My main LF antenna for other than WWVB is an AMRAD-design active voltage
probe antenna, with a 1 meter whip and a fiercely linear front-end (the
transistor runs with something like 70ma bias to keep distortion
products down). It actually works very well for LORAN, with strong
signal strength and very low noise numbers.
The problem, though, is that the broadband power (measured on an HP
3586C selective voltmeter, in broadband mode) is about -6dBm, changing
to about -3dBm at night when the BCB antenna patterns change. That high
power level was apparently changing the operating parameters of the
LORAN receiver front end, and when the power changed by 3dB at night, it
caused about a 30ns phase shift. Adding a low pass filter knocked the
broadband power down by about 50dB, and solved the problem.
I recently got an official Austron loop antenna of eBay, and plan to get
that installed outside this summer. But with the BCB filter in place,
the AMRAD antenna does a really good job (and it's useful for other
things as well -- I've also used it to receive WWVB).
73,
John
----
David L. Mills wrote:
John,
Get a couple of nice big ferrite cores and take a couple of turns with
your downlead. Also, I hope you are using the LORAN loop; the whip is
far inferior. My Austron receivers have both failed and my homebrew
LORAN works onlu pm ISA bus.
In the twenty years I have been here the RFI/EMI all over the LF, MF and
HF bands has grown steadily worse. The W3UD ham radio club HF gear is
essentially unusable.
Dave, w3hcf
John Ackermann N8UR wrote:
Hi Dave --
We've chatted about your WWVB problems in the past, and you seem to have
a real collection of noisemakers that cause problems. On the other
hand, I get really good reception here in Ohio. I'm using the
Spectracom ferrite antenna (with internal preamp) up on the roof of my
house, and don't have any major spark generators nearby. I do, however,
have challenges of my own with a few AM broadcasters that put in very
strong signals. The BCB power coming down the coax of my wideband LF
antenna is so great that my LORAN receiver shows a diurnal phase shift
when the broadcaster changes antenna pattern at sunrise and sunset.
I get very good signal strength, and the 8170 loses lock maybe once
every 10 days; I've never actually seen the "lock" LED go off when I was
there.
I just looked at the NIST WWVB page, and it looks like the experiment
they were doing with the modulation depth has become permanent -- the
advertised power drop is now 17dB instead of the 10dB it used to be. I
wonder if that might have a negative effect on some users, particularly
if they are competing with a lot of local noise.
I guess from Dave's, and my, experience you can draw the conclusion that
WWVB reception quality may be very dependent on the local environment.
By the way -- despite my fondness for LF, I'll readily acknowledge that
GPS is certainly a better time source in a whole bunch of ways, as long
as you can see the sky.
John
----
David L. Mills said the following on 04/15/2006 11:31 PM:
John, et al,
You are not going to like this.
I;ve been running several WWVB clocks since 1981 using Spectracom 8170
and Netclock/2 receivers with the NTP WWVB driver. NIST Time and
Frequency Services, NIST Special Publication 462 (Revised 1990), claims
nominal timecode accuracy of 100 microseconds, and that's what I got in
the beginning.
The WWVB signal has become degraded in recent years and now all WWVB
receivers here have been retired. Here in Delaware we are on the 100
microvolt per meter contour (before the WWVB transmitter upgrade) and
normally this would be sufficient for good accuracy. However, at least
here in Newark, there is another strong signal on 60 kHz that interferes
with WWVB. I chased this down to power-line conducted EMI and found the
interferiing signal was well above the noise several miles from campus.
It's not clear where it originates, but the prime suspect is a power
inverter for an arc welder at the Chrysler plant in town.
This was bad enough, but then we started installing banks of UPS units
in the machine room and they scream like banshees on 60 kHz, even with
the antenna on the roof of an outbuilding. The result was total failure
of all our WWVB radios, at least on campus.
Until recently I was getting good results at home, but now I see the
signal has failed there as well, probably due to increased crud at 60
kHz due some incidental radiator. I conclude WWVB is no longer useful
here and I suspect in any machine room, unless the antenna is far away
from the UPSes and with suitable ferrite decoupling.
A solution that would probably work for business is to locate the
antenna and radio in a doghouse on the roof and run serial cable to the
machine room. Lemme tell you how hard that is with a plastic sheet roof
and a landlord that believes rooftop space rental for tenant machinery
is a significant revenue stream.
Dave
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