martin,
I also heard on an old NPR broadcast that XERF would bring close 
flying birds out of the sky to their demise.  And I believe, in their 
heyday there were a number of "super power" stations just across our 
southern border.
Pat, K9JAUAt 12:00 PM 1/24/2011, you wrote:
>         I recently finished reading a book about one of the
>biggest medical fraudsters in US history who snake-oiled his way
>to riches during the 1930's in the United States and then from a
>powerful AM radio station on the Mexican border. His name was
>John R. Brinkley and he called himself a physician though he had
>no valid medical degree.
>
>         Most of the book is not directly radio-related, but
>there is a brief description of Brinkley's Mexican boarder radio
>station which later became XERF and I am sure some of you
>remember the preachers selling all kinds of stuff that one could
>hear after dark on XERF.
>
>         XERF's transmitters were just across the river from Del
>Rio, Texas and, at one time, were 500,000 Watts directional in
>to the United States.
>
>         People report that at night, the antenna towers arced
>with green flashes. XERF leaked in to telephones in town, could
>be heard on things that weren't even electronic such as bed
>springs, and made the headlights of cars come on even when they
>were supposed to be off as they drove around town.
>
>         XERF transmitted on 735 KHZ in the mid thirties and many
>of the radios were TRF or Tuned Radio Frequency receivers. These
>were the predecessors of the superheterodyne radios we are more
>familiar with. In a TRF radio, you have 1 or more tuned circuits
>which you peak and tweak until they are centered on the
>frequency of the radio station you want to listen to. There is
>no IF, no conversion, no nothing; just tuned tank circuits of
>sometimes dubious Q factor between the antenna and detector.
>
>         If you ever had a crystal radio or have seen an old
>radio from the early part of the last century, it was a TRF.
>
>         In the 1930's, XERF had such a power-house signal at
>night that people trying to listen to other radio stations in
>other parts of the AM band suffered interference from Brinkley's
>transmitter no matter where they tried to tune.
>
>         It was a combination of tremendous AM transmitter power
>coupled with basically poor quality receivers.