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Re: LF: 136KHz propagation

To: <[email protected]>
Subject: Re: LF: 136KHz propagation
From: "Alan Melia" <[email protected]>
Date: Fri, 3 May 2013 10:56:08 +0100
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Hi Markus, Spiros, there is another interesting effect at dawn. The weak rays of the rising sun actually strike the ionoshere first from underneath !! this means that the first photodissociated ionisation is very highly absorbing because it is formed at lower levels where the pressure is higher and the electron mean-free-path is very short. This has the effect of producing a deep dip in levels at dawn at mid-path. That is dawn at an altitude of 50km or so, not ground level. This effect is most noticable on N<>S paths which I think is the direction of the grabbers from Spiros.
An hour or so later the Sun will have risen sufficiently to illuminate the path from above and the daytime D-layer will build to provide daytime skywave. The signal levels in daytime usually show a gentle "dome" shape with the peak at mid-day at mid-path.
in the early days of 136kHz it was though that dawn would be a good time for DX because the noise was lowest. However we eventually realised that the noise was low because the distant noise which propagated by skywave was wiped out at dawn by this effect. Thus the best DX times are often (but not always :-))   )  early evening or about an hour before dawn.
Great Fun this Propagation stuff :-))
Alan G3NYK
----- Original Message ----- 
Sent: Friday, May 03, 2013 8:36 AM
Subject: Re: LF: 136KHz propagation

Hi Spiros,
not really strange: the rising sun is suddenly ionizing lower ionospheric layers and increasing the absorption. 
Here's a somewhat simplistic explanation of diurnal propagation: At night, LF is mostly reflected by the lower boundary of the E-layer, at a height of about 100 km. After sunrise, radiations starts to penetrate deeper and ionize lower layers. So the conductivity of the D-layer increases so much that it starts first absorb and later reflect LF. 
This is somewhat similar to a shunt resistor inserted in a transmission line: During the night it's a large resistor which will let most of the wave pass through, absorbing only a small fraction of the power which depends on "conditions" (like DST index, describing charges leaking down from a storage ring). At sunrise, the shunt suddenly conducts more ("closer to 50 ohms") so most of the wave is being absorbed. At midday when the sun is highest, the conductance is approaching a "short circuit" which reflects the wave from a lower altitude (about 80 km).
After sunset, the electrons and ions in the lower layers recombine quickly so the D-layer becomes transparent again. However at high altitudes, the gas density is so low that charges take long to find a partner, thus ionisation of the E and F layers persists throughout the night. 
Best 73,
Markus (DF6NM)

Sent: Friday, May 03, 2013 5:58 AM
Subject: LF: 136KHz propagation

GM to all.
The same strange propagation tonight as every night.
At around 03.00z my signal disappear from DF6NM, DK7FC, YO/4X1RF & G4WGT
I have better prop earlier and during the day with day light.
73, Spiros

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