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Re: [TowerTalk] Using Stubs to Reject Harmonics

To: "'Tower and HF antenna construction topics.'" <towertalk@contesting.com>, <TexasRF@aol.com>
Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
From: "Joe Subich, W4TV" <lists@subich.com>
Reply-to: lists@subich.com, "Tower and HF antenna construction topics." <towertalk@contesting.com>
Date: Tue, 2 Feb 2010 19:48:44 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
> 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@contesting.com 
> [mailto:towertalk-bounces@contesting.com] On Behalf Of TexasRF@aol.com
> Sent: Tuesday, February 02, 2010 6:33 PM
> To: garyschafer@comcast.net; towertalk@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@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@contesting.com  [mailto:towertalk- 
> > bounces@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@AOL.COM
> > Sender: owner-cq-contest@tgv.com
> >  Reply-to:       W8JITom@AOL.COM
> > To:   cq-contest@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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> > TowerTalk@contesting.com  
> > http://lists.contesting.com/mailman/listinfo/towertalk
> 
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