[Elecraft] T1 Antenna Considerations

[M1MGD1 at aol.com]

Hello
I have just received my T1 kit and i notice that under the above heading in  
the owners manual under random-wire antennas it states(avoid lengths close to 
a  multiple of 1/2 wavelength on any band) please can someone explain why this 
is  important,also i wish to use this tuner on the following bands 20/15/10m 
so a  suggested wire length would be appreciated I only have 40ft of space to 
play  with.
 
Thank you.  

Mick M1MGD  K2#4785 K1#1532
G-QRP 11355 ARCI  12343