Whence the nighttime interference on 1030?

[Rob Landry]

The most likely cause of interference to WBZ is WBZ itself. There are two 
types of emissions from AM stations: a surface wave, which follows the 
ground, and a sky wave, which follows a straight line into the atmosphere. 
When it hits the "D" layer of the ionosphere it is absorbed and 
disappears.

However, the "D" layer is only present during the day. At night it 
disappears, and the sky wave continues until it hits the "E" or "F" layers 
of the ionosphere, which reflect it downward. That is why people hundreds 
or even thousands of miles away can hear WBZ at night, but not during the 
day.

WBZ's surface-wave coverage is very impressive; driving to visit my father 
in upstate NY a few years ago, I had a listenable signal from WBZ all the 
way to Schenevus, NY, on I-88 just short of Oneonta. That's about 150 
miles, all of it over land -- excellent for an AM staion in daytime.

If you live at the outer edge of WBZ's daytime coverage area, you will 
often receive both the surface wave and the reflected sky wave, and they 
will interfere with each other. In the worst case they can completely 
cancel. If they do, you'll hear anything else that happens to be on 1030 
but not WBZ.


Rob

On Wed, 13 Feb 2019, Doug Drown wrote:

> I live on the upper midcoast of Maine, where WBZ's daytime signal comes in
> like gang busters --- absolutely crystal clear, no hiss or interference, as
> though the transmitter were just down the road.  One the sun sets, however,
> it deteriorates rapidly, especially at this time of year.  What is/are the
> station(s) I'm hearing on 1030 kHz?  They seem to interfere with one
> another, such that I can't make anything out plainly.  Am I correct in
> thinking that 'BZ's signal is nondirectional?  If so, the presence of these
> other stations' signals surprises me.
>