Tuesday, November 25, 2008

Low-band DX'ing: The high-angle mode hypothesis - G0KYA contribution

In my previous post, I gave a few links mentionning the "high angle mode". I found extremely interesting G0KYA's paper "the twilight zone".
This is the introduction: Steve Nichols G0KYA, of the RSGB's Propagation studies Committee, believes that propagation around sunrise and sunset is not fully understood. Here he outlines the mechanisms behind grey line and other twilight propagation modes and a research project to help us understand them.
And a particulary interesting paragraph
Now imagine a radio wave hiting the ionosphere at about 75-85 degrees to the earth - a near vertical incidence wave (NVIS). Below the critical frequency, it would be returned. At some frequency close to fof2 it could be refracted through a large angle and could end up travelling almost parallel to the earth, giving a very long first skip distance. This is the condition for the Pedersen or critical ray, discovered in 1927, characterised as being high angle, long distance and close to and probably above the fof2 frequency. As there would be no intermediate ground hops the signal strength could be very high indeed.

Following this reading, I exchanged a few emails with Steve (one of them, reproduced here with
his authorization)
Hi Patrick, Thank you very much for your e-mail. I have had a quick look at your blog and it is very interesting. Your message was actually very timely as I have been looking at WSPR as a greyline beacon tool, but had put it to one side to concentrate on other things - I will pick it up again! Here are some thoughts and observations that might be useful.
1. VOACAP is not very good at predicting openings below about 5MHZ. It was never intended for this and is usually very pessimistic. W6ELProp is better.

2. You are quite right - it doesn't take into account sunrise enhancements I did some work on signals from VP6DX into the UK on 80m and using VOACAp looked at the number of modes (hops). It showed clearly that there are more modes around sunrise and the radiation angle goes higher. I can send you this if you want. I am therefore convinced that greyline has more to do with higher angle signals due to ionospheric tilting and multiple modes than the old "D layer not illuminated/F layer illuminated" model.
This also explains why you get good greyline openings at right angles to the terminator.

Thanks for thinking of me.
Steve G0KYA
RSGB Propagation Studies Committee
You will find other articles and powerpoint presentations on Steve page, definitely a "must read" for whoever is interested by those phenomenas (the above pictures are from Steve presentations).
Steve's VP6DX presentation is available as a PDF.
By the way, the "high angle mode" has a name: The Pedersen Ray... and nothing new, since it was discovered in 1927...

1 comment:

F6AOJ said...

Hi Pat,

I Don't Agree Comparaison between 40m-Ace versus G-40m QSO stats.
It is not a good method because it is several reasons explaning qso's spread versus time
witch have no connection with propagation:
- Sleeping period, Gauss curve centred on 2.30 AM for 95% of ham population
- Working peroiod, Starting 8 AM for 80% of ham population ( retired 15%)
- Competition knowledge of Ham poulation regarding other dxers group.
ie : Why to loss time to fight with Russia hen they are are their SS
- Most part of dxers know what advantge is propagation in S/N RX at SS.

The comparation to be good, that mean without human factors,
should be done with S/N Rx fron UK versus time.

In that case the global profile will be probably globaly the same,
except around UK SS time.

I never read in propagation models manuals,
how they take care in a circuit prediction of short propagation change around SS and SR.

Those short particular cases have no use for professional communications
who want only stable circuits for long periods.
In consequence for this reason, model designers never try to solve the problem,
and also because they dont know how to integer it
by loss of knowledge explaining theory .

By the way the work of the Propagation studies Committee, is remarkable
I have no comments about explainations they are sugesting,
but it is not surprising me to learn that it is more complex as the basic, now old, theory declare.

Everything on the edge is much more complex than elsewhere,
a diode PN jonction is good typical exemple.

73 Jeff F6AOJ