[TowerTalk] Using Stubs to Reject Harmonics
Joe Subich, W4TV
lists at subich.com
Tue Feb 2 16:48:44 PST 2010
> 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,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com] On Behalf Of TexasRF at aol.com
> Sent: Tuesday, February 02, 2010 6:33 PM
> To: garyschafer at comcast.net; towertalk at contesting.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.
>
> I bet everyone already knew that.
>
> 73,
> Gerald K5GW
>
>
>
> In a message dated 2/2/2010 2:06:16 P.M. Central Standard Time,
> garyschafer at comcast.net writes:
>
> Hi Jim,
>
> 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
> low impedance.
>
> 73
> Gary K4FMX
>
> > -----Original Message-----
> > From: towertalk-bounces at contesting.com [mailto:towertalk-
> > bounces at contesting.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 study 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
> > anonymous,
> > 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 my
> > Titan Amp, whose output stage is a Tee network (inductor
> output), and
> > listened
> > 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
> > away
> > from the amp.
> >
> > My question to the list: Have others seen this, tried it?
> Measured
> > the result in a meaningful way?
> >
> > 73,
> >
> > Jim K9YC
> >
> > = = = = = = = =
> >
> > >Question 1): it would seem that placing the stub as close to the
> > >linear
> > is
> > >the best place for it
> >
> > W8JI: The best place to put a hi-reject stub is exactly a 1/4 wl
> > from the source, if the source has a low pass filter in the output
> > (like an ampolifier).
> > 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
> > the
> > stub is pl;aced 1/4 wl away (at the harmonic F) the
> transmission line
> > inverts
> > 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
> > the
> > amplifiers tank (a low shunt Z at the harmonic) looks into
> a very high Z
> > load
> > 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
> > know
> > >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 the 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
> > affects 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 at AOL.COM
> > Sender: owner-cq-contest at tgv.com
> > Reply-to: W8JITom at AOL.COM
> > To: cq-contest at tgv.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 1/4 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 from the source at the reject frequency. This is the
> usual case,
> > and varies with
> > 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 port. This is a rare case.
> >
> > I can't find any exceptions to these general statements.
> >
> >
> >
> > _______________________________________________
> >
> >
> >
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