Signals and mountains

High radiation angles give us short hops and low radiation angles give us long hops. High radiation angle energy can make it to the DX, but because many more hops are required to get there, the signal will be weak or non-existent.

-- Rick Hiller, W5RH, in Low Band DX Antennas
http://bvarc.freeshell.org/w5rh/low_band_dx.htm

I know virtually nothing about radio waves and antennas, but this page outlines how I think about signals arriving at my antennas. It explains why my contest scores were so bad from my first forays in 2002 till March 2004.

What is a "good" radiation or take off angle for DX contesting? The general feeling is: the lower the better. If you can hear them, concensus is you can probably work them.

The strongest long-distance DX signals arrive at an angle of incidence of 5 to 15 degrees (that's the red line). Geometry dictates that the same station's signals arriving at 25 degrees (green line) have to hop more times to get to you. Yes, you might still hear them, but they will be a lot weaker.

Let's say my yagi antenna hears best when signals come in at 25 degrees. If the station is a long way off, there isn't much energy in those 25-degree signals because they have to hop many times, each hop taking away more energy. Still, my yagi scoops up enough of the 25-degree energy (gain) so I can hear and work those stations -- although often only marginally.

The 5-degree signals are a lot stronger, having taken only a couple of energy-sapping hops. The yagi does hear these low-angle signals, but it has very little gain (not collecting much energy) at low angles. Bottom line: every bit of those low-angle signals you can get into the antenna helps you hear better.

Mountains and Blocked Signals
Now, with a mountain blocking European and North American signals below 13 degrees (as my mountain was -- see map), I had no low angle signals to work with. I was only hearing signals arriving at angles above 13 degrees -- meaning anything I heard was getting to me at high angles, which for DX takes lots of hops (blue line). On-air experience bears this out very well.

With an improved visual horizon toward Europe now allowing signals down to 0 degrees, and North American signals down to 5 or 6 degrees, my antennas at least have a chance to hear those stronger (fewer hops) signals from a long way off, and the strong high-angle stuff from closer in, too.

Again, I'm no expert source -- real ionosphere jockeys won't think much of my halting analysis but this isn't intended to be a scientific treatise; it's just a study of my situation and on-air experiences. It quite nicely explains the on-air performance of my station pre-April 2004 and what I hope to hear after April 2004.

For good reading about how signal angles affect your ability to hear and work DX, check these Web pages:

Angle of Radiation: What Is It?
eHam article by K2WH

Antenna Basics
Mildly technical, a good primer

Radio Wave Propagation
Great tutorial and very complete glossary of terms