[GreenKeys] 'Russian woodpecker' of old...

[Randy and Sherry Guttery]

  On 10/14/2010 11:42 PM, WA5CAB at cs.com wrote:
> Another thing to keep in mind before going gaa-gaa over 
> the numbers quoted is that pulse-echo RADAR is inherently 
> low duty cycle. 
Yes - and duty cycle determines the ratio of Average Power 
to Peak Power... However - RADAR power is conventionally 
(usually) quoted as average power.  Since (typically) the 
storage of a RADAR transmitter's energy is in the modulator 
- the power supply is only required to provide power at 
something slightly above average power (average plus factor 
in an allowance for the efficiency of the modulator / final 
stages). More on "the numbers" below... but I'll respond in 
turn to other points in the order made:
> You send out a pulse and then you sit and wait a while for 
> returns.  Other things being equal, the higher the duty 
> cycle the shorter the maximum range. 
PRR (Pulse Repetition Rate - also known as PRF - Pulse 
Repetition Frequency) is the determining factor of the 
maximum unambiguous range of a RADAR system... Pulse 
Width(t) is the determining factor of the minimum range of a 
RADAR system. Peak power is the determining factor of the 
maximum range of a RADAR system.  Duty Cycle (the ratio of 
the two) only speaks to the "Peak to Average" power ratio - 
and does not of itself give either factor.
> If returns come back after you start transmitting again, 
> you are not going to hear them. 
Unless they return after that second "pulse" has stopped - 
and the system is again "listening".
> And significant returns from a previous (to the most 
> recent) pulse are confusing and difficult to sort out. 
This used to be true - however - modern RADAR systems employ 
"staggered" PRRs to detect a return that past the 
unambiguous range. By staggering the PRR - a target beyond 
the unambiguous range "jitters" rather than being steady on 
the display. Further - such returns can be processed by 
computer and accurately placed at their correct range time 
(distance).
> So in the sense of radio transmitters operating 
> continuously, your normal assumptions will be off by an 
> order of magnitude or two.
If one considers that the power quoted for the Russian 
Woodpecker is indeed quoted as Average Power (as noted above 
- as is "usual") - then comparisons to continuous wave power 
are reasonable.  If, however - the power being quoted for 
the Woodpecker is indeed Peak Power - then comparison to 
continuous carrier would be off by a very large amount (I'll 
admit to assuming it to be Average Power).

Also keep in mind - unlike "traditional" microwave RADAR 
(i.e. hard pulse "simply" hitting a magnetron) - the 
Woodpecker was AM modulated - and quite possibly other forms 
of modulation as well.  It's been reported by several people 
to have detected intelligence via modulated AM in those 40Kc 
wide pulses- and some speculate there may have been 
additional modes contained as well. High AM modulation 
presents it's own "demands" on power supplies...

just my .02

-- 
randy guttery

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