OBSERVATIONS FROM NORWAY & SCOTLAND   02 January 2008   

A short  Joint Article by LA2QAA & GM1SXX

Passive Radio reflectors... or 'Getting out of a hole!"

Much confusion and misunderstanding exists regarding the usefulness (or otherwise) of using passive objects such as insulating plane surfaces to reflect Radio Waves and it seems nowhere more so than on the amateur satellite scene.Well folks, there's nothing quite like practical experimentation to see what works and what doesn't.

For quite some time, LA2QAA has been operating via amateur radio satellites from his QTH in Frei Island, Norway.

 He's in a lousy location for normal VHF/UHF working thanks to the presence of a few  'obstacles', just one of which is pictured here.

Now many people would just call it quits and take up Tiddleywinks or Monopoly to while away the time.

LA2QAA doesn't give up so easily though and has used satellites from this QTH for a long time.

His 'secret'?  That big lump of rock you see in the picture.  Contrary to what some may believe, such surfaces can and will assist in the propagation of radiowaves in the VHF and UHF part of the spectrum. Consider for a minute normal police and fire-brigade handheld radios in an urban environment. Such environments sport a plethora of surfaces suitable for reflecting radio waves such as supporting steelwork, and many flat straight edged surfaces of varying size. If insulating plane surfaces *really* stopped radiowaves in their tracks, rather than by scattering them, these radios would not work inside steel framed buildings full of steel reinforcing bar and concrete but clearly they do indeed work, as clearly witnessed by the many people who use them in such environments.

The FCC's GMDSS Examination Page  outlines some useful facts in Questions 14, 15 & 16 where 'Knife Edge Propagation and VHF and UHF frequencies are covered. http://members.aol.com/ab0di/el3c2.html

LA2QAA says.... I have done a fair amount of AO-7 work Stateside using knife-edge propagation (refraction) because I am COMPLETELY blocked in that direction by 'Freikollen' ... the main Mountain here on the Island.   I have also worked PH7PCF when AO-7 was 90 degrees !!! to the mountain by 'firing' the 70cm array at the rock-face at a 45 degree angle ... this is mentioned in one of the previous OBS (direct reflection). 

So yes, you CAN use big lumps of rock as a passive reflector on VHF & UHF.

Knife edge propagation can allow you to get a signal out of a 'hole in the ground' provided it is a large hole with sharp edges!  OK, It's a lot of fun trying to work out the best place to point your aerials during a pass, but that only adds to the challenge of working satellites.    Wikipedia also makes mention of Diffraction effects in http://en.wikipedia.org/wiki/Radio_propagation#Diffraction

 LA2QAA suggests the unconverted may like to try a little experiment...

Items required.
1 x shoebox
1 x torch
1 x black card
1 x white card
Spray inside of shoebox BLACK. Shine torch across edge of box. Note varying magnitudes of light on inside surface. If light does it - so will radio waves ... SSNW.

SSNW? Same shit, new wrapping. (Don't forget  ... light is also an electromagnetic wave... GM1SXX).

We won't go into how 'knife edge diffraction' works in this little 'bite sized' article, but LA2QAA for one is rather glad that it does.  The reader is encouraged to do a little research of  his/her own in understanding why this useful propagation mode works.