[time-nuts] LORAN-C antenna

WB6BNQ wb6bnq at cox.net
Tue Nov 13 05:05:44 UTC 2007


Didier,

Here is the USCG web site that has a lot of technical explanation on LORAN.

http://www.navcen.uscg.gov/loran/

LORAN is a series of pulses that are transmitted from a master and several slave
stations for any particular chain.  Each chain has its particular repetition
rate.  Your spectrum display looks about right as you are most likely be seeing
more then a single chain.  The aggregate of all those pulses would look like a
bell curve unless you were able to speed up the spectrum analyzer fast enough to
catch the individual pules.

The left side bar on the above web page has 2 pages worth looking at as a
starter.  The first would be the "LORAN-C User handbook" and then the "LORAN-C
Signal Spec" links.

Bill....WB6BNQ

Didier Juges wrote:

> I get this (see picture) with the spectrum analyzer and my wire antenna.
> That looks a lot cleaner than what I hear on the HP 3586.
>
> The spectrum analyzer was in peak hold, because the signal has on/off
> modulation at several Hz. I got the picture after about one minute.
>
> The picture is here:
>
> http://www.ko4bb.com/Timing/Loran.jpg
>
> (sorry it's 2.2 MB)
>
> Is that the Loran signal? Seems too narrow, based on your comment (20%
> bandwidth)
>
> If so, I would like to find something smaller and maybe more portable than
> my 20m wire up the tower :-)
>
> Didier KO4BB
>
> > -----Original Message-----
> > From: time-nuts-bounces at febo.com
> > [mailto:time-nuts-bounces at febo.com] On Behalf Of Carl Walker
> > Sent: Monday, November 12, 2007 8:31 PM
> > To: Discussion of precise time and frequency measurement
> > Subject: Re: [time-nuts] LORAN-C antenna
> >
> > You can build your own LORAN-C antenna coupler without too
> > much trouble.
> >
> > A lifetime or so ago, I was one of the analog design team at
> > the company that made Northstar LORAN-C receivers for marine
> > and aircraft navigation. The first generation of receivers
> > used an active coupler (MOSFET amplifier) with some high
> > frequency roll-off to avoid BC band overload. These receivers
> > were quite simple, with bandpass filters and a couple tunable
> > notch filters to eliminate interference close to the LORAN-C
> > band - before some hard limiting to allow the uP and sampler
> > logic to process the information. This basic type of antenna
> > coupler is what I'm using at home (with a distribution buffer
> > amplifier) for the 2100F, 2000C, and the various WWVB
> > receivers; this has been quite satisfactory - given the
> > low-pass filtering in the coupler allows both 60 KHz and 100
> > KHz signals through quite nicely.
> >
> > Based on your location, you may or may not have interfering
> > VLF signals in the neighborhood of LORAN-C; there's only one
> > real way to find out - have a look with the spectrum analyzer
> > at the output of whatever you devise for an antenna coupler
> > amplifier and see what's there. Also bear in mind the
> > receiver itself is generally designed with filtering of its
> > own (may or may not have internal, fixed notch filters for
> > close in interference in addition to some band-pass
> > filtering), and may not require that you do all that much
> > external filtering in the coupler itself. I must admit I've
> > not snooped around in either Austron for some time, and the
> > details of the those receiver designs escape me at the moment.
> >
> > If LORAN-C is all you're interested in receiving, you'd do
> > well with a bit of bandpass filtering before the amplifier
> > stage in the antenna coupler to avoid overload and
> > interference both above and below the desired signal. The
> > energy in a LORAN-C pulse is very broadband (a 20% bandwidth
> > pulse), so making a filter that's as flat in amplitude and
> > group delay distortion over the 90-110 KHz band helps
> > preserve pulse envelope shape and zero crossings; liner-phase
> > filters work quite well here - although the skirt selectivity
> > might not be all that you'd like.
> > Preserving pulse fidelity is the key here. Pulse envelope
> > shape is often critical - since many receivers use the
> > envelope shape of the pulse to determine which zero crossing
> > to track when cycle-selecting. The other thing to bear in
> > mind is that if you'd like to use a short length of wire for
> > your receiving antenna, the impedance of the input bandpass
> > filter needs to be quite high; as an example, we used 8 foot
> > CB-type whips for marine applications - and to approximate
> > this antenna length with 50 Ohm signal sources, we used a 20
> > pF series cap at 100 KHz.
> >
> > It's also interesting to note the diurnal effects due to
> > sky-wave contamination of the pulses that was mentioned
> > earlier. Depending on amplitude and delay of this sky-wave
> > signal, it's quite possible to get vector-sum effects that
> > cause the perceived zero crossings of the pulse to shift in
> > time. Since the ionosphere isn't stable in height, and the
> > sky-wave signal often is greater in amplitude than the ground
> > wave signal by 10 to 20 dB, the point at which your receiver
> > is tracking may appear to be time displaced in a jittery
> > sort-of way (based on delay and amplitude of the sky-wave
> > signal) - and the receiver tracking loops will follow this
> > displacement early and late in time - making the oscillator
> > appear to be unstable. I believe this to be one possible
> > cause for the degradation of stated accuracy by the 2100F for
> > a given oscillator during the evening hours. I see the
> > degradation clearly here - whether the receiver is driven by
> > the Austron xtal oscillator, or the HP5061.
> > Changes of two to three orders of reported magnitude are not
> > uncommon for the frequency offset display on my receiver
> > between daytime and nighttime operation.
> >
> > Here we have another reason to maintain pulse fidelity -
> > since too narrow a filter selectivity will tend to distort
> > and suppress the rise time of the pulse envelope, causing a
> > receiver to select a zero crossing later than desired in the
> > pulse to track - late enough in the pulse to allow the
> > sky-waves to have more effect on the zero crossing its trying
> > to track.
> >
> > I'd be happy to share some ideas on LORAN antenna couplers
> > and their design if anyone is interested - drop me a line.
> >
> > -Carl WA1RAJ
> >
> >
> >
> >
> >
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