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?
= = = = = = = =
>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
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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