[time-nuts] LORAN-C antenna

[Didier Juges]

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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