enough some folks in s.e.d told
me that transistors don't work well with low bias currents, but that may
only apply to bipolar amps. They make time code receiver chips to do
this on a few hundred microwatts and have an internal amplifier. So
obviously it can be done. I just can't find a low enough power opamp
for a 60 kHz signal.

Also this a learning exercise for me. So reinventing something would be
ideal!

--

Rick

rickman wrote:
On 10/28/2014 6:14 PM, Ralph Mowery wrote:
"rickman" wrote in message
...
I have a project in mind that would need a very good antenna in the
frequency range of 60 kHz. Originally I looked at loop antennas and
liked
the idea of a large shielded loop made of coax tuned with a
capacitor. My
goal is to get as large a signal as possible from the antenna and
matching
circuit to allow the use of a receiver with very low sensitivity...
in fact
an all digital receiver.

I spent some time simulating antennas in spice and was able to get a bit
of a feel for the circuit, but I'm not convinced it would work the way I
want. Just before I set the project aside I was told I needed to model
the radiation resistance. That has the potential of wrecking the Q
of the
circuit. I am counting on the high Q to boost the output voltage. If
the
radiation resistance is at all appreciable I would lose the high Q and
need to start over.


I don't think I would try and reinvent that type of antenna. There are
several designs on the web that use a loop about 3 feet in diameter and
several turns of wire inside the shield. In most cases a low noise
preamp
is needed, but that shold be simpleand inexpensive to build.

Go to this page and go toward the bottom for some loop antenna ideas.
http://www.w4dex.com/lf.htm

I have known Dexter for around 40 years.

I am not sure what you mean by "reinvent" that type of antenna. Every
antenna can be optimized for a given design. My requirements are very
unique. I need as much voltage from the antenna as possible. My
receiver input impedance can be very high (~1 Mohm) which is very
different from a typical receiver.

I have already gone down the road of looking extensively at loop antenna
designs. I have not found a significant difference other than the ease
of construction. That is one reason why I chose to use coax rather than
wire within a shield like pipe or a bicycle rim (as I found in one
project).

My current design is 100 feet (the 50 feet I said originally was due to
my poor recollection) wound on a 2 foot diameter spoke arrangement of
wood which turned out pretty well for a first pass. I have yet to
characterize the antenna which may be the easier path than trying to
construct a good model from theory and the known details.

Several people have suggested that a preamp will be required. That may
be possible. But this is not an analog receiver and don't need a lot of
SNR for it to work. The time code signal is modulated at 1 bps using
both phase and amplitude modulation and pulse width bit encoding. I
will need a resolution of no worse than 100 milliseconds to decode the
bits. So I figure a bandwidth of 10 Hz should be plenty enough. This
means I can vastly over sample the signal and get lots of gain digitally.

So the tricky part is to overcome the poor analog characteristics of the
differential digital input. I only need it to turn the input signal
into a one or a zero, but it needs to be sensitive to a very small
signal. With the various imperfections of input offset, hysteresis,
etc., I will be lucky if it works with very low voltage signals at all.
I could rig up a test circuit and see just what signal levels are needed.

The other part is that the purpose of this design is to receive the
signal digitally on as low a power level as possible. The entire power
budget is a couple hundred microwatts. I have yet to find an amplifier
that will fit this power budget. Oddly enough some folks in s.e.d told
me that transistors don't work well with low bias currents, but that may
only apply to bipolar amps. They make time code receiver chips to do
this on a few hundred microwatts and have an internal amplifier. So
obviously it can be done. I just can't find a low enough power opamp
for a 60 kHz signal.

Also this a learning exercise for me. So reinventing something would be
ideal!


For commercial designs, I keep seeing references to a
ferrite core with a winding on it, as an antenna.

The article here, describes two kinds of receivers. One
is sensitive to AC pickup, so would only be a candidate
in special physical circumstances. The other uses the
high impedance input.

http://home.pon.net/785/equipment/build_your_own.htm

It suggests to me at least, you want plenty of gain
on the input stage, plus enough filtering to reject
louder noise sources. Your digital processing section
can provide the selectivity. But if spurious out of
band signals saturate your gain stage, you might not
get the desired result.

It would all depend on the tradeoffs you want to make.
You'll always require a gain stage.

Perhaps the antenna of your choice (not your final design)
and a spectrum analyser that works in that range of
frequencies, you can do a survey to see what is possible.
What noise sources are immediately evident, and so on.

No big antenna here. The antenna is one of these.

http://www.maplin.co.uk/p/ferrite-rod-aerial-lb12n

http://www.burningimage.net/clock/20...0khz-receiver/

I think by "sensitive" what they meant was "it picked
up the signal I wanted". The circuit diagram would
have been labeled "insensitive" if no signal was
found. Or if it didn't oscillate at 60KHz on its
own (like a couple amplifiers to drive speakers
have done here) :-) I think some audio circuit
I built, checking with a scope later on, indicated
a nice fat signal at 500KHz. Great.

Perhaps using your big loop of wire, you get to
remove one of the op-amps.

*******

The circuit above uses TL-081, with gain bandwidth product
of 3MHz. So I guess that's why there is still a bit of gain
at 72KHz.

In school, were were shown an example of a filter that
used only resistors. An example is seen on Fig 2.27(c)
on PDF page 70. The neat thing about this topology, is it
was working at 50KHz on a pair of $0.25 opamps. It uses the pole
of the output stage of the opamp, as a filter element. We
had some afternoon lab to do, with this circuit as part
of the work.

http://www.springer.com/cda/content/...022-p174507347

9780817683573-c1.pdf 3,791,230 bytes

The book table of contents is here. It's by Mohan, P.V.A.
With ISBN 978-0-8176-8357-3. I was hoping the topology
had a name, but I don't see one.

http://www.springer.com/cda/content/...069-p174507347

So the circuit could be in range of some opamps. And then
you might not need a huge antenna.

HTH,
Paul