On Thu, 11 Oct 2007 06:23:44 -0700, Roy Lewallen wrote:
Frnak McKenney wrote:
. . .
Okay... ARRL Antennas, Chapter 8: Multielement Arrays. We've got
an (approximately, given skip) vertically-polarized 10MHz signal, so
the E-field is moving up and down and the wavefront is a circular
ripple (nearly a straight line by the time it gets to Richmond)
travelling roughly west-to-east, that is, it's hitting my house
end-on.
. . .
--snip--
While you're looking at the ARRL Antenna Book, look over the chapter on
propagation. You'll find that when receiving a signal by ionospheric
skip (as you are), the polarization will be randomly oriented. So
there's no point in choosing your antenna orientation on the basis of
some supposed wave polarization. Its orientation will, however, have a
striking impact on its pattern, so you should choose the orientation to
get the most favorable pattern.

Ah. So even if it starts out in vertically polarized in Fort
Collins 'way out thataway (he says, gesturing faintly west-ish)
WWV's signal might be polarized north-north-west by the time it gets
ro Richmond.

The fading in and out of the WWV signal you described in an earlier
posting is very likely due largely or at least partially to polarization
shift -- the signal fades when the polarization rotates to be crosswise
to your antenna, and gets loud when the polarization lines up with your
antenna's. I've seen tens of dB difference switching between a
vertically and horizontally polarized antenna, with the change going the
other way after a minute or so when the polarization rotates. If your
receiver needs a constantly strong signal, you're going to have a hard
time getting it what it needs.

Hm. Wonder if anyone has built an antenna whose polarization shifts
to "best match" the incoming signal? (No, not _this_ weekend!
grin!)

I haven't followed the thread closely, so please pardon me if I've
missed something. Your initial description of the problem sounded like
receiver overload. A sharp preselector would help a lot, although it
sounded like you were using a tuned loop which, if carefully balanced,
should provide that function.

A minor update: It seems that I was _mis_tuning my antenna,
adjusting it for the strongest signal (highest stack of LEDs lit).
Over the past two days either I've finally tuned it _correctly_ or
I've done that _and_ the signal has improved. Whatever the
cause(s), I can now -- at times, in fact for an hour at a time --
hear the tocks fairly clearly and even understand the voice. (Who
knew the announcer's phrase for UTC "Coordinated Universal Time"?).

How good? Well, I've unplugged the clock to reset it and it has
then received an "acceptable" WWV signal (it started showing digits)
eight times in the past two days. It might have been more times,
but I don't watch it constantly, and I've noticed that twiddling the
tuning knob tends to make sync-ing a little harder. ("Ack! It's
fading! See if I can tune the antenna _just_ a little better!"
grin!)

I _do_ know that the point of best reception is much narrower that I
thought it when I was tuning in that "other" signal. Turning the
tuning capacitor's knob a "minor wiggle" either way can decrease the
signal strength by an LED or 2, and I need to compensate for the
effect of moving my hand near the antenna to tune it. grin!.

Oh, and the MAC-II seems to be a lot pickier about what it will and
will not accept than my ears are. The microprocessor is driven by a
9.216MHz crystal and it's an 87C52 (an 8051-alike) which (as I
recall) means it only gets around 0.768 MIPS (clk/12) to do all its
work, so I doubt it's doing much "DSP" itself; its interrupt lines
are tied to a 567 tone decoder IC for WWV's "start of minute" and
BCD data subcarrier tones (1000Hz and 100Hz).

The MAC-II seems to be very "picky". Part -- but not all -- of its
requirement for starting the clock digits running is that it receive
a recognizable 100Hz signal for a full minute, that is, between one
1000Hz tone and the next. In other words, if WWV's signal is fading
on a 5- or 30-second cycle, odds are good that at one point the
signal will be come "bad". I've seen it recognize the start of a
"frame" by lighting its CAPTURE LED, but then turn it out ten or
forty seconds later when part of the frame has faded a bit; my ears
can still recognize the tones, but the're better designed and have a
better processor than the MAC-IIgrin!. This pickiness I suspect
is part of the reason it takes to long for the clock to start
running.

... If a preselector isn't enough, the next
step is to add an attenuator -- I have to use one between my TV and its
antenna, in fact. You should consider the possibility that the 10 MHz
WWV signal itself is overloading the receiver, in which case an
attenuator is necessary, and the last thing you'd want to do is use a
better antenna. A directional antenna can be used to reduce the strength
of interfering signals if they're coming from directions different than
WWV. But making an antenna which has good rejection in the right
directions can be something of a project.

I do a lot of reading in comp.dsp (sometimes it's fun just watching
the phrases fly back and forth grin!), and one common topic there
is the difference between "noise" and "signal". For me, "signal" is
"what I want", "noise" is "everything else", and the fun(?) part is
figuring out how to get as much of the former as I can while
downplaying or being able to ignore the effects of the latter. My
next step is to add a "line out" jack to the MAC-II so I can capture
long stretches of the signal to disk; when reception goes bad again
I'll be able to use Scilab or Matlab or something to play "Beat the
Heathkit!" with my own algorithms.

And if I get tired of that, I can unsolder the 87C52 and wire in one
of Atmel's $20 Butterfly boards in its place, adding an LCD and my
own algorithms. And _then_ I can... and _then_ I can... grin!

Anyway, "It's feeling _much_ better now." grin!


Frank
--
The first Zen master in Japan to write extensively on good and
evil was Dogen Zenji... Dogen was one of the most adamant of
those who rejected the widespread use of Buddhism for social,
political, and material power; and he was driven out of the
capital area for his trouble.
-- Thomas Cleary / The Japanese Art of War
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)