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