Quote:
According to this page:

http://www.crompton.com/wa3dsp/hamradio/antcalc.html

the half wavelength length is 63 centimeters



Depending upon which end of the frequency range you look at. I used the same page to check my calculations.

Quote:

Are you saying that there is nothing to be gained in increasing the length of the damp peice of string beyond this ?


Basically, yes. Extending the string wouldn't improve things. Might be worth investigating a different aerial though. I'd imagine earthing the ground of the aerial to the ground of the dab module would suffice. Suppose you need to investigate the impedance values of your module and aerial.
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Cheers,

Andy M