Topband: Testing Beverages with AM BCB signals

[Rick Karlquist]

I often use BCB signals as a sanity check for my beverage antennas.
I have three of them in a 400 foot equilateral triangle that can
be driven at either end to give me beams at 30, 90, 150, 210, 270,
and 330 degrees.  I found a great site that shows transmitter
locations:

www.radio-locator.com

Radio stations that were a
good distance away (25+ miles) were received best on the beverage pointed
in their direction and their signal strength dropped down
progressively as I selected beverages pointing to the side
and then reached a low on the beverage pointed away.  20 to 40
dB front to back was common.  You might think 400 feet is
too "short" for BCB, but actually the beverages are great for
BCB DX'ing.  I once got reception of WCBS in New York from here
on the west coast using a beverage.  I can often receive 2 or
even 3 different stations on the same frequency by switching
beverages (and none can be heard clearly on my vertical).

I noticed a curious phenomenon:

Nearby stations cannot be "DF'ed" by using the
beverages.  They seemed to not be localized.  The closest station
(6 miles away) has the vaguest directional characteristics.  It's
60 over S9 on any beverage; not much difference.  BTW,
I have actually seen this station's transmitting towers, so I know for
sure what direction they are in, in case there is an error on
the site I mentioned.

These tests were made at noon to minimize sky wave effects.

Does anyone have any explanation for this observation?
Has anyone else encountered this?

I'm thinking it might have something to do with tilting of
the wavefront of the ground wave.  Maybe the waves have to
propagate for a goodly distance before they acquire "tilt".
I don't understand this very well, but books explain that
the beverage receives vertically polarized waves using a
horizontal wire because of this "tilt".

Rick N6RK