Loading

The collector output may be used to drive other circuits (like the differential amplifier in the previous circuit in place of the crystal oscillator) but it may be desirable to add a few thousand ohms in series to prevent excessive loading of the oscillator.

So, loading maybe where too big or strong a signal/voltage is impressed onto a device/circuit.

http://www.techlib.com/electronics/amxmit.htm

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Archimedes wrote:
> When I increase the antenna length to say 30cms, from the recommeded
> 6inchs or 15cms, the circuit does not work.

The device is a super-regenerative receiver. The antenna
is part of the oscillator circuit. If you change the
length of the antenna, you change the resonant frequency
and/or possibly cause the oscillator to stop oscillating.
Either of those results will cause the receiver not to
work right.

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Antenna tapped to Inductor in a Tank Circuit

The antenna acts like a capacitor.  Hence the antenna is like a capacitor connected to the inductor in series.
The capacitance of the antenna is reacting with the inductance of the coil to resonate at the frequency of the radio station.  Increasing the antenna length increases the capacitance and throws off the resonant frequency of the tank circuit.

The resonance frequency is dictated (essentially) by the contribution of the complete coil length.
The reason for the tapping is to better match the aerial impedance to the tank circuit, also (in
this case) to reduce the influence of the aerial upon the resonant frequency. You may find best
performance at an even higher tap ratio.

Best
Drew