[DX] Merenranta-QTH vs. sisämaa

[T&M Ritola <ritola@xxxxxxxxxx>]

Alla pätkä keskustelusta NRC-listalta koskien merenranta-QTH:n paremmuutta 
sisämaan vastaavaan, nimenomaan keskiaaltojen osalta. Herättäisikö 
mahdollisesti mielipiteitä tällä listalla?

> *** The signals bounce off the water somewhere in the mid-Atlantic, then
> back up and then again down to the rx. I'm more inclined to believe that it
> is a combination of the completely clear horizon and the excellent ground,
> although the amount of difference observed shouldn't be as great as it is.
> It may be that this combination permits a multiplicity of signal paths only
> minutely different from one another to somehow congregate.
>
> I do know that when Ron Schiller DX'ed from Monmouth Beach NJ, he was on an
> inland waterway and less than a mile from the ocean, and he had that 
kind of
> enhancement. I believe that he did the same thing years later from
> Lighthouse Point, FL. And, given the differences I've seen between my 
former
> Northern NJ location and this one, ( a result of the increase of land mass
> at this end of the signal path of about 150+ miles for much of Europe ) the
> addition of ground at this end in substitution for ocean is very
> substantial....
>
> For some as-yet-not-fully-understood reason (s) this occurs. I don't recall
> anything in any of the literature I've read about this, although it is
> certainly no coincidence that many of the early radio pioneers used seaside
> sites for their experiments.

The effect is known as "sea gain", and it often appears as one of the factors
in empirical formulas for estimating the field strengths of MW skywave signals.
Essentially, a skywave signal coming in at a low angle becomes a groundwave
signal for the last part of its travel (and the first part near the transmitter
site too, of course).  Attenuation of the signal over these segments of the
path is therefore much the same as for groundwave signals - very low over salt
water, and much higher over land (especially land with low conductivity).
The difference is the "sea gain" factor for receivers located close to the
coast.  A book I have ("Propagation of Radiowaves", from the IEE in the UK)
shows the sea gain plotted as a function of the ground distance for the path.
Since the gain is greatest when the skywave is at a low angle, it peaks when
the distance is near the maximum for one hop propagation (about 2000 km), two
hops (~4000 km), and so on.  In between it dips a bit lower - e.g., near 4000
and 6000 km it's about 10 dB, but it's only about 4 dB at around 2700 km and
6 dB around 4800 km.  According to the book, ITU-R Recommendation 435 gives
a procedure for calculating sea gain in more complex situations (receiver
located inland, irregular coastlines, etc), but I don't plan to delve into
it that deeply, since I'm nowhere near a coast. :-) 

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