Tom has it exactly right. Think of it this way:
Let's say that your quarter wave stub provides a 10 ohm impedance at the
harmonic frequency. It the impedance of the amp is say 25 ohms at the
harmonic frequency, then you have 10 ohms in parallel with 25 ohms.
Now if you put a quarter wave length of line between the amp output and the
point where the stub hooks up, the quarter wave length line transforms the
25 ohms amp output to a rather high impedance. That high impedance point now
connects to the 10 ohm impedance of the stub.
Any time you use a stub it is much more effective being placed across a high
impedance than it is across a low impedance.
The same thing is done with uhf and vhf cavity connections when notch
cavities are cascaded. A quarter wave line is used between each cavity. The
first cavity provides a low impedance (just like your stub), the quarter
wave line transforms that impedance to a high impedance where the next notch
cavity connects. The notch is working against a high impedance rather than a
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of Jim Brown
> Sent: Tuesday, February 02, 2010 2:32 PM
> To: Tower Talk List
> Subject: [TowerTalk] Using Stubs to Reject Harmonics
> I'm building stubs for a DXpedition, and also for my own shack. I always
> the wisdom of others before undertaking a project, and came across this
> gem on
> K1TTT's website. He's quoting W8JI. The questioner in Tom's post is
> and may be Tom's alter ego.
> Last night, I built a 160M stub using very low loss coax, carefully
> tuned it
> with an HP generator and spectum analzyer, and stuck it at the output of
> Titan Amp, whose output stage is a Tee network (inductor output), and
> on another radio to the 2nd harmonic. The stub didn't do much to reduce
> the 2nd
> harmonic. As noted below, Tom strongly recommends staying 1/4 wavelength
> from the amp.
> My question to the list: Have others seen this, tried it? Measured the
> in a meaningful way?
> Jim K9YC
> = = = = = = = =
> >Question 1): it would seem that placing the stub as close to the linear
> >the best place for it
> W8JI: The best place to put a hi-reject stub is exactly a 1/4 wl from
> source, if the source has a low pass filter in the output (like an
> Thereason is a shorted stub is a low impedance, if you just place it
> across the
> amp output the low shunt Z of the stub barely improves the bypassing. If
> stub is pl;aced 1/4 wl away (at the harmonic F) the transmission line
> the impedance to a high impedance. We not only have the advantage of a
> low Z
> stub shunting the line at the stub location, we have the andvantage that
> amplifiers tank (a low shunt Z at the harmonic) looks into a very high Z
> at the harmonic frequency! The improvement in supression can be many dB!
> >Question 2): Okay I 'm gonna put the stub/trap outside. Is there a
> length of
> >coax running to the rig from the stub that would be better or worse? I
> >some have talked about using two stubs and a crtical spacing between
> the two
> >exists (is it 1/4 wave or 1/2 wave ??).
> W8JI: 1/4 WL at the harmonic fy, always! Unless the source is high Z at
> harmonic, like a series "C" T network!
> >Question 3): If I understand correctly, hardline is a much better
> choice for
> >this type of a "filter", as opposed to say RG213 with a woven shield.
> W8JI: The shield QUALITY has little to do with anything except as it
> line loss. A lower loss line will present a more extreme impedance at
> the far
> end. Use a low loss line for the stub! Loss is critical to performance.
> 73 Tom W8JI
> Subj: Re: Stub QTHs
> Date: 96-03-31 11:55:51 EST
> From: W8JITom@AOL.COM
> Sender: firstname.lastname@example.org
> Reply-to: W8JITom@AOL.COM
> To: email@example.com
> Like anyone would be interested, but I reviewed the data again.
> Optimum attenuation occurs when cascaded stubs are place 1/4 wl apart
> (or odd
> multiples thereof) *at the REJECT frequency*.
> Optimum SWR bandwidth *at the pass frequency* occurs when pass stubs are
> wl apart at the pass frequency.
> Optimum harmonic suppression, if the source favors a low Z load at the
> harmonic's frequency, occurs when the first reject stub is 1/4 wl away
> the source at the reject frequency. This is the usual case, and varies
> the PA's internal layout more than anything else.
> Optimum harmonic suppression, if the source favors a high Z load at the
> harmonic's frequency, occurs when the first stub is right at the output
> This is a rare case.
> I can't find any exceptions to these general statements.
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