[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

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