Frnak McKenney wrote:
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.
Not exactly. The wave will still be nearly planar, that is, the
orientation of the E field will be in a plane which is perpendicular to
a line between you and the effective point in the ionosphere where the
wave is coming from. But the E field can be rotated in any direction
within that plane. So you want your antenna to have substantial gain in
the direction of Fort Collins and at the elevation angle of the arriving
signal (the latter will vary somewhat). But the polarization is a crap
shoot.
Hm. Wonder if anyone has built an antenna whose polarization shifts
to "best match" the incoming signal? (No, not _this_ weekend!
grin!)
Sure, many. Polarization diversity is an old idea. In a previous life I
worked on a phased array radar (cf.
http://www.globalsecurity.org/space/...an-fps-85.htm). The
transmitters have only horizontal dipole antennas so they transmit only
a horizontally polarized signal. But each of the 4660 receivers has two
dipole antennas, one vertical and one horizontal. This gives the
receiver information about whether an object is tumbling or rotating,
for example, by the way the polarization is shifted by the reflection.
Amateurs would have to use polarization diversity at both ends of a
contact, since there's no way to predict which polarization would be
best at a given moment for transmitting to a fixed polarization
receiving antenna.
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!)
Be cautious in generalizing about your accomplishments. Day-to-day
propagation differences can be extreme. Unless you can do an immediate
A-B comparison or take many, many measurements over a very long period
of time, there's no way to distinguish between antenna and propagation
changes.
. . .
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.
The whole objective to receiving system design is to maximize the
signal/noise ratio, where "noise" is "everything you don't want". Making
both larger by the same amount accomplishes nothing you can't do with a
simple amplifier.
. . .
Roy Lewallen, W7EL