TopBand: Ducting

Mon, 7 Jul 1997 00:00:17 +0000

KL7Y wrote:

> From my experience of watching the ionosphere as delineated by a large HF
> backscatter radar, Pederson rays rarely outperfrom normal modes.  For freqs
> considerably below the MUF (as 1.8 Mhz normally is), the absorption is
> considerably increased the longer a signal remains in the ionosphere.

That has been my experience also, working with an OTHR system on the 
east coast. Enter the ionosphere at a low incident angle far below 
the MUF, and absorption is much greater than the same path with 
multiple hops.

Years ago I did BC engineering, so I had a chance to play with high 
antennas at BC stations. Half-wave or 5/8m wl verticals were poor DX 
antennas compared to 1/4 wl antennas. A 300-350 ft  high dipole was a 
killer antenna.

There is such a thing as too low a wave angle on 160, unless the MUF 
is REALLY low.

Both of these areas makes me seriously doubt a low incident angle 160 
signal grazing along the ionosphere in a "duct" would be very strong, 
let alone ever existing in useful amounts.

> preclude any chordal hops.  Furthermore, near the MUF how do you tell the
> difference between a chordal hop that just misses the earth and a hop that
> grazes the earth's surface?  The time difference is insignificant and the
> reflection loss over salt water is very small compared to overall path loss.

Bingo. There is no way to prove or disprove the duct, except by 
logical de-duct-tion. For every technical statement supporting ducts, 
there is a equally worse technical contradiction present.

But just like the UFO following Hale-Bop, we can't prove it doesn't 
exist. There are those who will believe and those who won't. At this 
point I would say I'm skeptical, because few or none of the 
explainations fit the effects I've seen first hand in over 30 years 
of 160 work and antenna experimentation.

> I am no ionospheric physicist, either, but a couple of years of staring at
> backscatter displays taught me a couple of things:
> 
>         1.  HF signals really do bounce off the earth.

Who would doubt that? The "rays"  bounce off the Moon at UHF, they 
bounce off the earth quite well in the FAR field with antennas, why 
wouldn't they bounce off the earth?

In all likelihood, they bounce off most areas of the earth with MUCH 
less loss than they do off  the ionosphere.

>         5.  For the same distance, a 2 hop high angle signal can be
> considerably stronger than a single hop low angle signal due to the effects
> of the E layer.

VERY true

73, Tom W8JI 

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