Knife edge refraction!

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Knife edge refraction!

Post by Gregtheham » Sun May 13, 2018 7:38 am

Those new to broadcast V/UHF may be surprised to learn that you can sometimes see “over” a hill or mountain using knife edge refraction.

I am amazed I get deep fringe channels from my location north of Toronto, on the “ blocked side” of the Oakridge Moraine (a high landform that separates Lake Ontario and Lake Simcoe- like the Great Divide ). Toronto and Buffalo are also in the low Lake Ontario basin .

Even worse, I am also in a very low valley and completely blocked to the south - towards the signals of Toronto and Buffalo.

Yet, with my 60 ft tower and yagi with corner reflector (gleaned from a vhf-uhf deep fringe ant) I get most UHF channels from Buffalo ! (and Toronto- tnx to the CN Tower)

It sometimes depends on WX, but I believe it is more due to knife edge refraction off the newmarket hilltops ( top of the valley) and/or the Morain , and NOT Tropo.

Give it a try if your buried in a low valley or on the backside of a high landform.- u may be surprised.

I haven’t bothered trying it on lowband VHF- I don’t have a suitable antenna (or room on the tower) I’d love to get channel 7 - WBBZ.

Note: This effect is also used on VHF radio communications -with 10 Watts and short boom yagi I can talk to Ripply NY and Erie PA

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Re: Knife edge refraction!

Post by tek2000 » Mon May 14, 2018 6:04 am

Well, I'm just outside of Toronto (near Lake Ontario) and I've got two pairs of bow-tie feeds at the convergence region of my 12' c-band dish and I can pickup up all 1st-edge, 2nd-edge and most Tropo signals from Peterborough, Mid-Western Ontario, Rochester, Buffalo, Erie, Hamburg and a few others. It helps to have your dish motorized (azimuth, elevation, skew), so you can scan the whole sky.

I've scanned nearly 150 "local" channels (the norm for Toronto with a standard 8-bay antenna is about 50 channels if you are lucky) into the ZGemma and I'm sure there are hundreds more. You just need a really narrow-beam and pin-point accuracy to pull them in. Most Tropo signals have a signal-to-noise ratio that can vary by 10-15 dBs over a few hours as atmospheric conditions change dramatically. You'd be surprised whats "up" there...

I bet a motorized 14ft or 16ft parabola could potentially scan over 300 channels from places as far away as Detroit and Cleveland, even Chicago, NY and Montreal. Got to try it someday. :cool2

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