To: The Microwave Group From: Dick, K2RIW 7/15/03. Re: Ku Band LNB Conversions.
INTRODUCTION -- In the past, many microwavers have made the serious mistake of removing the WR-75 waveguide assembly from the front end of a Ku Band LNA (LNBF), and replacing it with a coaxial structure. I think they have done this for reasons of esthetics, or for making the assembly more compact, or for the mistaken belief of lowering the RF loss.
BIAS VOLTAGE -- But, many microwavers didn't realize that most LNB designs have the first stage transistor's DC bias voltage present on the WR-75 probe. Since the WR-75 is a nearly perfect High Pass filter (with a cut-off frequency of 7.87 MHz), this is an ideal arrangement.
ESD VULNERABILITY -- However, when the WR-75 assembly is removed, and direct connection is made to the probe of the first stage, then the assembly becomes extremely vulnerable to any ESD voltage that may be introduced into that probe, or to inadvertent grounding of the bias voltage. Some microwaver's have attempted to alleviate this problem by placing a series coupling capacitor in that line. THIS IS NOT A SOLUTION. The ESD "spike" will simply walk right through that coupling capacitor and damage the transistor. Some help would occur if an RF choke to ground was followed by the coupling capacitor into the transistor, but the new impedance match is likely to degrade the original beautiful Noise Figure.
LEAVE IT ALONE -- My suggestion is to not disturb the WR-75 assembly at the LNB's input. Knowing that WR-75 is usable down to 7.87 MHz should remove that worry. In fact, even WR-62 is usable (marginally) down to 9.52 MHz.
NF PEAKING -- If you have an ideal 10.368 GHz Noise Figure setup, you could place a tuning screw in the WR-75 line to "peak up" the Noise Figure at 10.368 GHz versus the 12 GHz of its original tuning. But, I doubt that you will make more than 0.1 dB of true improvement.
VSWR EFFECTS -- Bear in mind that most GASFET LNA's are extremely sensitive to the VSWR that is presented to them. Therefore, achieving a good VSWR of the rest of the feed system is very important. Also, remember that the reflected "signal" from the apex of a non-offset parabolic dish can considerably affect that horn's VSWR. Therefore, tune the horn's VSWR while the dish is present. And, if the horn's position is change for the reason of peaking the Dish Gain, then the horn's VSWR should be rechecked.