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Re: fail safe




Bob  W7LRD wrote......

>To: AMSAT-bb@AMSAT.Org
>
>Hello	
>In "resetting"up my station for mode "B" & "A", as well as "S"band
>receive  (Drake).
>I realized that if I'm not careful, I could easily apply RF to the output
>of pre amps and or converters.  I would be curious to know what kind of 
>"fail safe" measures some of us use to prevent such catastrophes.
>73...Bob...W7LRD
>----

Bob,

  One effective way to "fuse" the outputs of gasfet RX amps is to make
  small (and Cheep) amps from MMIC amplifiers.....they take 10 or so
  watts ONCE without passing the power along to the more expensive
  preamps....use them liberally at the output of receive converters
  and preamps to give about 15Db gain....you can even afford the
  luxury of a 10Db pad for additional isolation.  Replacement MMICs
  are about the cost of good quality fuses!
  
  The most effective method I use, especially when mast-mount amps
  get expensive is a T-R relay at the antenna just before the preamp
  then cheep coax into the shack to the receive converter input and
  a *SEPERATE*  low-loss coax from the power amplifier up to the
  TR switch at the antenna; a separate dry contact at the antenna TR to
  provide a TX interlock and a relay sequencer round out the method.
  
  IMHO this is the only fool-proof method to prevent smoking mast-mount
  preamps.....I lost several EME contacts as a portable by NOT going
  the distance and separating the feeds! After three or four preamps
  get destroyed, you can immediately see the benefit to the two-
  feedline method. First you save on one mast-mount high power relay
  (and its loss) and secondly since the preamp establishes system
  noise right after the TR relay at the antenna you can run RG-58
  or similar coax for a considerable distance before you loose system
  RX noise figure.  I.E. if you have 20 Db gain at the preamp you can
  run about 200 ft of RG-58 at 144 before the loss adds into the noise
  figure.
  
   Hope this helps.
   
                                           Cheers!
                                           
                                              kermit
  



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