On 11/29/2012 10:21 PM, Arid ace wrote:
On Thu, 29 Nov 2012 14:43:33 -0500, wrote:

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.

I used Al strip, bent in a circle, for the loop (used from 0.1 to 30 MHz). It
was cheap and easy but making a multi-turn loop would have been quite a task.

I am making a shielded loop so I'm using coax to make the construction
simple. Multiturn won't be a problem.


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 antique booklet I have on potcores (Soft Ferrites, October 1973) lists the Q
realized with a core of certain dimensions as a function of ue of the material.
At 60 KHz, a P 11/7 core with material FXC 3B7/3H1 and ue=68 has the lowest Q,
220. The use of material with ue=220 delivers a Q of 420. This might serve as an
estimate of what could be expected: the 11/7 is the smallest potcore and the
bigger ones result in higher maximum Q, to well over 1,000.

I'm not sure I understand. This sounds more like a ferrite core antenna
where the ferrite is an integral part of the antenna itself. I was
asking about the type of ferrite for a ferrite loop to use as the core
of a transformer to couple the output using multiple turns to boost the
voltage.


When using a FET device as input, at 60 KHz the input impedance can be
neglected.

This will be driving an input on an IC so I don't have much info on it.
I expect it is very high impedance just like the FET.


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).

The impedance of a source-follower will be much higher than 10 M - at 60 KHz.

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

With a one turn loop coupled to the one turn primary of an RF transformer,
there's no need for a shield. It's fairly easy to provide the piece of wire used
for the primary with a center tap that will be grounded so the loop is
symmetrical. That also obviates the need for a differential amplifier. A 50 feet
loop is huge.

The loop won't be 50 foot in diameter, it is 50 feet of coax wound in
some 8 turns give or take. At 8 turns that is 2 foot diameter or 300 mm
radius.

As to the shield, I'm told the shield will be a big help in eliminating
local interference from E-field sources. I read that household
appliances are a major source of electrical noise and I know from
experience that my radio controlled clocks seem to have trouble working
near my computers.


I once (in Europe) designed a frequency standard synchronized to
DCF77 and reception was OK at distances over 2,000 Km with just a ferrite rod of
12 cm length and 1 cm diameter. In order to keep it easy for constructors, the
coil had just 140 turns with 2n2 to resonate the LC. I don't think the approach
for WWVB at 60 KHz could be much different (apart from the modulation format).
I had designed the ferrite antenna amp as a small device that could be fed via
coax, so one could install it in a place with the lowest noise.

Yes, if I can get away with it, I'll use a ferrite rod. But I want to
eliminate the front end amp and will be driving the input directly from
the antenna. The inductance of this coil will be much lower than a
ferrite core and will need a lot more capacitance to resonate. The coax
coil is what I'm starting with and can always back off to a ferrite
antenna if needed.

Thanks for the input.

Rick