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Re: [TowerTalk] Using Stubs to Reject Harmonics

Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
Reply-to: "Tower and HF antenna construction topics." <>
Date: Tue, 2 Feb 2010 18:32:40 EST
List-post: <">>
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 
I bet everyone already knew that.
Gerald K5GW
In a message dated 2/2/2010 2:06:16 P.M. Central Standard Time, 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  
connects to the 10 ohm impedance of the stub.

Any time you use a  stub it is much more effective being placed across a 
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 
cavity connects. The notch is working  against a high impedance rather than 
low  impedance.

Gary  K4FMX

> -----Original  Message-----
> From:  [mailto:towertalk-
>] 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:
>  Reply-to:       W8JITom@AOL.COM
> To:
> 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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