[IRCA] FSL Antenna: pulsating 610kHz in video - slightly off-frequency co-channel interference?

[Stephen Airy <pianoplayer88key@xxxxxxxxx>]

First off, thanks, Guy & Gary, for shooting & posting the FSL demo video. :)
 
I noticed someone on the ultralightdx group commented on the pulsating reception on 610 kHz.  I'll quote part of the message:
 
---- begin quote ----
<< 1) Would the pulsating reception on 610 Hz have been clearer if you
would have adjusted the Variable Capacitor on the FSL Antenna? >>

The 8" FSL's variable cap was already adjusted to the optimal position
for 610 kHz before the video was taken. The PL-380 (and all the Tecsun
Si4734 DSP chip models) typically have this pulsating reception on
weaker AM-DX signals, as the DSP chip samples the station's signal
level every few seconds. As you can see on the video, the S/N reading
cycled between about 12 and 24 (one short of the PL-380's maximum S/N
reading) as the FSL provided its serious inductive coupling boost.
---- end quote ----
 
It's not just the Si4734 chip radios that have that pulsating reception, and from what I can tell in my experimentation it's not necessarily because of the weak signal.  My theory is co-channel interference - from two stations mixing with each other that are slightly off frequency.  (How far off frequency determines the speed of the pulsing.)  In the video the pulsating was a constant rhythm, and Gary even commented that there was another station audible in the background(which I could also hear).
 
Here are a few demos I recorded yesterday and Wednesday using the PL-606 and the SRF-59, some barefoot and some with the Select-A-Tenna...
 
http://www.mediafire.com/?9r28jnur48eqy14 - PL-606 - 1150 KTLK + Mexicali? - barefoot (45,00) then SAT (3208-3213)
http://www.mediafire.com/?yyj37775081dbz5 - PL-606 - 1190 KGBN + Mexicali - SAT (39,16) then barefoot (43,00) then SAT (25,12 - 27,08-19)
http://www.mediafire.com/?zmnutn2b0cpdkwq - SRF-59 - 1190 KGBN + Mexicali - barefoot then SAT
On 1170, I have a 50kW local (KCBQ) 9.3 miles north of me which indicates about 80-81,25 on the barefoot PL-606.
 
http://www.mediafire.com/?4b8ckf5btagsb2n - PL-606 - 1290 KZSB (KKDD in background) - barefoot (24,06-10) then SAT (31,14 - 34,22)
http://www.mediafire.com/?7atz6zgqo19ltv7 - PL-606 - 1290 KKDD (KZSB in background) - barefoot (32,01) then SAT (32,10 - 34,08-16)
Note about 1290:  KZSB is 500 watts and 195 miles from me, and KKDD is 5 kW (sending about 6.4-6.5kW toward me) and 95 miles distant.  KZSB's signal comes in better, in spite of being farther and much lower power, due to the saltwater path between Santa Barbara and San Diego.
 
http://www.mediafire.com/?cd7atczup1zz4w7 - SRF-59 - 1340 KCLU + Mexicali - barefoot then SAT
http://www.mediafire.com/?97nw6ac1ylyqaz6 - SRF-59 - 990 XECL + KTMS - barefoot then SAT
http://www.mediafire.com/?vy8i48v7v1v4x32 - SRF-59 - 790 KABC + XESU - with Select-A-Tenna.  (XESU was too weak barefoot.)
http://www.mediafire.com/?8x3tp5eaqg3aw0o - SRF-59 - 530 unID Caltranss TIS (somewhere in San Diego county) + WNHV296 (LAX Airport TIS) - with SAT
 
 
73,
 
Stephen (near El Cajon / La Mesa, CA)
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