From: MarkusVester@aol.com
Message-ID: <118.698000.285d4165@aol.com>
Date: Sat, 16 Jun 2001 19:10:29 EDT
Subject: LF: Loops and steep skywaves
To: rsgb_lf_group@blacksheep.org
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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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