The basic D-104 (without amplifier) is a crystal microphone, designed
for very high impedance (>100K ohm), and delivering fairly high output
voltage. It worked well with the vacuum tube amplifiers used when the
D-104 was new. The K3 (and other modern rigs) have much lower input
impedance and will load down the crystal mic element considerably.
There were several amplified versions, the "power mikes" of the CB
craze. These have low output impedance (a few K ohms, varies with the
output level pot setting) combined with high output voltage. Their
output level can over-drive radios designed for dynamic microphones
unless the gain setting is kept very low, or an attenuator is placed in
the output.
Links:
Adapting D-104 to modern radios (using an op-amp buffer)
<http://www.w6ier.org/images/9908034%5B1%5DAdapting%20the%20Astatic%20D104%20Mic.pdf>
Schematics for amplified Astatic mics
<http://www.barovelli.com/projects/radio/radioastatic.htm>
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