[time-nuts] LORAN-C antenna

Carl Walker carl at icmp.com
Mon Nov 12 21:30:35 EST 2007


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
 






More information about the time-nuts mailing list