Gary, That is very good information.
We're going back up to the site today and will try that method. > > 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 >
