Topband: Antipodal Focusing Of RF Signals

[Ronald Sekkel]

Hello gentlemen !

Living in a place (Sao Paulo, Brazil) where the antipode is very RF active 
(Japan - actually the exact antipode would be a little south of Okinawa) I 
must say contacts with JA's are not the most difficult from here - and I've 
worked them from 160 up to 6m. Actually several JA's travel to Okinawa just 
to QSO with PY's on 6m, as openings on that band to Brazil happen much more 
often from JR6 than from JA main land, probably due to the antipodal 
focusing. Sometimes stations like JR6HI are s9+40db here on 6m - it's amazing.

Of course the QSO on 160m is much more difficult than in any other band. 
The main difficult to work JA's on 160m is the short darkness window (about 
15min max, around our sunset and sunrise), even during our summer when 
conditions are better, and the usual summer QRN of S9+ - there's always a 
thunderstorm around. As I keep the radio QTH in a small ranch 80km north 
from Sao Paulo city (where I work and spend my week days), even my wife 
knows during the summer we must leave earlier to the radio QTH on Friday's 
in order to be there before the sunset...

There's a JA broadcast in 3925Khz (Radio Tampa, I guess) I use to reference 
the JA sigs on 80, which is a good info for 160m too, as I've never seen a 
160m opening to JA that did not happen at the same time on 80m. I was told 
Radio Tampa uses 25KW into a dipole (??) and it ranges from S4 to S9 here, 
almost every day during our summer.

I understand one big advantage for the antipodal QSO is there's always a 
path not going over the pole, where absortion is quite high, in special 
when sigs must cross the South Pole. Australia and New Zealand are not so 
far each other, but the short path from Sao Paulo to VK goes through the 
South Pole, and to ZL it doesn't - and that makes a huge difference, as QSO 
to VK from here is quite difficult on all bands, even on 20m if we are not 
in the top of the sunspot cycle.

My 2c.

73's and DX

Ron




At 18:56 29/03/2004 -0300, you wrote:
>I - and a lot of DX'ers who specialize in international MW DX - would 
>disagree with this. There are many examples of incredible antipodal 
>receptions, with recent examples being the 300 kw 1557 Taiwan station 
>received with some regularity in Brazil.
>
>One advantage that the MW DX'ers have is that they have a larger range of 
>targets, they are on the air 24 hours a day 365 days a year, and use 
>powers ranging up to 2 MW. That leads to some receptions that would 
>surprise a lot of hams.
>
>
>Chuck
>
>
>
>
>>True antipodal focusing of transmitted RF signals does occur at HF 
>>frequencies. BUT as Bob Brown NM7M pointed out in an earlier post the 
>>ionosphere at medium frequencies is to heterogeneous to support antipodal 
>>focusing.
>
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Ronald J. Sekkel - PY2FUS
py2fus at integral.com.br
Grid locator: GG66pt