On Wed, 05 Nov 2014 20:50:31 -0500, rickman wrote:
Not sure why you can't discuss this in the right thread of this group.
I've posted my reply to your post in the loop antenna thread.
Because I prefaced my comments by mentioning that a 60 KHz loop is on
my "agenda". I guess that's a bit vague. What I meant to say was
that I'm not very well read on the technology involved, a total clutz
with LTspice, and I haven't built another loop so I can measure how it
acts. In other words, I'm not ready to discuss it (unless you can
tolerate my guesswork).
First, I'm not sure what you are talking about connecting high impedance
antennas to condensation and salt fog. If you are transmitting, then
maybe you could get such high voltages as to attract microscopic
objects, but this is a receiver design.
Well, a 33:1 turns ratio is a 1000:1 impedance ratio. Using 75 ohms
as the coax cable and the characteristic impedance, that's 75K ohms.
In general, board leakage and conduction problems start around 100K
(depending on trace spacing etc), so I suspect you can make it work,
at least on the bench. However, in the typical marine atmosphere,
with ionic crud in the water, there will be leakage issues. I don't
recall the typical sheet resistivity for a standing salt water puddle
on a PCB, but I suspect it will be a problem. Of course, you can
conformal coat the board, hermetically seal the package, wax dip it,
or pot the antenna amplifier in epoxy to avoid the problem. However,
the favored method is to design with low impedances and not create new
problems with conformal coatings and sealed boxes.
There are also some PCB layout tricks that will help. For example,
here's part of a book on PCB design issues:
http://www.analog.com/library/analogdialogue/archives/43-09/edch%2012%20pc%20issues.pdf
See Pg 12-15 to 12-19 on "Static PCB Effects" with examples of PCB
guard patterns.
Incidentally, my unofficial test for decent design was to immerse the
radio in a bucket of genuine San Francisco Bay salt water. If the
board continued to operate normally, it passes. If not, I get to
spend the evening with the bucket and a megohmmeter looking for the
culprit.
If you're building this loop as an academic exercise, you can probably
ignore all the aforementioned comments on PCB leakage. However, if
you're going to sell it, think carefully about such environment
problems.
Also, the antenna is not high impedance, just the input to the receiver.
The transformer I am looking at is a high turns ratio current sensor.
It spans the right frequency range and is a nice compact package easy
to mount on a PCB.
Why not just make it a 40 KHz tuned xformer? You get the same
impedance transformation with the added bonus of additional bandwidth
reduction (increased Q) to eliminate as much atmospheric and man made
noise as possible. It's also much less lossy than a broadband
xformer.
My main concern is lowering the Q because of the loading from the
receiver input, especially with the change in impedance as reflected
through the transformer.
Well, you're stuck with matching the loop to the receiver input
anyway, so there's no way around that with passive components. You
can insert an emitter follower to do the impedance transformation.
Incidentally, the typical loaded Q for such loops seems to be around
100. Some claim 200 or more, but for small loops, 100 seems to be the
target. At 40 KHz, that's a -3dB bandwidth of 200 Hz, which is rather
wide for a 1Hz wide WWVB signal. You could probably increase the Q
somewhat, mostly be reducing the resistive losses, but that might
create drift and tuning accuracy problems. Higher Q is possible, but
I suspect will require a much more rigid and beefy design.
I think when I simulated it, I found the max
signal strength came with a 25 or 33:1 turns ratio because with higher
turns ratios the Q was spoiled enough to bring the voltage down at the
receiver input.
This simulation didn't include the effect of the radiation resistance,
so I will need to add that in. I expect this will lower the Q as a
starting point which means the affect from the receiver input loading
will not be as significant, possibly making a higher turns ratio in the
transformer more useful.
I can't comment on that without seeing the design. Actually, I'm not
sure seeing the design will help as I need to do some more reading
before I can understand exactly how it works.
11:30PM. Time for dinner.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558