An article by K2PO in QST Feb 2020 entitled An SWR shifting T
illustrates the pitfalls in naive design and implementation of transmission line matching systems. I say naive because the article does not address the matter of loss, yet QST publishes it as an example. Continue reading Stub matching loss can bite you
Category: Antenna Tuners (ATU)
A thinking exercise on Jacobi Maximum Power Transfer #4
The article A thinking exercise on Jacobi Maximum Power Transfer #3 discussed Kurokawa’s power reflection coefficient as in indicator of mismatch at a system node.
Above is a demonstration circuit in Simsmith, a linear source with Thevenin equivalent impedance of 50j5Ω. The equivalent voltage is specified by useZo, which like much of Simsmith is counter intuitive (as you are not actually directly specifying generator impedance):
Vthev and Zthev are chosen so that ‘useZo’ will deliver 1 watt to a circuit impedance that equals the G.Zo. Zthev will be Zo*.
Continue reading A thinking exercise on Jacobi Maximum Power Transfer #4
The transmitter matching problem
In the article The system wide conjugate match stuff crashes out again I worked through an example proffered in an online discussion to show that Walter Maxwell’s teachings on system wide simultaneous conjugate match do not tend to occur in practical systems.
Why are hams so obsessed with conjugate matching?
The answer is on the face of it quite simple. Continue reading The transmitter matching problem
A thinking exercise on Jacobi Maximum Power Transfer #3
At A thinking exercise on Jacobi Maximum Power Transfer #2 I posed the question of a metric for the mismatch at the L2L1 junction in the following network where the calculated values L2L1_lZ is the load impedance at the L2L1 junction (looking left as Simsmith is unconventional), and L2L1_sZ is the source impedance at the L2L1 junction (looking right). The left three components are the fixed antenna representation.
Common practice is to speak of a “source VSWR” to mean the VSWR calculated or measured looking towards the source, and very commonly this is taken wrt 50+j0Ω which may be neither the source or load impedance but an arbitrary reference. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #3
A thinking exercise on Jacobi Maximum Power Transfer #2
At A thinking exercise on Jacobi Maximum Power Transfer I posed an unanswered Q2:
Keeping in mind that C2 and L2 are an adjustable matching network, usually adjusted for minimum VSWR as seen at the source G. So, the questions are:

Does the system take maximum available power from the source G when the load impedance seen by source G is equal to the conjugate of its Thevenin equivalent source impedance (ie C2.Z=G.Zo in Simsmith speak)?

Does that ‘matched’ condition result in maximum power in the load L?
Above for reader’s convenience is the model conjugate matched at the GC2 interface. The calculated Po figure (lower right) is the power in the load L to high resolution. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #2
A thinking exercise on Jacobi Maximum Power Transfer
At The system wide conjugate match stuff crashes out again I discussed the failure of Walt Maxwell’s teachings on system wide simultaneous conjugate match using an example drawn from an online expert’s posting.
The replicated scenario with matching with an L network where the inductor has a Q of 100, no other loss elements is shown below. (Quality real capacitor losses are very small, and the behavior will not change much, the inductor loss dominates.)
Above is a model in Simsmith where I have adjusted the lossy L network for a near perfect match. I have used a facility in Simsmith to calculate the impedance looking back from L1, often known as the source impedance at a node but in Simsmith speak the calculated L1_revZ on the form, (ie back into the L network) from the equivalent load. Continue reading A thinking exercise on Jacobi Maximum Power Transfer
The system wide conjugate match stuff crashes out again
Walt Maxwell (W2DU) made much of conjugate matching in antenna systems, he wrote of his volume in the preface to (Maxwell 2001 24.5):
It explains in great detail how the antenna tuner at the input terminals of the feed line provides a conjugate match at the antenna terminals, and tunes a nonresonant antenna to resonance while also providing an impedance match for the output of the transceiver.
Walt Maxwell made much of conjugate matching, and wrote often of it as though at some optimal adjustment of an ATU there was a system wide state of conjugate match conferred, that at each and every point in an antenna system the impedance looking towards the source was the conjugate of the impedance looking towards the load.
This was recently cited in a discussion about techniques to measure high impedances with a VNA:
WHEN the L and C’s of the tuner are set to produce a high performance return loss as measured by the vna, then in essence, if the tuner were terminated (where the vna was positioned) with 50 ohms and we were to look into the TUNER where the antenna was connected, we would see the ANTENNA Z CONJUGATE. Wow, that’s a mouth full. The best was to see this is to do an example problem and a simulator like LT Spice is a nice tool to learn. Or there are other SMITH GRAPHIC programs that are quite helpful to assist in this process. Standby and I will see what I can assemble.
The example subsequently described set about demonstrating the effect. The example characterised a certain antenna as having an equivalent circuit of 500Ω resistance in series with 4.19µH of inductance and 120pF of capacitance (@ 7.1MHz, Z=500j0.119, not quite resonant, but very close). A lossless L network (where do you get them?) was then found that gave a near perfect match to 50+j0Ω. The proposition is that if you now look into the L network from the load end, that you see the complex conjugate of the antenna, Z=500+j0.119.
I asked where do you get a lossless L network? Only in the imagination, they are not a thing of the real world. Continue reading The system wide conjugate match stuff crashes out again
Series match for a half wave dipole
An online poster was demonstrating the effect of varying line length on a half wave dipole on VSWR_{50} and by mistake configured the line be of of Zo=75Ω.
He asked the question
In the general case, if you are trying to match 50 Ohms, would you be better off feeding a normal backyard dipole with 75 Ohm coax if you are willing to prune it to a specific length after the fact?
Transmitter / antenna systems and the maximum power transfer theorem
Jacobi’s Maximum Power Transfer Theorem
Jacobi’s law (also known as Jacobi’s Maximum Power Transfer Theorem) of nearly 200 years ago stated
Maximum power is transferred when the internal resistance of the source equals the resistance of the load.
Implied is that the internal resistance of the source is held constant, it does not work otherwise. The source must be one that can validly be represented by a Thevenin equivalent circuit. This was in the very early days of harnessing electric current, direct current initially.
Later adaptation dealt with alternating current and it became
Maximum power is transferred when the load impedance is equal to the complex conjugate of the internal impedance of the source.
Again a necessary condition is that the source must be one that can validly be represented by a Thevenin equivalent circuit. Continue reading Transmitter / antenna systems and the maximum power transfer theorem
Walter Maxwell’s teachings on system wide conjugate matching – a SimSmith example
I have written on Walt Maxell’s proposition about simultaneous system wide conjugate matching in antenna systems. I will repeat a little to set the context…
Walt Maxwell (W2DU) made much of conjugate matching in antenna systems, he wrote of his volume in the preface to (Maxwell 2001 24.5):
It explains in great detail how the antenna tuner at the input terminals of the feed line provides a conjugate match at the antenna terminals, and tunes a nonresonant antenna to resonance while also providing an impedance match for the output of the transceiver.
Walt Maxwell made much of conjugate matching, and wrote often of it as though at some optimal adjustment of an ATU there was a system wide state of conjugate match conferred, that at each and every point in an antenna system the impedance looking towards the source was the conjugate of the impedance looking towards the load.
This is popularly held to be some nirvana, a heavenly state where transmitters are “happy” and all is good. Happiness of transmitters is often given in online discussion by hams as the raison d’être for ATUs, anthropomorphism over science. Continue reading Walter Maxwell’s teachings on system wide conjugate matching – a SimSmith example