[TowerTalk] Using Stubs to Reject Harmonics

[Gene Fuller]

I think this may be why I designed my amp with a Pi L output circuit, 
although I'm not a big contest station and have never really measured the 
harmonic output.
Gene ./ W2LU


----- Original Message ----- 
From: "Rick Karlquist" <richard at karlquist.com>
To: <k3lr at k3lr.com>; "Tower and HF antenna construction topics." 
<towertalk at contesting.com>
Sent: Tuesday, February 02, 2010 3:04 PM
Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics


> Tim Duffy K3LR wrote:
>> Hi Jim:
>>
>> I always wanted to test the 1/4 wave "insertion" theory, but never got
>> around to it. I would love to hear what the difference is - if you can
>> experiment.
>>
>> 73,
>> Tim K3LR
>>> 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
>
> What Tom says is right on the mark, as long as the last component in
> your amplifier is a capacitor to ground, which is probably the case
> 90+% of the time.  In Jim's case, the last component is a series
> inductor, in which case Tom's advice needs to be modified to
> placing the stub at the amp, or a multiple of a halfwave away.
> All of this confusion is yet another one of many reasons, IMHO, to build
> filters out of lumped components instead of coax stubs.  And
> the filters should be elliptic function low pass filters, not bandstop
> filters.  For examples of these filters, study the output filters
> in any solid state transmitter.  For example, see pages 3 and 4 of:
>
> http://www.n6rk.com/COM_1000_schematic.pdf
>
> I have been using this linear on the air for the last 10 years.
> It is so clean harmonic wise, I didn't need a low pass filter on
> it for TVI purposes back in the analog TV era (we have no low VHF
> stations in our area now).
>
> Rick N6RK
>
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