Antipodean Focusing - heard of this?

[VK2FAK]

Hi all...

Looking for a definition of the term....Antipodean Focusing allowance.. in percentage terms...

Have looked in my books.. and did a search online ......nothing...

Is this just a fancy term for something with a more common name...?

Ok...just found a pdf about antipodal focusing...so I have some reading.....learn something new all the time...still not sure how this goes into percentage terms...but...reading..

any ideas where to look....?

John

[VK3YE]

This is a stab in the greyline, but I'd be looking in G4ZU Les Moxon's books, particularly where he discusses QRP tests between the UK and VK/ZL, chordal hop, low radiation angles and operating on ground sloping towards the favoured direction. I think VK3MO has also done work in this area.

When locations are at exact antipodes (eg G and ZL) beam headings could vary. Signals may take a variety of paths from A to B and at times several may arrive at the receiver at once. Signals may add and subtract in strength and phase and odd fading or echoes may be heard. And beam headings are less clear since distances are all about the same, whatever the direction.

It's been demonstrated that antipodean paths have propagation benefits. DXers who live at the antipodes of heavily populated areas can exploit as the following notes explain:

http://www.nsa.gov/public_info/_files/t ... gation.pdf

http://ieeexplore.ieee.org/xpl/articleD ... er=1140329

http://member.melbpc.org.au/~jadcock/ch ... ordal.html

http://www.antennex.com/prop/prop1206/prop1206.pdf

http://onlinelibrary.wiley.com/doi/10.1 ... 5/abstract

http://k9la.us/Propagation_to_the_Antip ... isited.pdf

http://nrcresearchpress.com/doi/abs/10. ... 3bw6fmSxEw

[VK2FAK]

Hi all...

Thanks for all the links...they will keep me reading for a while....I found one in your list that gave me the general idea...and I think the others will expand a bit on it...

Thanks again...Peter

John

[VK3ZAZ]

As I understand it and from my experience to the Azores my antipodes, the phenomena makes it really easy to work because all signals cross no matter where you point so you only need a sniff of prop in any direction.
I have worked my antipodes on every band from 80 through to 6 metres and often there is nil mid path just the guy at the far end at your antipodes.

Zl1RS had such a path on 6M at 19,900K a few weeks back.

There are others that say the focussing is due to the whispering gallery effect, which after you read all the gunk about target and focussing you can read about the similarities between 160M and 6M wrt the signal skipping along the underside of the upper f layers and dropping out at same time same place 11 and 22 years apart.

[VK3QI]

John,

Two early papers on antipodal focussing were the following:

(1) Pipp and Webster, Radio Science, Volume 68D No 3 March 1964

Using a 15.9 Mhz beacon between Perth W.A. and St George Bermuda, which is within 90kms of Perth's antipodal point


(2) Bold, IEEE Transactions on Antennas and Propagation, Volume AP-20 No 6 November 1972

Using 15.27 MHz and 11.79 Mhz between the VOA site in Tangier Morocco and 2 points in New Zealand ((Ardmore in the North Island 156kms from the antipodal point) and Invercargill at 1288kms from the antipodal point between the two sites

Bold also published an earlier paper in the Journal of Atmospheric Physics Vol 31 November 1969 on the same topic.


Both publications can direct you to further references.

Worth chasing up these articles.

Cheers

Peter VK3QI

[VK2ZRH]

Antipodean focussing is a geometric effect, not an ionospheric propagation phenomenon.

You will recall that a "great circle" line drawn on the surface of the Earth has its centre at the centre of the Earth.

From any point on the Earth, all great circle lines at any azimuth (compass direction) will converge on the point that is geometrically (and thus, geographically) opposite (the antipodean point). Hence the term - antipodean focusing.

Posted in the interests of dispelling myths and woolly, half-recalled thoughts.