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Monday, January 19, 2015

When Your Local Oscillator Could Sink Your Ship!


Thomas KK6AHT is the intrepid radio amateur from France who moved to California and successfully built a Minima as his first homebrew project.  Yesterday we got additional evidence that Thomas definitely has THE KNACK.  It seems that he has been looking through really old radio magazines (clearly a symptom).  He sent us this add from 1942.  Yikes!  Imagine getting your ship sunk because your Local Oscillator is not quite local enough!  
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Hey!
I thought you would get a kick out of this 1942 ad. Sounds like those guys had a some good reasons to fight QRM ... Who knew the important role played by the FCC during the war?
Now tell me: why were the receivers so noisy at the beginning of the war? What did they change? Happy new year to you both! May the gods of radio (and digital) bring you much fun on the air.
73, Thomas  

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Hi Thomas,
Well Thomas, I will dip my toe into some very deep water and attempt an answer for you which also is an important clue about QRP operations.
  1. Today we have many many signals co-existing in the radio spectrum. During the war there was much less radio garbage and the military lit up the ether with transmissions very sparingly. But that is on the transmit side. That said the local oscillators (much like you have with the Si-570 on your Minima) used in receivers also produce RF that unless is minimized in some fashion is passed right through to the antenna and can be detected. Regenerative receivers are especially prone to this. Yes some military equipment used regenerative receivers. In fact the famous Paraset had to be constantly moved so it would not be detected.
  2. This receiver generated RF into the antenna was addressed by companies like Scott by shielding everything. That receiver in the photo probably weighed about a 100 pounds or more. Much attention was paid to RF bypassing and grounding. The cheap table top radios were RF generators par excellence.
  3. There was another approach developed in WWII to solve that problem and forms the basis of what makes work that little device sitting in your pocket. The odd part it was invented by a famous movie star. Look up Heddy Lamar in wikipedia. She and a co-inventor came up with the concept of frequency hopping and spread spectrum technology. By jumping frequencies it would be hard to pinpoint a transmitting station. That concept forms the backbone of our cellphone system
  4. Now the QRP part – if the RF output from a receiver local oscillator (milliwatts) can be detected from afar – then it follows QRP works!
Have fun. 

Pete



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