On 11/27/2012 11:01 PM, Arid ace wrote:
On Mon, 26 Nov 2012 16:25:37 -0500, wrote:

Just to give an update...

[...]

I did the calculations and at 100 uV/m with 50 foot of coax in a two
turn loop I get a bit over 1 uV, not so good. I'm going to see what I
can find about the signal level from ferrite cores. I had the
impression they would be lower, but I seem to recall better numbers than
this in the ones I've seen before.

Rick

When I was working with loop antennas, it became rather obvious that
optimal receiving loops too are 1 turn, with material of the largest practical
circumference. That one turn loop was coupled to one turn as the primary of a
coil / tuned transformer and on VLF / LF that concerned a potcore of the proper
material. The amount of turns for the secondary could be as much as 100 and
with a Q factor of 100 or more that meant at least 10,000 times the voltage of
the one turn loop - more than enough.

Jan

I have come to a similar conclusion, but not about using a single loop.
The single loop is the best antenna if your constraint is the total
length of cable. But if your limitation is the size of the loop, it
will be improved by more turns at the same diameter.

When I wrote the message you replied to I had left the Q factor out of
the equation. I calculated the Q factor at slightly over 100
considering just the resistance of the center conductor. I am also
aware of the idea of coupling using a ferrite core. I don't know
anything about ferrite cores (I suppose I'll need to learn that soon)
and I don't know what the optimal turns ratio would be. I assume that
the resistance of the load reflects back into the antenna as a parallel
resistance? Using your numbers that would mean a 1 Mohm input impedance
would result in a 10 kohm load. At the frequency of 60 kHz and with an
antenna inductance of about 90 uH that gives a reactance of 34 ohms. So
10 kohms shouldn't be a problem. But I read one reference that
indicated the Q needs to be factored in as well which means it would be
100 ohms vs. 33 ohms, and so 1 Mohm might not be high enough.

The input impedance of the device I am using is not characterized. I'll
have to measure it, but I expect it will be rather high, easily 10 Mohm
which should be good enough (30x).

I'm also not sure if there will be a loss of Q due to capacitance of the
cable. I haven't found a clear reference on this. Some say
inter-winding capacitance will reduce the Q, but I don't think this will
be subject to inter-winding capacitance because of the shield. So the
question is will the distributed capacitance to ground create any sort
of problem? This cable has 16.2 pF per foot and I plan to use 50 feet.
I expect this will simply appear as part of the resonating capacitance.

Rick