Gary, Just a follow up to the preamp questions from last month.
Your advice was fantastic. We added attenuation right to the point where the sensitivity started dropping off and it was perfect. By putting in the attenuator, it actually increased the receive range of the repeater by about 30%. We're going to do this with every site that we have! Dwayne Kincaid WD8OYG > Hook your signal generator up to your system at the antenna port and measure > receiver sensitivity with and without the preamp. Then with the preamp in > circuit start adding attenuation between the preamp and the receiver. When > you just start to loose sensitivity stop adding attenuation. That should > give you near optimum sensitivity without excessive gain. Too much gain in > the preamp overloads the receiver mixer and front end amp if it has one. > > For every db of gain you add in front of the receiver you reduce the IM > performance of the receiver. > > You only want enough preamp gain to overcome the noise figure of the > receiver. Although the noise figure of the receiver and preamp are > cumulative the preamp is the biggest contributor in setting system noise > figure. In other words putting a hot preamp on a very hot receiver will give > you a better overall noise figure than putting that same preamp on a poor > receiver but the difference will not be great. > > You may not be able to realize the full benefit of the preamp if you have > excessive IM. You may have to add more attenuation to where it further > reduces receiver sensitivity. When you get down to the point that the > sensitivity is the same as it was without the preamp, then throw out the > preamp. But you may be able to find a happy medium where the preamp does > help some without destroying your IM performance. > > If you still have excess IM problems you can add attenuation ahead of the > preamp by raising the insertion loss of the loops on your band pass filter > as others have suggested. By raising the insertion loss on the loops it does > the same thing as adding an attenuator ahead of the preamp but with the > added benefit of steeper skirts on the band pass filter. > > By the way don't worry about adding adaptors between the preamp and > receiver. After all you are looking to add attenuators anyway. But adaptors > really make no measurable difference in attenuation at vhf or uhf. They may > give a slight impedance mismatch but you probably don't have anything that > will measure the small amount of loss from them. > > 73 > Gary K4FMX >
