Reverse Blind Scan With F15 ?

[moonbase]

Hi,

I did a blind scan of 34.5W on C band with an F15 receiver and I noticed that the scan proceeded in decreasing frequency, ie, it started at the higher frequeny of the blind scan range and scanned downwards to the lower frequency. Is this normal, I was using a Titanium LNBF with flat scalar feed and dielectric plate inserted for circular polarisation?

Rgds

 

[scopus]

It's the same on the Openbox high to low on C band, but low to high on Ku.

 

[jeallen01]

Is it the way the receivers are programmed to scan C-band transmissions (sounds unlikely if you are using a normal sat Rx?), or is the LNB LO frequency of a C-Band LNB higher than that of a Ku-Band one with respect to the actual transmitted ones, and therefore the difference between the two, which is what the receiver detects, decreases as the transmitted frequency increases? (Don't know myself, never having looked at C-Band stuff!)

 

[Channel Hopper]

Is it the way the receivers are programmed to scan C-band transmissions (sounds unlikely if you are using a normal sat Rx?), or is the LNB LO frequency of a C-Band LNB higher than that of a Ku-Band one with respect to the actual transmitted ones, and therefore the difference between the two, which is what the receiver detects, decreases as the transmitted frequency increases? (Don't know myself, never having looked at C-Band stuff!)

#2
Go to the front of the class.

 

[jeallen01]

A guess based on basic 1940's+ Superhet theory (an oldie but goldie)

 

[Llew]

C- Band 3.7GHz -4.2 GHz. LO 5.15GHz. Blind scan can only count away from the LO, in other words increasing the actual frequency stepped from 4.2GHz to 3.7 GHz.
Simply put, 5.15 minus 4.2 = 950MHz, 5.15-3.7 = 1450MHz. That's the way your box steps, increasing the frequency count, just as it does stepping through the Ku Band, only with Ku it steps upwards from an LO of 9.750 to the highest frequency.

 

[moonbase]

Thanks for all the replies.

Rgds

 

[jeallen01]

FWIW, whilst looking for some other info, I came across this page on Satsig.net which seems to outline the issues quite well, and includes a chart of C/Ku/Ka bands frequencies and LNB LOFs - and illustrates the underlying reasons for the answers to the OP's questions quite clearly.

OTOH, there appear to be a few instances - marked "inverted" in the Notes column of the chart - of where the LOF is ABOVE the sat transmission frequency in both the Ku and Ka bands; so why would that be done?