[TowerTalk] Making stubs - questions

[i4jmy@iol.it]

The quarter wave stub behaves like a short on the even harmonic(s) and 
a quarter wave line to the transmitter reverses the stub impedance to 
that of an open circuit, this as seen by the last amplifier of the 
transmitter chain.
Under this viewpoint the advice to place the stub a quarter wave 
(referred to the harmonic) from transmitter (PA) is the most correct 
possible because summing the shunt effect of the stub with the minimal 
energy transfer of the transmitter to what's seen as an open circuit. 
In a real world, things are slightly more complicated since stubs and 
antennas are hardly non reactive devices along the used band.
Ham system are reversible and PAs can be switched off but kept in line. 
The transmission line that's 1/8 wavelenght from PA can easily become 
close to a quarterwave up to the RTX and maximally enhancing the stub 
and line mismatch on the fundamental frequency.

73,
Mauri I4JMY


> ---------- Initial message -----------
> 
> From    : owner-towertalk@contesting.com
> To      : <towertalk@contesting.com>, "Robert Shohet" <kq2m@mags.net>
> Cc      : 
> Date    : Sun, 17 Sep 2000 19:55:30 -0400
> Subject : Re: [TowerTalk] Making stubs - questions
> 
> 
> 
> 
> > 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

> 


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