[Lowfer] Guided Waves

[Bill Ashlock]

Lyle and group,

>I agree with Bob that the "Guided Waves" text is more applicable to >VLF 
>than to LowFER frequencies. Figure 11 tells the whole story in a nutshell. 
>The curves in Figure 11 >show that there is indeed a "skip distance" for LF 
>skywaves

Adcock's Figure 11 indicates that for a 'winter night' there is only about a 
6db variation in signal strength over distances of 375 miles to 1200 miles 
from the source. I don't consider a distance this broad to be much of a 
"skip distance" - at least in the HF sense where it can be less than 100 
miles wide. In fact I see the peak in the signal strength of the skywave 
occurring at approximately 1000km (625mi) to be more the result of the 
cancellation effects (to the left of the peak)from the surface wave 
interference and other effects resulting from steep angle reflections.

>At 200 km, the skywave signal is typically at least 40 dB below the
>surface-wave signal. Obviously, very little skywave signal is returned >at 
>high angles of incidence.

This is true, but 200 km (125mi) is really not an area that would be 
expected to be supported by skywave propagation. By 400 km (250mi) the 
situation improves by 15db.

When I look at Figure 11 from a distance what I see is a lessening signal 
strength with distance that is pretty much uniform from 0 to 2000 miles with 
a relatively small dip in smoothness of the curves in the 300-600 mile 
region. In this respect Lewis's article and the Adcock article are not that 
different in their explanation of what happens to the signal strength with 
increasing distance from the source.

What has perked a lot of our interests relative to skywave propagation, 
lately, is the bazaar signal strength variations in the 220 mile to 400 mile 
range (IE: the dip in Adcock's Fig 11). What we are seeing must be all the 
Adcock-referenced effects thrown together! Before the common use of digital 
receiving techniques this distance was not explored by US Lowfers, because 
signals were mostly too weak to be heard, or more importantly, recorded.

Lyle, I appreciate your comments on this. At least it forced me, and I hope 
others, to re-study the Adcock article - a thorough treatment of the 
subject.

Bill A

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