-----Original Message-----
>From: "Joe Subich, W4TV" <lists@subich.com>
>Sent: Feb 2, 2010 4:48 PM
>To: "'Tower and HF antenna construction topics.'" <towertalk@contesting.com>,
>TexasRF@aol.com
>Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
>
>
>> 1/4 wavelength at the second harmonic is only 1/8 wavelength at
>> the operating frequency. If you make the connection 1/4 wavelength
>> away at the operating frequency, the described transformation does
>> not happen at the second harmonic.
>
>Which is why W2VJN's stub cookbook shows some interesting behavior
>with 1/8 wave connecting lines <G>. Without getting the book off
>the shelf, I believe he was using 1/8 wave between the amplifier
>and first stub and 1/8 wave between first and second stubs in some
>cases to optimize 2nd harmonic rejection.
>
>1/16 wave would be another interesting connecting length if the
>4th harmonic were the bigger problem ... the combinations are
>endless <G>.
>
>73,
>
Which is why a computer to model it is nice<grin>...
Or a set of decent design equations.
I'll bet there's a nomogram or set of templates for a Smith Chart that does
this kind of thing, at least for a fairly simple 1, 2, or 3 stub setup.
It's not much different than using a multistub tuner at microwave frequencies.
After you do it a few times, you get a feel for what the various stubs are
doing. If you have a network analyzer, it's even more fun, because you can see
the S11, S22, and S21 all at the same time.
I will say, though, that for multiple stubs, doing without explicit calculation
is somewhat of an art. There are people who can tune up a multi section filter
quickly, and those who can't.
Jim
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