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Re: [TowerTalk] Using Stubs to Reject Harmonics

To: "'Tower and HF antenna construction topics.'" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
From: "Gary Schafer" <garyschafer@comcast.net>
Reply-to: garyschafer@comcast.net, "Tower and HF antenna construction topics." <towertalk@contesting.com>
Date: Tue, 2 Feb 2010 15:55:10 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Jim,

You should be able to get an idea of the output impedance of the amp at the
second harmonic frequency by checking with an impedance meter at the output,
like with an MFJ. You may need to place a resistor from the plate to ground
of the tube to simulate the tube load. Do all this of course after the amp
has been tuned up on the operate frequency and then shut down.

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 3:40 PM
> To: Tower and HF antenna construction topics.
> Subject: Re: [TowerTalk] Using Stubs to Reject Harmonics
> 
> On Tue, 2 Feb 2010 13:49:36 -0600, 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.
> 
> Hi Tim,
> 
> OK. As noted, my Titan 425s (original) are not a good output stage to
> test this, but I've got a Hercules II that does use a traditional pi-
> section (C-L-C) as the output filter. When I have time, I'll set that up
> and see what I can learn. But as I said in an earlier post, I find no
> flaws in Tom's logic.
> 
> Note, however, that Tom's logic applies to TX. RX is a different story,
> so a stub tuned to reject a TX on the band below (that is, an 80M reject
> stub on a 40M line) should be fairly effective near the RX input. I say
> "fairly effective" because RX inputs are not always well matched to the
> lines that feed them. :)
> 
> 73,
> 
> Jim K9YC
> 
> 
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