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## Re: [TowerTalk] Using Stubs to Reject Harmonics

 To: "'Tower and HF antenna construction topics.'" , Re: [TowerTalk] Using Stubs to Reject Harmonics "Joe Subich, W4TV" lists@subich.com, "Tower and HF antenna construction topics." Tue, 2 Feb 2010 19:48:44 -0500 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 . 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 . 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. > > > > > > > > _______________________________________________ > > > > > > > > _______________________________________________ > > TowerTalk mailing list > > TowerTalk@contesting.com > > http://lists.contesting.com/mailman/listinfo/towertalk > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread Re: [TowerTalk] Using Stubs to Reject Harmonics, (continued) Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Brown Re: [TowerTalk] Using Stubs to Reject Harmonics, Gary Schafer Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Brown Re: [TowerTalk] Using Stubs to Reject Harmonics, Rex Lint Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Brown Re: [TowerTalk] Using Stubs to Reject Harmonics, Joe Subich, W4TV Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Hargrave Re: [TowerTalk] Using Stubs to Reject Harmonics, Gary Schafer Re: [TowerTalk] Using Stubs to Reject Harmonics, TexasRF Re: [TowerTalk] Using Stubs to Reject Harmonics, Joe Subich, W4TV <= Re: [TowerTalk] Using Stubs to Reject Harmonics, Paul Christensen Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Lux Re: [TowerTalk] Using Stubs to Reject Harmonics, Richard (Rick) Karlquist Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Lux Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Lux Re: [TowerTalk] Using Stubs to Reject Harmonics, TexasRF Re: [TowerTalk] Using Stubs to Reject Harmonics, Paul Christensen Re: [TowerTalk] Using Stubs to Reject Harmonics, Jim Brown Re: [TowerTalk] Using Stubs to Reject Harmonics, Michael Tope Re: [TowerTalk] Using Stubs to Reject Harmonics, TexasRF