Re: digital modes on 2m 70cm


Hi Graham
You brought back a memory of when I was working for the County as a radio tech. I was troubleshooting a pesky problem with a speed radar gun used by the Sheriff. The needle would hang up about half scale instead of falling to zero. Out of desperation I installed a headphone jack in the Doppler Radar audio path. I found the switching power supply was introducing a whine which was seen as a "signal." After I repaired the power supply, I took it on a test run. With the headphones I could hear the Doppler shift as I panned the dish antenna. Pan the signals up a stairway and I could hear the steps. I could tell where walls were. (Note: this could be developed into a useful tool for the blind) If I aimed it at a vehicle that was using a stick shift I could hear the shift points. Hmmm I thought what would it do with Military aircraft landing at a nearby Navy base? We drove to a frontage road that crossed the landing pattern, stopped and waited. Sure enough, a "Tom Cat F16 was landing. There are no "sights" on the gun to aim the marrow beam - but I was able to get a return and the audio tone went thru a screaming 8-10 KHz and the meter pegged hard well above 100 MPH!! I did not stick around for the MP's to show up! I wonder if the F16 IFF alarm went off?!
73 Bruce WA6DNT       

On 10/21/2020 1:34 PM, Graham wrote:

Interesting  point on the  military aircraft , popular rumer had/has  it , that one of the  local  police , out on traffic speed  duty , close to a  scottish air base , out of curiosity , pointed his  hand held 'speed gun' at one of the  aircraft coming  into  land , which lit up the  counter measures suite , end of career .. 

Its more to  re-radiation , most of the  aircraft to  day are  composite hulls ,  [one of the reasons for the  11 meter  ww2 radar , was aircraft wing was round 35 feet , so it gave a  good  return  signal]  so the  signals are not as  good as  in the past , polarisation ,  probably Horizontal , I would  expect the  return  signal to  be modified  , dopper plays a part as well , so the  decoder needs  to have a  wide  acceptance 
bandwidth , I think the  doppler shift is up to  x2  if the  target is  advancing ,  first shift  tx> a/c  then a/c >> rx  , at 600 mph , that's  round  +150 Hz on 2 meter's  ?

Ive  found this  write up  by two  spanish  stations  using a  plotting/ prediction software package to  give the  expected  angles / range , google translate makes a  good  job  of converting  to  english , just noticed it was  back in  2014  , when the  decoder was re-worked  during the  eme  testing

This was a  scatter  spot , from a French  10 watt  70 cm  beacon, using a  halo round  Ae to  south of france   to paris 704 Kmt , confirmed by 
the  keeper 

19:56 432418 F5ZAS de F1RJ Op05 -19 dB F:2% in JN18AT 1390,6 Hz

>> Foxsix Htj Yes, Aircraft Scatter, big airplane above Clermont Ferrand (matching with AirScout software).


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