[NJARC] Hedy Lamarr

[Harry Klancer]

Nick,

There was an article about Hedy Lamarr in "Invention and Technology" 
magazine about
7 or 8 years ago.

In frequency hopping, instead of using a fixed carrier frequency, a 
transmitter uses a number
of carrier frequencies in the same band, first it sends info using one 
carrier frequency for a
period of time, then it changes to use another carrier frequency for a 
period of time, then another
frequency for a period of time, etc. The transmitter and receiver have 
to have an agreed-upon
arrangement for what frequencies will be used, and for how long. In 
addition, they must be
synchronized so that the receiver is looking at the right frequency at 
the time when the transmitter
is using that frequency.

The agreement between sender and receiver can be in the form of a fixed 
arrangement of frequencies,
for example, f1 then f4 then f2 then f5 then f3 then etc. Or you can use 
something called a
pseudorandom number generator, and I'm sure there are other schemes. My 
understanding is that Hedy's
scheme used a piano roll to tell the transmitter and receiver what 
series of frequencies to use.

Some advantages of frequency hopping is that the receiver is less 
susceptible to narrowband noise.
Suppose that there is a lot of noise around frequency f4. The receiver 
is only listening around frequency
f4 for a short time, and then it moves on to another (presumably 
quieter) frequency. Another advantage
is against eavesdropping - unless an intercepting eavesdropper knows the 
pattern of carrier frequencies
that will be used, it can't follow the communication between transmitter 
and receiver since it doesn't
know where to look.

Hope this helps.

                                                     Harry K



NICHOLAS SENKER wrote:
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> An article in yesterday's Newark Star Ledger about Hedy Lamarr (not to be confused with Hedley Lamar in 'Blazing Sadles') discribes the sultry beauty as not only a  great actress but also a 'brainy beauty'.  This is not an April Fool's joke!
> To quote the article (p 29):
> 'in the early 40's Lamarr came up with the concept of frequency hopping. Her invention was 40 years ahead of its time. It was a secret communication system.  It was the first time wireless radio transmissions could be sent and received without being intercepted.  Her children stress how the wireless world uses frequency hopping today with PDAs, wireless phones, and Wi-Fi. Composer George Antheil implemented Lamarr's concept.  He and the Vienna born actress received a patent which they gave to the US government.'
> Amazing!
> Can someone explain frequency hopping?
> Nick Senker
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