[time-nuts] LORAN-C antenna

Tue Nov 13 04:23:56 UTC 2007

I took another shot with the analyzer set to 10 kHz RBW, and the signal now
looks more like 20 kHz wide.

See http://www.ko4bb.com/Timing/Loran-2.jpg

Didier

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Didier Juges
> Sent: Monday, November 12, 2007 10:02 PM
> To: carl at icmp.com; 'Discussion of precise time and frequency 
> measurement'
> Subject: Re: [time-nuts] LORAN-C antenna
> 
> 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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