TopBand: Ducting

[w8jitom@postoffice.worldnet.att.net]

Hi Bob,

> You will agree there is lots of conjecture in science (called 'hypothesis' I
> guess.) Who measured the multiple hops? 

Along the same lines, who measured ducting? when I asked if there was 
any way to measure ducting, the answer was no.

So I guess no one measured either, except with OTHR you can 
see ground reflection images, and no evidence of ducting.

In response to Peter, when I say signals get much better in my mobile, 
I'm not OVER or around saltwater. The race tracks I attend are 
inland, many miles from the ocean.

But the effect you noted is very real, mobile signals are greatly 
enhanced when the mobile's near field is over saltwater. That's 
because there is several dB of near field loss when the antenna is 
exposed to lossy earth, whether it's a few elevated radials or a 
vehicle ground, insulating the ground system from earth does nothing 
to get rid of these losses. But parking over saltwater or laying down 
a large radial system sure will.

 >Time measurements for
> round-the-world signals have shown that some signals do not bounce up and
> down in the conventionally assumed manner.

Is that data for frequencies far below the MUF, and what is the 
source of that data? When I was designing some RF equipment for an 
OTHR system, I asked other engineers many questions about how the 
OTHR works. They plainly said the signals were multiple hops, 
because the system I designed equipment for was intended to spot low 
flying aircraft and boats sneaking around under the regular radar.

I would think they would have had a problem doing that with ducts, 
and any ducts would have shown up in the reflected signals. 

> With my horizon towards VK at 22 degs above the horizontal, I can believe
> this morning's 439/339 with VK for ten minutes involved lots of bounces. But
> I can't believe the S9+25 both ways with VK3EW on 28 May was via a ray that
> bounced up and down as many times as geometry would dictate, salt water or
> no salt water.

What was the MUF on both days? A grazing signal on 160 has high loss, 
while a higher angle signal has less loss. The exception is near the 
MUF, where the high angle signal cuts right through.

Ten hops can easily be lesss loss than one grazing hop far below the 
MUF, especially over a saltwater path where the ionospheric loss  
dominates the path loss.

>There has to be another route, and SKYCOM shows a probability
> of extended hops using an accredited ionospheric profile. Creating the
> multiple hops usually shows the signal falling below workability before it
> gets halfway along the path.

I can't comment on that, because I have no idea how Skycom works 
and what type of data it is based on. Maybe someone could explain the 
base data used and how the program applies that data.  Does Skycom 
allow you to put in ground types under each hop?

Does it allow different ionization levels along the path? And where 
did its loss data come from??

73, Tom W8JI 

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