[TowerTalk] Making stubs - questions

[Tom Rauch]

> 2) If I understand correctly, W8JI suggests that the best place to put a
> stub is 1/4 wave away from the amp.  I am having trouble conceptualizing
> this.  Each of my antennas comes into a 2 position switch as 
follows:

What is the purpose of the stub Bob? 

If it is to eliminate even harmonics, the best place to install the 
stub is 1/4 wl away from the amplifier on the 2nd harmonic 
frequency. That would be 1/8 wl away on the fundamental.

If you add a second stub for additional 2nd harmonic suppression, 
it should also be spaced the same distance as above.

The reason for this is the 1/4 wl line (at the frequency of the 
harmonic) will reflect an "open circuit" back to the amplifier port on 
the harmonic. The loading capacitor will have a larger shunting 
effect for the harmonic, and less harmonic current will flow out into 
the transmission line.

A similar effect happens with stubs in tandem. 

The description someone gave on using a rig, T connector, SWR 
meter, and load is a good clear simple explanation. It will also work 
using any antenna analyzer instead of a rig.

If you aren't trying to trap the 2nd harmonic, disregard my advice on 
stub spacing.


73, Tom W8JI
w8ji@contesting.com

--
FAQ on WWW:               http://www.contesting.com/FAQ/towertalk
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com