KJ6ER Primer 2: The Same 100 Watts Must Mean the Same Thing

After discussing Greg Mihran, KJ6ER's first antenna primer, I turned to his Antennas Primer 2: Antenna System Efficiency. Moving from the antenna alone to the complete transmitter, feedline and antenna system is a useful step. It also makes consistent bookkeeping essential: the watts leaving one block must be the same kind of watts entering the next.

My concern is not that every equation is wrong. Several are right. The problem is what happens when a calculation based on forward power is followed by one based on net accepted power, while both still appear to start with the same 100 W. Add a promise that an antenna tuner prevents ALC operation, and the reader can leave with the wrong picture of both the power budget and the transmitter.

First, Keep the Distinction That Page 12 Gets Right

Return loss and mismatch loss are not interchangeable. Greg distinguishes them correctly on page 12. For a 2:1 standing-wave ratio (SWR), at one measurement plane on a nominally real 50 Ω line, the reflected-wave voltage magnitude is one third of the incident-wave voltage magnitude. Power follows the square, so one ninth of the incident power travels back.

The voltage-reflection magnitude is |Γ| = (SWR − 1)/(SWR + 1). Return loss is −20 log₁₀|Γ|; mismatch loss is −10 log₁₀(1 − |Γ|²). These equations describe different ratios, not two competing estimates of cable heating. Keysight's reflection-measurement guide explains the underlying incident/reflected-wave quantities.

With 100 W incident at the load, 88.89 W is accepted and 11.11 W is reflected. That relation remains valid in steady state when the readings describe the complete waves at that plane. It is not merely an incomplete first pass waiting for another batch of watts to be added.

The Meter Position Is Part of the Number

Forward power describes the wave travelling towards the load. Reflected power describes the wave travelling back. Net power is their difference at the same cross-section. On an ordinary 50 Ω directional wattmeter:

This is the relationship given in the Bird Model 43 manual's load-power explanation. A reading of 100 W forward and 25 W reflected means 75 W net crossing that plane towards the load. It does not mean 100 W net plus 25 W waiting to be reclaimed.

Move the meter along a lossy cable and the forward and reflected readings change. A reading beside the transceiver is not automatically the reading at the antenna. Nor is power accepted by the antenna automatically power radiated: some accepted power may heat wire, soil or matching components.

The Primer's Two Answers Describe Two Different Inputs

Pages 14 and 16 give us a particularly useful example: 100 feet of RG-213 at 14 MHz. Page 14 rounds the matched-line attenuation to 0.73 dB; page 16 uses 0.734 dB, which I use below. Keep that cable assumption and choose the 5:1 SWR row at the antenna end. Page 14 gives about 47 W delivered; page 16 arrives at 68.7 W after adding a recovered contribution.

I recalculated the example using the same cable attenuation and load mismatch. The result is not that 68.7 W is impossible. It is that 46.9 W corresponds to 100 W total forward power, while 68.7 W corresponds to 100 W net input power.

Both columns conserve energy; small sum differences reflect rounding. In the first, 68.30 W enters the cable net, of which about 46.92 W reaches the antenna as accepted power and 21.39 W heats the cable. In the second, 100 W enters net and divides into 68.69 W accepted by the antenna and 31.31 W of cable heat.

The second column requires 146.41 W forward, not the unchanged 100 W forward shown in the first. A calculation establishing that operating condition is not a promise that a particular 100 W transceiver will produce it. Its output network and control system determine what it actually does.

The smaller 1.5:1 example follows the same pattern: 81.07 W at the antenna for 100 W forward, versus 83.45 W for 100 W net input. The difference is less conspicuous, but the changed reference is the same.

The Calculation Behind the Table

Let t be the fraction of a travelling wave's power that survives one trip through the cable, and r the reflected power fraction at the load. For this example:

Here P_F and P_R are the total steady-state forward and reflected powers at the cable input; P_load is net accepted power at the antenna end. The factor t² appears because the reflected portion traverses the cable in both directions before reaching the input again.

These are ordinary single-mode transmission-line calculations for a uniform reciprocal cable with approximately real 50 Ω characteristic impedance. They assume the stated attenuation and the load-end SWR; they do not include extra connectors, tuners or antenna losses, and they do not prescribe the transmitter's response.

This also explains why simply adding matched-line attenuation and mismatch loss needs a label. Their sum gives the reduction from input forward power to load-accepted power in this model. It is not all heat, and it is not the same ratio as net cable input to load power.

Re-Reflection Is Real, but a Forward Reading Already Includes the Forward Wave

Greg explicitly assumes no source absorption or foldback in the introductory reflection examples on pages 10–11; page 11 also neglects coax loss. Those qualifications matter, and I am not treating them as absent. What they do not provide is a complete source-boundary model that permits every returning watt to be added to an unchanged steady-state forward reading.

A wave arriving back at a transmitter may be reflected again. How much, and with what phase, depends on the electrical boundary it meets. In the elementary linear model, a source resistance matched to the line does not re-reflect the arriving wave. A transmitter designed to deliver rated power into 50 Ω is not, merely from that specification, proven to behave as a 50 Ω absorbing termination for a returning wave. MIT's bounded-line treatment shows explicitly why the source boundary matters.

There is another important distinction between separated pulse echoes and continuous RF. With a continuous carrier, contributions travelling in the same direction overlap. Their voltages add with phase before power is calculated: two equal in-phase contributions reinforce; two equal opposite-phase contributions cancel. Adding their individual watt figures cannot generally replace that calculation.

Once a directional meter reports the complete forward wave, any contribution from re-reflection is already in that reading. If it raises the forward wave, the reading changes. That is the missing step in the account that keeps 100 W forward and then adds recovered watts separately.

The lesson is neither that every reflected watt becomes transmitter heat nor that coax loss is the only possible destination. Keep the boundary, the phase behaviour and the actual operating condition consistent. The cable's heat is the difference between net power entering and net power leaving it.

ALC Does Not Mean Only High-SWR Foldback

Page 3 groups several protective functions under Automatic Level Control, or ALC. Page 13 then goes further: a matched transmitter output is presented as preventing ALC operation and ensuring full rated output on all bands. That conclusion does not follow.

ALC regulates the transmitter's signal level during normal operation too. High-SWR power reduction is a protective response; depending on the radio, it may share control circuitry with other limiting functions or be handled separately. A matched antenna does not remove supply, drive, temperature or current limits.

A concrete counterexample is the Elecraft K4 operating manual, revision D14. It uses ALC indication during ordinary microphone and data-input adjustment and documents power reduction at inadequate supply voltage. That is enough to rule out the universal claim that a good match means no ALC activity. It is not an instruction to use a K4's meter settings on every other radio.

The foldback curve on page 3 therefore needs an identified radio and test condition before its numerical values can guide an operator. There is no universal curve mapping SWR alone to remaining transmitter power. Follow the relevant manufacturer's operating limits; do not disable protection to make a power-budget example come true.

A Feedpoint Tuner Has a Real Advantage, Not a Universal Guarantee

The strongest part of page 13 is its basic placement argument. An antenna tuning unit (ATU) at the feedpoint can present a much better termination to a long coax run. That reduces mismatch-related cable loss and standing-wave voltage/current stress on that run. A tuner at the shack can present a good load to the transmitter while the cable beyond it still operates at high SWR.

That is a genuine reason to prefer a suitable remote tuner for a substantially mismatched antenna fed through appreciably lossy coax. It does not establish that remote placement is always the most efficient complete arrangement. The gain from improving the cable condition must exceed any extra tuner and connection loss; a short, low-loss feedline can give a different trade-off.

The pictured Icom AH-730 specifies tuning accuracy of 2.0:1 VSWR or less, with exceptions, rather than guaranteed exact 1:1. A successful tune may be better than that specification, but the manufacturer's stated boundary is not the perfect termination assumed in the diagram.

Likewise, the slide's 0.3–0.5 dB tuner loss is not a universal fixed allowance. Matching-network loss depends on the transformed load, frequency, topology and component losses. The ARRL laboratory's AH-730 review demonstrates this with different resistive and reactive test loads and explains the component-loss mechanism. The loss belongs in the complete power budget once, but its value does not become load-independent because the tuner is at the antenna.

What I Would Carry Back to the Station

I would keep Greg's transmitter–feedline–antenna framework and the correct return-loss/mismatch-loss distinction. I would keep the practical case for reducing excessive coax loss and, where appropriate, putting the tuner at the feedpoint.

I would change the conclusions built on switching power references. Record whether the starting value is forward or net power, name the measurement plane, and keep accepted power separate from radiated power. A feedpoint tuner can solve a cable-mismatch problem; it cannot make the transmitter's control system disappear.

These are not objections that require a new antenna model or an argument over somebody's preferred vertical. They can be settled with a consistent power balance and the relevant equipment documentation. The same 100 watts must keep the same meaning all the way through the explanation.

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