The simplest way of getting on the air with phone is to convert a single transistor crystal controlled TX into an AM transmitter by putting a carbon mike in series with the poer supply. This is illustrated by a desin in the G-QRP Scrapbook (p??). There are several disagvantages to this approach, the main practical one being that depth of modulation is very low, I would estimate about 15%, and so the effective audio power is only going to be a few milliwatts. It does work after a fashion however, and short distance contacts can be made.
A more elegant approach is to use DSB. At QRP levels this is a perfectly respectable mode even on narrow bands such as 40m and 17m. It is more efficient than AM and not that much harder to construct.
A more conventional design was given in Sprat by ZL2BMI. Although this uses a carbon mic a dynamic or electret mic may be substituted.
The design below uses a ceramic resonator, either 3.395MHz or 2MHz for 160m, which can be pulled over a useful portion of the respective bands.
Cm acts as a high pass audio filter. A value of 1microF is suitable, but can be between 100nF and 10microF.
The `crystal' is a ceramic resonator, either 3.695MHz for 80m or 2MHz for top band. For top band a 22 microH series inductor should enable the oscillator to cover most of 1.9 to 2.0MHz.
Cv is any conveniently available twin gang capacitor. A large airspaced 2 x 500pF device will give coverage of much of the 80 phone section, typically from 3.6 to 3.74MHz, a compact `polyvaricon' rather less.
A description of the PA is here
I built this in `modules', see photos, starting with the exciter. This is best in a screened box to ensure carrier supression. It was built on a piece of perforated board. The version in the picture uses only a single variable capacitor (the left hand half of Cv) which gives only about 40kHz of pulling range, but was what I had to hand.
The amplifier was built using (very) ugly construction. The BD135 heatsink is a piece of thin copper sheet (all metal, not pcb material) fixed with a suitable bolt. This must NOT touch an earther case or ground plane.
Everything is linked up using phono plugs and sockets. A beginner can build the exciter, connect up the microphone, put a short piece of wire on the output, tune in on a receiver and hear himself speaking. In fact, this constitues a single chip TX with an output of a few hundred microwatts. It should be possible to use it very short range qsos!
One thing I'd particularly like to do is to make a 12m version. This idea arises from some experiments with two 25MHz crystals in parallel (and they're cheap enough!) along with a series inductor of about 2.2 microH. This will pull over the whole of this narrow band. However, the current linear amplifier runs out of puff well below these frequencies and I have yet to get round to finding or concocting a more suitable design.
I definitely must get round to this, because there aren't many simple designs for this band, and it is one on which QRP phone can be very successful.