Bear,
Guess my math-speak was a bit more obscure than the duck-speak in my
direct-conversion receiver. In less mathy technese:
1. While it looks like a pi network, it operates more like an L
matching network. That's because it uses the loop's inductance
partially compensated by the distal capacitor as the inductive leg.
2. No terminating resistor is needed. It's actually a disadvantage
to use one.
3. The impedance of free space only plays in the physics of finding
the loop impedance from first principles—something I've never seen
done outside of university exams. An impedance bridge will do the
trick.
4. The main advantage over the Colpitts-like matching network using
the same parts values is that both capacitors have one side returned
to common. That does not help much in the broadcast band where the
proximal capacitor has a very large value. Above 15 or 20 MHz real
variable capacitors can be used. With traditional variable caps,
being at common becomes a decided advantage for easy construction
and somewhat reduced hand capacitance.
Walking the southern 3 km of the Bothell-Everett highway yesterday
(I was thinking about the next experiment with my 1-evening direct
conversion receiver and got on the wrong bus), I noticed a few
salmon berries looking ripe. The Himalayan blackberries are in
flower, so have a couple weeks before the berries are ready to
harvest. With this year's fashions the lady bears (at least around
here) have blue tummies. So unless you're going to claw your way
into big-leaf maple sap, you might as well work on loops for the
next couple weeks. By the way I arrived in Bothell just in time to
see the correct bus disgorge a passanger who'd been waiting next to
me where I'd gotten on the wrong bus. The way some buses zig-zag
through the industrial parks, walking can be faster.
Regards,
Steven
--- In loopantennas@..., "qrpbear" <qrpbear@y...> wrote:
Steve,
Ahh... thanks for that. I'm still puzzling over the math but
perhaps
they point to a possibility of success.
For those who may not be aware of the "Street Sweepings Loop" it
is
based on the idea of feeding the loop directly from 50 ohm coax
using
the loop inductance itself as part of a Pi-Network. The most
important remaining questions are... what would be the output
impedance of the Pi-net? 377 Ohms (the impedance of free space) or
some other value? Would a resistive output termination be
required?
All this preliminary to building such an animal of course. The
real
answer would be to build it and compare it against a conventional
loop of the same size but Summer is coming on and 'Bear' is more
concerned with eating berries and catching salmon to fatten up on
for
the coming Winter's sleep.... and mebbe finding Lady Bears, too
<G>.
73,
'Bear' NH7SR (Yo! Shoggoth!! Fetch me a beer!!! No... not St.
Pauli
Girl you dumb beast!!!!)
--- In loopantennas@..., "Steven S. Coles"
<steven_coles@y...> wrote:
Bear,
I tried to answer you on the GCC Loop Group. For some reason
the
post didn't appear. So:
Bear,
My shoggoth upchucked allowing me to provide the following
example:
An impedance analyzer gives an impedance of 3 + j215 ohms at 1.6
MHz
for a certain Pioneer AM loop. Let's make the proximal
capacitor
(Cp) 7900 pF and the distal capacitor (Cd) 490 pF. That gives
capacitive reactances –j12.6 ohms and –j203 ohms respectively at
1.6
MHz.
The loop in series with Cd gives
3 + j215 – j203 = 3 + j12
Using the parallel impedance formula gives
(0 – j12.6)(3 + j12)/[ (0 – j12.6)+(3 + j12)] = 50.9 – j2.4 ohms
That's not too bad for the shoggoth and me making some wild
guesses.
Cp could be a 5100 pF cap with a dip switch to add 200, 400,
800,
1600, and 3300 pF in parallel. Cd could be a 360 pF variable
with
200 pF in parallel.
Actually the shoggoth used
and transformed the negative inductor in the upper right network
into a capacitor.
Regards,
Steven
