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Re: LF: MF propagation over sea water

To: [email protected]
Subject: Re: LF: MF propagation over sea water
From: DK7FC <[email protected]>
Date: Thu, 18 Jun 2015 02:01:50 +0200
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Hi Alan,

Well, it is not a new effect that i've discovered, maybe i should have added this, as usual.
I'm here talking about MF, i.e. the 630m band and there is no daytime range of 1000 km above land even with more power then you may expect. The typical daytime range over land is 300...400 km. This is what i see since several months now.
For example: No chance to get a decode by F6GEX in daylight, while i am quite detectable at night:
Timestamp Call MHz SNR Grid Reporter RGrid km
 2015-06-05 01:48   DK7FC   0.475682   +8   JN49ik   F6GEX   IN97na   758 

Of course it depends a bit on the ground conductivity and hills and so on. But i never (!) made 800 km over a land path in daytime. So there is not really a skywave on MF in daylight. OK it may still be there but 60 dB weaker, not to mention for our purposes.
So in that specific situation, the decode by TF3HZ, it must be a combination of skywave reaching the sea water and then travelling above the water as a ground wave, 2400 km in sum.

I found that quite interesting and the effect is nicely shown in the capture of the WSPR map, thus i thought i'll share it :-) This is the point i made here.

73, Stefan



Am 18.06.2015 01:18, schrieb ALAN MELIA:
Hi Stefan, I'm not sure what point you are making here....maximum skywave daytime range is about 1000km so there is no ground reflection. However a receiiving station near seawater can get a "coastal effect" which is a bit like focussing receibing the direct skywave plus a slightly shorter one which is reflected off the coastal water. This effect  is not available on land because of the larger variations in "roughness" over land as well as its absorption. Also over water at 400km there is substantial groundwave, though the effect of this depends on the power radiated and S/N at the receiver For commercial station (Big antennas) the ground and skywave are around the same strength at about 700km. Amateur station do not radiate ground wave so efficiently so the groundwave is maybe about 6dB weaker (this was what we measured at 136 about 10 years ago.)  Would that help explain it??

Alan
G3NYK
----Original message----
>From : [email protected]
Date : 17/06/2015 - 23:48 (GMTST)
To : [email protected]
Subject : LF: MF propagation over sea water

Hi MF,

While there is nearly no chance to leave decodes above 400 km in 
daylight above land, it seems to be no problem above sea water. As soon 
as the first hop (?) hits the ground on water, the wave can travel to 
the RX site. At least this attachment shows this effect. The distance 
crossed in daylight is about 800 km in this picture.

73, Stefan


  
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