Continuing from the discussion of 10m propagation modes,what role does diffraction play at these frequencies (10m thru 2m)? Will a nice sharp cliff-edge or knife-edge ridgeline cause RF ‘knife-edge’ diffraction the same way as a many-magnitudes smaller sharp edge diffracts light?
There is a location near me in the deep Teviot valley where, for about 10km, you can pick up the Stewart Island repeater on ESS116 (142.4500MHz / 139.4500MHz) 200km to the south. Reliably, every day. And often open it from a mobile set. This is a work (DOC) repeater, not an amateur one, but I’ve been puzzling over the effect for many years. The frequencies are close enough to 2m for the propagation to be similar.
The elevation profile of the path is as follows (with the repeater on the RHS).
The hills above the Teviot Valley to the south and west (Old Man Range) rise steeply, with many bluff escarpments and tors on the summit that lead me to wonder if diffraction is the explanation. The range to the north-east is a very gentle climb into the Lammerlaws and there is nothing there that would make reflection an obvious explanation.
“The Snowdon Effect - an interesting case of VHF propagation” by J. David Last, PhD, MIEE, GW3MZY
RadCom Feb 1983 pp 136-138
EDIT
I posted whilst having a morning cuppa and was not properly awake!
The article explains some knife edge diffraction and how a 2m repeater can be heard some considerable distance away when blocked by the bulk of Wales’ biggest mountain Snowdon. (Snowdon should now be known by its Welsh name but I can’t remember it yet, the old neurons are slower to learn as you get older!)
De fraction is what’s left in a bottle of good red wine left in my care.
It’s also how Aircraft Enhancement works.
If your repeater can “see” the ridge and the ridge subtends a couple of degrees or more at the Rx site then enhancement can occur. Going mobile should give a clue. The signal should fade out and then maybe return and fade out.
If you and the repeater are far enough from the ridge then it has no effect.
