[ARC5] LDE

[George Maier]

Great subject.......The possibility of a monster trans global duct exists,
but it's pretty doggone low on the probability scale.  Having spent decades
in microwave systems engineering, the idea of ducting is pretty much a
routine exercise in some areas.  Surface ducts appear when the refractive
index drops into the cellar and traps radio signals close to earth's surface
over the length of the duct.   An elevated duct can occur when signals are
trapped between different layers with opposite indicies.  Again, not
impossible but really unusual, especially at HF frequencies where the duct
size would be really big and would have to remain stable long enough to
allow enhanced propagation.  GTE Lenkurt published a good book microwave
propagation decades ago; I still have my original 1972 second edition:
Engineering Considerations for Microwave Communications Systems by Robert F.
White.   There's a good on-line explanation of ducting by VK3KAQ here:
http://vhfdx.radiocorner.net/docs/GTPaper2004V2-1.pdf

I should think the HF OTH radar guys would have a really good handle on
this, based on years of observations.  Personally, I think that one, or even
a series of tall and coincident surface ducts could have been be the
mechanism that Dennis is referring to.    BTW - one trip around mother earth
at radio (light) speed is a bit more than 130 milliseconds, so 1/2 second
echos are taking a pretty unusual path!

73, George - W1LSB


-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of D. Platt
Sent: Friday, July 19, 2013 6:14 AM
To: ARC-5 Maillist
Subject: Re: [ARC5] LDE

Gents,

A number of yeasr ago, I worked for a lab (ITT-EPL) that was in the
forefront of Over-the-Horizon Radar (OTH) and ionospheric sounding
development.  We used frequencies up to the lower VHF region, but primarily
in the HF spectrum, of course.  The thing is, I don't recall return delays
into the multi-second region?  Many return signals appeared to be mostly to
be "extensions" of the original pulses, to the human ear.  Of course, those
signals coming back from Eastern Europe and the Middle East and Asia were,
of course, predictably longer.  That I never heard very long dalay signals
indicates nothing as I would have had to be at one of the sites at the exact
time one of these phenomena occurred.  The power we used was in the 3-10 KW.
My first S-Line transmitters were much-modified 32S-1's the company
surplussed out.....  
That was in the early days, of course.  I recall we used Rhode &Schwartz
programmable signal generators, later.

Jeep - K3HVG
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