In the circuitry. Look at the resistoe capacitors around the T/R circuitry of 2N7000s.

Mike, the Rx line is derived by inverting and delaying the tx line.

In the sbitx.c, there is an array that defines the band edges. You can edit them to get to your desired frequency. Remember to rebuild the program after editing it.

Juan,
There is lurking danger with the t/r circuitry.
As the entire receiver is bidirectional, when you go from transmit back to receive, there can a millisecond or less of time when the rx power line capacitors still hold power and the tx circuitry is powered up.
This can cause the entire power amp and the if aml to go into self oscillations. These can be very large oscillations that can blow up the finals.
Be careful to time the T to R transitions carefully. When starting tx, the rx line should go down, you wait a millisecond or so and then the tx line powers up.
The transition from T to R should be similar. Power donw the T line, wait for a millsec or so for everything to discharge power, then turn on the R line.

The ideal place to tap would be between the crystal filter and the mixer.
This can be a very interesting and powerful second receiver.
Here is how it will work.
1. Consider that an rtl sdr is connected to the point between the if filter and the mixer.
2. Let us assume that the main radio is tuned to 7 mhz. The local oscillator is now at 47 mhz.
3. If the rtl sdr's own sdr, running seperately is now? tuned to 68 mhz.
4. The rtl sdr can now listen to 21 mhz band (68-47 = 21 mhz). It can also display a spectrum of upto 2 mhz wide.

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