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80 Meter Increase Sensitivity Mod

This mode adds a 18 pf cap between LO and Antenna Input circuits

If you have modified your CONAR receiver as discussed you might have discovered decreased sensitivity on 80 meters compared to the rest of the bands. I am at a loss to explain it. I have worked on three CONAR receivers and they all have decreased sensitivity. I have substituted 6BE6's from GE, Motorola and Hitachi and this made no difference.

I have then tried to rewind the 80 meter antenna input coil. I added additional turns to the antenna input circuit. No difference noted. I also tried to adjust the input capacitance in and modified the input coils in order to affect the "Q" of the circuit. Again, no improvement.

I took an RF voltmeter and measured the Local Oscillator output level. The level was the same on all bands so the problem is NOT in the LO circuit.

I varied the local oscillator coil slugs far above and far below the 80 meter tuning range and the receiver does pick up signals above and below the 80 meter band very well.

The last thing which I have yet to do is substitute the 6BE6 with another tube. According to my tube substitution book, a 6CS6 or a 6BY6 pentagrid amp tube should work as well. The 6BE6 was made to be a mixer in both AM (LF) and FM (VHF) radio sets. But the frequency response may not linear across the frequency range.

Further experimentation remains!

In the meantime, this mode might help. I added a 18 pf disc across the coil of the 80 meter LO coil into the 80 meter antenna input coil. Basically I coupled some RF energy with the incoming signal, thus turning the mixer into a "semi regenerative" mixer. This is a common procedure for parameteric amps used on the microwave bands. It increases sensitivity but not as much as I would have liked. Still, it's better than a stick in the eye. Here is the schematic:


You should notice that input antenna peaking capacitor C9 is now a part of the LO circuit (remember RF is a 2-way street!). Turning it will PEAK the Signal and slightly vary the LO frequency thus FINE TUNING the incoming signal as well. It does take some getting use to. But once you do, you'll find tuning in SSB during Nets very easy. Here is a pic showing how easy it is to do it.



Clearly, this is an area in which I will continue experimentations!


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