Looking at W2VJN's book, page 28 caught my attention. Specifically, the use of
type 3 stubs, where we use two 1/8th wave stubs in parallel instead of a single
1/4. Such stubs have higher attenuation at the expense of twice the loss (0.08
db vs 0.15 db).
Other than being twice the work, is there a downside to using 2 X 1/8 instead
of 1 X 1/4 stubs?
Rudy N2WQ
Sent using a tiny keyboard. Please excuse brevity, typos, or inappropriate
autocorrect.
> On Jul 26, 2016, at 7:26 PM, Jim Brown <k9yc@audiosystemsgroup.com> wrote:
>
>> On Tue,7/26/2016 3:17 PM, Steve NR4M wrote:
>> Jim,
>>
>> Been following this thread since it started (this time) several days ago. I
>> have lots of interest in it as I have a multi multi station here in VA.
>> I have built coaxial stubs in the past using my AIM 4170, FSJ4 heliax, and
>> K2TR's data as a good place to start. I think after screwing with them for
>> years, I've gotten pretty good at putting the nulls where I want them. And,
>> they seem to work, but no real data on how well they work. My 40m 3rd
>> harmonic on 15 is about S-5 and I can get withing a couple of KC, or so.
>> It's just not much of a problem any longer.
>>
>> Been quite interested in your suggestions on how to place them for maximum
>> attenuation of the undesired RF. I, like many, I guess, have installed them
>> at the output of the amplifier, because that would be a 50 ohm point in a
>> properly tuned amp. But, now I see that it's more important to know what
>> the impedance of the harmonics is on your band of choice.
>
> Yes. This was an important realization that George, W2VJN brought to our
> attention with his NCJ piece a couple of years ago. Up to then, I don't think
> any of us had given it any thought. I had not.
>
>>
>> What got me up and moving on this is your comment in a previous post today
>> about it being the second harmonic that's important. I have been building
>> stubs, per N2TR, that covered multiple bands, without really focusing on a
>> particular stations 2nd harmonic.
>
> Well, the focus on second harmonic as opposed to others is that it tends to
> be stronger than higher order harmonics, but the same logic certainly
> applies. Jeff, AC0C said it perfectly -- he looks at isolation using his VNA,
> pays attention to what he hears on the air in SO2R mode, and works on those
> things one at a time.
>
>>
>> I pulled the coax off the stub set on the back of my 20 meter amp, connected
>> a Rig Exper AA54, and started looking up my coax toward the antenna.
>> On 10 meters, I saw R101 -J14.6 and on 15 (for the heck of it) I saw R112
>> j-36.
>>
>> Following your instructions in your pdf on locating the stubs, I am just
>> about where I need to be. If I added .44 feet of Coax, it would be right on
>> the voltage peak.
>
> If it's that close to the voltage peak, there's no need to move it.
>
>>
>> Do these numbers above look as they should? I was expecting higher R.
>
> What kind of antenna? Is it a tribander, or a monobander? Monoband antennas
> tend to look like pretty high Z at the second harmonic at their feedpoint
> (because it's a full wave dipole at the harmonic), so should be pretty high
> VSWR. By contrast, a tribander should be fairly close to resonance on all
> three bands.
>
> If the antenna is resonant on the harmonic, placement of the stub with
> respect to the antenna doesn't matter, but placement with respect to the
> amplifier does.
>
> Another important point about "higher R" from an analyzer measurement.
> Virtually all of the analzyers we use (including the best VNAs) make
> "reflection-based" measurements using S11, and derive impedance from that
> data. The math involves the difference of two numbers that are relatively
> close in size, so a small error in the measurement can result in a large
> error in the computed Z. A good rule of thumb is that reflection-based
> measurements have fair accuracy if the unknown is within a 1:5 or 5:1 ratio
> of the system impedance, and they become increasingly inaccurate as that
> ratio gets larger. In other words, with our 50 ohm analyzers, Z is reasonably
> good for ACTUAL values between about 10 ohms and 250 ohms.
>
> There's also the issues of stray capacity, and of calibration of the
> instrument. Many years ago, W8JI published calibration instructions for the
> popular MFJ-259 and its descendants. I don't know if there are comparable
> procedures for your analyzer. The effect of stray C is to move resonant
> impedance peaks down in frequency.
>
> 73, Jim K9YC
>
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