Re: High SWR on all HF Bands


Dennis Klipa
 

Steve,

I assume that you know a lot of what I am about to share but I include it for others reading the thread who may not know  A lot of what you said is true about the transmission lines acting as impedance transformers and matching sections.  However, I respectfully submit that the statement that the SWR, in a lossless feedline terminated in a balanced load will change with cable length is not true.  In a feedline of at least 1/4 wavelength, attached to a balanced load the SWR does not change with cable length. 

According to many sources, including the 2014 edition of the ARRL Handbook, p 20.4, the Standing Wave Ratio, SWR, is a measure of the relative amounts of forward and reflected power.  It is often described in terms of voltage and is thus a VSWR or current (ISWR) and is the ratio of the highest voltage (current) anywhere along the line to the lowest voltage (current) anywhere along the line.  Thus it is a characteristic of the system, not a characteristic of a point on the line.  Measuring the voltage minima and maxima along the line is difficult for coaxial cable but forward power and reflected power are easily measured by a directional coupler or reflectometer.   SWR is dependent on the complex reflection coefficient, p, which is calculated from the characteristic impedance of the feed line (Zc) and the impedance of the load (ZL) as p =(ZL - Zc)/(ZL + Zc).  In terms of Forward Power (Pf) and Reflected Power (Pr) the reflection coefficient is defined as p = SQRT (Pr/Pf).  Regardless of how you do the calculation you get the same answer.  SWR is defined as  SWR = (1 + p)/(1 - p).  Any losses in a transmission line would only reduce the measured SWR.  There is no mention or implication in these expressions that the SWR varies with line length.

It is true that if you were to measure the impedance that you see at various lengths along the transmission line, the impedance would vary.  This is clearly seen on a Smith Chart in an article in the files section; "Matching Networks.pdf" by Terry Rogers, WA4BVY (https://groups.io/g/ic7000/files/Matching%20Networks.pdf), page 32, Figure 19, which you uploaded on February 7th.  The Figure shows two circles centered on the center of the Smith Chart which represent SWR ratios of 5:1 and 3:1. At all points along each circle the SWR is the same.   Moving around the Smith Chart in a clockwise direction starting at the intersection of the horizontal axis on the right side of the circle represents moving along the length of the coax.  Any point on the constant SWR circles represents the impedance you would measure at that point along the feedline.  So, yes, the impedance that you see along the feedline changes, while the SWR remains constant.  A changing impedance does not imply a changing SWR.

For a balanced load and a feedline greater than 1/4 wavelength (Reflections III, W. Maxwell, page 2-4, 2010):
"(22) The SWR in a feedline cannot be changed, adjusted, or controlled in any practical manner by varying or adjusting the line length."
"(23) If SWR readings change significantly when moving the SWR bridge a few feet one way or the other in the line, it indicates either"antenna" current flowing on the outside of the coax, or else an unreliable instrument, or both, or even a reliable bridge incorrectly adjusted to the line impedance, but it is not because the SWR is varying with line length.  Some suggest that the bridge must be placed and a 'lambda'/2 interval from the load to obtain a correct reading.  This is incorrect. All readings are invalid if they change significantly along the line, even though they may repeat at 'lambda'/2 intervals."

Byron Goodman in his April, 1977 QST, pg 40-42 article; "My Feedline Tunes My Antenna", says the same thing, although a little less delicately and more succinctly.

So with that I choose to stand by my original comment in an earlier post.

Best Regards,
Dennis Klipa, N8ERF

On Wed, Oct 28, 2020 at 8:58 AM Steve W3AHL <w3ahl@...> wrote:
Dennis,

The length of coax will affect the SWR seen at the radio.  Using a simple program like Transmission Lines for Windows, supplied with the ARRL Antenna Book, will show how much it varies.  Transmission lines are often used for matching purposes in antenna designs for single band antennas.  A quarter wave (electrical length) transmission line is an impedance inverter and can dramatically change the measured SWR, compared to a half wave line.  

But an unbalanced antenna with no common mode choke at the feed point will also exhibit the same behavior, along with distorted audio or other problems due to high RF in the shack.

Steve, W3AHL



--
Best Regards,
Dennis Klipa

Join ic7000@groups.io to automatically receive all group messages.