On Sun, 04 Nov 2012 08:55:50 -0600, amdx
wrote:
I did some searching for info for him but didn't find a lot on just a
loop resonant at 60 Khz. I was looking for turns and capacitance but
found no designs.
So I think he needs to pick a size that he is comfortable with and
calculate turns somewhere between 100uh and 200 uh and find some
polystyrene 10,000 pf caps put 4,5, or 6 in parallel and then find a
large air variable to fine tune it with. (135uh and 5000pf is resonant
at 60khz) Then he needs a matching pickup coil, I've seen a small coil
about 1/5 dia. of the larger coil used. There are other methods.
Should he use coax? Wrap it with foil when complete? (with gap)
You're thoughts? (maybe more uh's?
Ok, let's do the math (for the 2nd time as my PC crashed in the middle
of the first attempt).
The loop antenna is basically an impedance transformer. To feed with
50 ohm coax and no preamp in the antenna, the require turns ratio is:
( main-loop-turns / 1-turn-coupling )^2 = Xl-main-loop / 50 ohms
Using your 0.01uF tuning capacitor, which has a reactance of:
Xc = 1 / (2 * Pi * Freq * 0.01uF) = 265 ohms
At resonance, the inductive and capacitive reactances are equal.
Plugging in, I get a turns ratio of:
sqrt ( 265 / 50 ) = 2.3
which isn't very practical for a loop antenna. To get a higher turns
ratio, a smaller cap will be needed. For example, with 1000pF, the
reactances are 2600 ohms for a turns ratio of 7.2, which is somewhat
better. The main loop and the coupling loop would need to be rather
close together. No problem with a ferrite core, but not very
practical with an air core shielded loop. You could use your 1/5th
size coupling loop (as is used in a magnetic loop HF antenna), but at
60KHz, efficient coupling will not happen. The loops need to be
physically close (or wrapped around a ferrite rod). However, a tapped
loop would work if you insist on not using a preamp.
The more common way is to use a preamp at the antenna. It's purpose
is to amplify the received signal, but also to deal with the loop to
coax impedance conversion. See the schematic at:
http://www.ka7oei.com/wwvb_ant_1_1.gif
Note that the amp if connected directly across the main loop. The
0.4uF tuning cap yields a reactance of 6.6 ohms, so the 1K amplifier
input resistor will have no effect on Q. I would have used a smaller
capacitance, but since the author elected to use a bipolar xsistor
instead of a FET, the lower reactance will result in a smaller voltage
swing and will help prevent clipping.
Then, there's bandwidth or Q. The main loop will probably require
about 15ft of #26awg wire according to some of the construction
articles. At 0.041 ohms/ft, 15ft = 0.6 ohms.
Unloaded Q = Xl / R = 265 / 0.6 = 440
So, the -3dB bandwidth of the loop will be:
60Khz / 440 = 135 Hz
Since the bandwidth of WWVB is about 700Hz, that should work. It will
never really have a Q of 440 due to loading and losses, but that
should give a clue on how critical the tuning might become with a
huge, single turn loop. Incidentally, I was aiming for a Q of about 5
on the LORAN antennas which was required because the 100KHz LORAN
signal is 20KHz wide. Loops were possible, but amplified whip
antennas were much easier to deal with.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558