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Re: LF: Loops and steep skywaves

To: [email protected]
Subject: Re: LF: Loops and steep skywaves
From: "John Currie" <[email protected]>
Date: Sun, 17 Jun 2001 19:31:32 -0400
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Hi Markus,  As I have stated before I believe Loran could be used to obtain much
skywave information.  I  have personally done  many observations of Loran 
signals
with ranges of 300 to5000 nautical mile. .  Unfortunately most of my notes have
been lost.
    I do , however ,  remember that at 300 nm range the night time skywave delay
is about 600 microseconds.    The delay of the first hop skywave  is near 30
microseconds in the daytime and 60 microseconds at night.  this is at a latitude
of  40 to 50 degrees north and at a range of 800 nautical miles.    At higher
latitudes where the reflecting layer may be lower the minimum daytime delay may
be 25 microseconds or less.  The 25 microsecond # also can be observed when the
ionosphere is disturbed.
     The above data were obtained when the groundwave was propagating over salt
water.  When propagating over ground of extremely  low ground wave conductivity,
the grounwave can be both extremely attenuated and delayed.  Oslo Norway could 
be
a very good place to observe loran skywaves, because the Ejde groundwave is
severely attenuated and the skywaves are more easily seen .

      Observing loran -C ground and skywaves are also interesting to observe
during a solar eclipse.
      The methode I used for observing ground and skywaves was as follows :
      1  You need to phase lock to the incoming signals.
      2   The front end filtering is important. We used the same ant coupler I'm
using now with a 5 pole elliptical filter ( low Pass) cutting off at 150 kHz. 
The
input is untuned. It has  high impedance, very low (5 pico Farad) capacitance to
ground at the input
      3  The receiver must have a wide bandwidth prior to sampling.  We had 23
kHz
This can result in heavey  interference problems.  The Decca Master and red
stations were always a problem.  So notch filtors were used to help.
      4  In most Loran receivers there is a trigger which comes out just before
each of the 8 pulses is received.  This can be used to trigger the scope and 
with
the scope set for 50 or 100 microseconds/cm  The received pulse train can be
easily seen and photographed.
      5  It is not easey as the Loran pulse at the output of the receiver filtor
is 250 microseconds wide and the various hops can be 60 microseconds apart and
therefore they can overlap.  Still much info can be gained.  It might be easier
to try on a close-by loran tx as the first hop skywave will be separated by at
least 250 microseconds and there will be no overlap.

      One last tinkle from my memory banks.  At   more southern latitudes like
15 or 20 degrees,  the ionosphere is so reflective that one can see a whole 
train
of skywaves first , second.    up to sixth or seventh all  with strong signal to
noise ratios .
      Thats enough for now all I can say is that when I was busy designing and
selling navigation aids for LF skywaves were a pain in the butt.  Of course now
for amateur communications they are a necessity.

      One last memory drop At A range of around 800 nm over seawater, the
daytime skywave was 6 to 10 db over the ground wave.  The night time first hop
was 20 to 30 db over the groundwave
73 de John VE1ZJ



[email protected] wrote:

Hi LF-group,

yesterday I put together a small receiving loop for DF, and did a check
tonight on the broadband DGPS signal above 122.5 kHz from Frankfurt. I was
quite surprised to find that close to the loop minimum, the sound of the
noise-like modulation very much varied with the pointing angle, and the
effect was similar to selective fading on shortwave. In one position about 5
degrees offset, the spectrum contained a deep null, which moved across the
band and changed its depth over a few minutes. Apparently I am seeing the
superposition of the (attenuated) groundwave plus a variing-delay skywave,
whose polarisation has either been modified by Faraday rotation along the
path or by a horizontal current-component in the transmitting antenna.

This observation brings up another thought: Has someone tried to measure
skywave reflection on LF at near-vertical incidence?

Difficulties could be expected in separating the skywave from the
well-propagating groundwave, and sub-millisecond time resolution will be
ineffective with a narrowband amateur signal. LORAN could possibly provide
that, but their  Marconi antennas are useless here due to their zenith null.

However, transmitting with a magnetic loop over ground will produce vertical
skywave and groundwave radiation with equal efficiency. For example, Walter
(DJ2LF) has successfully operated a large rotatable tx-loop in his garden two
years ago. The attached graph lines out a proposed experiment to measure
skywave phase and amplitude at steep incidence:

The transmitting station orients its loop close to the ground-wave minimum,
e.g. 5 deg offset would provide 21 dB reduction. The receiver, say 20 km
away, operates two antennas,
- a magnetic antenna, precisely nulled to the groundwave, but almost
maximally sensitive to the (unrotated) skywave polarization (cos 5° = 0.996),
- a vertical electrical antenna which picks up the weak groundwave as a phase
reference. If needed, it may be decoupled from the loop using a
high-impedance rx input.

With a skywave path on the order of 200 km, both components will have
comparable fieldstrength. Amplitude and differential-doppler measurements
over several hours should show the reflectivity and changing altitude of the
D-layer.

If both stations can share a common clock (eg TV stn, VHF link), the
groundwave reference would be expandable, and the TX loop could also be
directed towards the minimum, providing further isolation.

73
Markus, DF6NM

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