faa  radar aliasing in band


A2010 radar harmonics measured 24mar05:

 When 100 Mhz 3 level data centered at 1384 Mhz is taken (a2010),   radar birdies appear in the band. The plots show data taken 24mar05:

Test data taken 03mar05:

    On 3mar05 i took some data with the wapps and alfa. The setup was 100 Mhz 3 level sampling, 1 second samples, with cfr at:
cfr birdie freq band limits
1380 1390 1330:1430
1385 1380 1335:1435
1390 1370 1340:1440

The image shows 60 seconds of data taken at each cfr (.gif).

A plot of the total power at 1350 and the birdie frequency (.ps) (.pdf) shows that

Data taken by a2010 when the 1350 radar is off (but the 1330 Mhz is on) does not have these birdies present so they need the 1350 Mhz signal.

The table below shows where the 1350 Mhz birdies sits in the 1st IF and at baseband. It also includes the location of the baseband 2nd and 3rd harmonics of the 1350 Mhz birdie.
1380 Cfr
1385 cfr
1390 Cfr
1350 birdie at 1st IF
(cfr-1350) + 250
1350 birdie at baseband
(cfr-1350) + 50
1350 birdie
2nd harmonic at baseband
1350 birdie 3rd harmonic
at baseband 
sky frequency of birdie

The location of the 1350 birdie in the 1st If and at baseband is also shown in the figure (.gif): The plot displays the locations when the center freq is 1380 Mhz (left panel) and 1385 (right panel). The bandwidth was 100 Mhz so the aliases at baseband starts at 100 Mhz. The blue lines are the normal mixing locations for the 1350 radar. The Red lines show where the baseband 2nd harmonic of 1350 occurs. The green lines are the 3rd harmonic.

    When the center frequency is 1380 Mhz, both the 2nd and 3rd harmonics at baseband alias down to 1390 Mhz. Shifting the center frequency to 1385 and then 1390 rules out the 2nd harmonic. So the alias of the radar is the 3rd harmonic.
    This could also be occurring at the IF1 in the amplifier after the mixer (and filter). The 3rd harmonic of the IF ends up at the same location as the 3rd harmonic at baseband.


processing: x101/050303/birdiefaa.pro