); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY 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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] 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 > > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com To unsubscribe, > go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.