There still seems to be some confusion about the 1/4 wave versus 1/2 wave
stubs for harmonic suppression. The confusion seems to be if the length is
related to the band you are operating or the band you are trying to suppress.
An example: while operating on 20m, the second harmonic at 10 meters will need
suppression. A 1/4 wave shorted stub on 20 meters will look like a near open
circuit on that band. This same stub will be 1/2 wave long on 10 meters, where
a shorted 1/2 wavelength looks like a near short circuit.
The other suggested method was to use a 1/4 wave *open* stub cut for 10
meters. This arrangement also will look like a near short circuit to 10m
harmonic energy and provides good attenuation. BUT, this arrangement is 1/8
wavelength on 20 meters and is no longer a near open circuit. In fact, it will
have a capacitive reactance with a value nearly equal to the characteristic
impedance of the coax the stub is made from!
The capacitive reactance can easily be cancelled by a second stub in parallel,
shorted on the end, which produces a positive reactance nearly equal to the
coax impedance. This arrangement is the basis of the 1/8 wave stub article in
Jan/Feb NCJ magazine which shows superior harmonic rejection and bandwidth
compared to the single stub design.
73 to all, Gerald Williamson, K5GW, Owner/General Manager, Texas Towers
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|