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