Re: [IRCA] Interesting dx conditions

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

> Now, that's DX, Don.....have you seen their pattern?....2 Megawatts
> equivalent to Amphion St., 12 watts equivalent to el casa de Kaskey
> (wild estimates).

One of the misleading things about AM directional patterns is that the
picture is only accurate at zero degrees elevation, AKA groundwave.  When
the takeoff angle increases, the pattern can be very different.

For example, the WDDZ-550 pattern is a cardiod aimed roughly SSE.  There is
a minor lobe off the back aimed at WDEV-550 in Vermont.  The angle to bounce
a signal off the ionosphere to WDEV is about 40 degrees at that distance.
The pattern plotted at that 40 degree elevation angle resolves into a null
toward WDEV.  The minor lobe has disappeared.  So, WDEV has a signal minima
while the people who live in northern RI get service from that minor lobe,
all in the same direction.  It also explains why stations can be stronger
via skywave than the ground patterns indicate.  What is a null on the ground
can be a significant amount of signal at higher elevations.  That null on
either side of the WDDZ minor lobe is long gone at a 40 degree elevation,
and a good amount of skywave exists.

The way this works is that when you look at the towers from an increasing
elevation angle, the effective distance between them is shortened.   The
phase relationship is not changed, just the spacing.  It makes a different
pattern.

This also can explain why some stations are weaker by skywave than the
groundwave pattern would indicate.  And why for DXers, the patterns in the
various books and online sources should be taken with a grain of salt.  What
we need is a program to calculate the point to point takeoff angle, and then
show the station's pattern at that distance and elevation.  An interactive
pattern book.  If I had time, I'd take a shot at it.  Maybe someday.

Craig Healy
Providence, RI

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