Howard fidel ?proposes to ?triple The output capacitance of the filters by dropping the impedance to 25 ohms. ? I suspect this requires rebuilding the output transformer and then adding an additional transformer to get back to 50 ohms.?
If his theory is right , the voltage showing it around the shelters might decrease to 1/third, and thus the power by 10 DB.?
What do others think of this? ??
What would it do to the power handling capability of the inductors & ?and their wiring??
It requires rewinding or creating two transformers, and adding probably 12 capacitors. ?
On another note, might I point out that if this rig had some kind of ALC, that might be a huge help if it is going into a linear amplifier following it! ? Most of the rigs that I have ever played with had some way for a linear amplifier to feed ALC back
to them. ??
On Aug 11, 2018, at 13:02, Howard Fidel <
sonic1@...> wrote:
toggle quoted message
Show quoted text
Hi Gordon:
Before you do all that rework, have you read my post of the ninth? My solution provides a 10dB improvement by design. You need to rewind the output transformer, change all the filter capacitors (uses the same inductors) and add a new 1:2 transformer. It can
be done with NO artwork changes.
Howard
-------- Forwarded Message --------
After much thought, and reading all the threads, and speaking to Allison (who has not commented to me on this yet) I think I have a good solution to the harmonics issue, not the spurs. We know that the problem is capacitance between input and output of the
filters. We estimated that it is 2 pF per a relay. For 80 meters, we have 4 relays, or 8 pF capacitance from input to output. You will note that the filter has a capacitor at its output. This acts as a voltage divider. The bigger we make this cap, the small
the energy will be that comes through, since the larger the cap is, the more attenuation we get from the voltage divider. To get a filter with more capacitance we need to lower the impedance of the filter, so I used 25 ohms. This means that we need to rewind
T11 to get a 25 ohm output impedance. The redesigned filter for 80 meters uses the same inductors as we now have, we just need to change the capacitors to the new values, and add a transformer to the output. If done on the board, this is the only artwork change.
It can also be put in line with the output coax, and then no rework of the PCB artwork is needed. The simulation shows that there is a peak in the filter response caused by the 8 pF cap on the original design at 12.5 MHz and -53 dB. With the new filter this
peak is at 10.5 MHz and is - 64 dB. I think a 10 dB improvement is all we need. The same can be done for each of the other filters, and the improvement will be the same, since the capacitance through the relays goes down as the frequency goes up (fewer relays).
I have attached the simulation files and the filter schematic. I have to design a transformer.?
Howard
<Instrumentsfix.pdf>
<Instruments.pdf>
<80Mmodfilter.pdf>
