This is a continuation of Portable preamps for 6m 2m and 70cm – part 3.
After some comments from Bob, KV3VO, I decided to make these preamps narrow band by using a shunt LC tuned filter. It increases the component count by two, but there was room on the board for it. The reason for this change is by having the preamp broadband, it effectively reduces the radio’s ability to reject signals by the same amount of the gain of the preamp – because these strong signals are going to be amplified by the same amount as the desired signal. By using a LC filter, the signals in the desired amateur band will still be amplified, but other signals will be subject to attenuation. I have experienced this problem, not with the FT-817 or FT-897, but with a Wouxun KG-UVD1P. When doing SOTA activations from a summit with commercial RF towers nearby, the Wouxun can be overwealmed. The FT-817 and FT-897 have been used in the same environment and their ability to reject signals is much better, but again, it is better if this preamp is not weakening that capability.
So, here is the insertion loss at 6m with a 39nH inductor and 240pF cap, both 0805 size:
Here is the insertion loss at 2m with a 6.4nH inductor and 180pF cap, both 0805 size:
And again for 70cm with a 2.7nH inductor and 51pF cap, both 0805 size:
The PCB is little changed. I have added the filter on the front end of the amplifier.
EDIT: It does not take long for bugs to be found! I have to move the shunt filter C6 and L4 to the other side of the coupling capacitor C5, otherwise I create a DC short across L4. Simple change to the PCB, will be able to get it in before it is loaded for fabrication on the first batch.