Matched and Mismatched Loads

Every amateur radio station has a goal when it comes to impedance: get the transmitter output to see a 50-ohm resistive load, so maximum power flows into the antenna system without reflections. When the antenna, feedline, and transceiver all have 50-ohm impedances, the system is said to be matched. When any component presents a different impedance, the system is mismatched, and some power is reflected. Understanding what actually happens in matched and mismatched systems — and what you can and cannot do about it — is essential knowledge for every ham radio operator.

A matched system (left) carries only a forward traveling wave with no reflections. A mismatched system (right) creates standing waves from the combination of forward and reflected waves, with the SWR determined by the impedance mismatch ratio.

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The Perfectly Matched System

A perfectly matched system is one where every component in the signal chain presents the same 50-ohm resistive impedance to each adjacent component:

• The transmitter's output stage presents 50 ohms to the antenna port
• The antenna port is connected to 50-ohm coaxial cable
• The far end of the coaxial cable is connected to a 50-ohm antenna (or a matching network that transforms the antenna's natural impedance to 50 ohms)

In this ideal case, the transmitter delivers its maximum rated power into the feedline, the feedline delivers all of that power (minus its own matched-load loss) to the antenna, and the antenna radiates it. The SWR is 1:1 everywhere in the system. There are no standing waves on the feedline — only a single forward-traveling wave carrying power from transmitter to antenna.

It is important to understand that "50-ohm impedance" at the antenna means the antenna's feedpoint impedance equals 50 ohms at the operating frequency — not that the antenna is physically 50 ohms (antennas do not have DC resistance of 50 ohms; they have an RF feedpoint impedance that depends on their geometry and the frequency).

A half-wave dipole at its resonant frequency in free space presents about 73 ohms at its feedpoint. This is close enough to 50 ohms that the SWR is only 73/50 = 1.46:1 — a modest mismatch that causes negligible additional feedline loss. Many operators feed half-wave dipoles directly with 50-ohm coax and accept the slight mismatch rather than add a matching network.

What Mismatch Actually Does

A mismatched load at the end of a feedline creates a reflected wave, which combines with the incident wave to form standing waves. We now know how to calculate the SWR and reflected power percentage from the impedance mismatch (using the formulas from lesson M13H). But what are the actual practical consequences for your station?

There are three main effects of a mismatched load:

1. Increased feedline loss: The elevated standing wave currents and voltages on the feedline cause additional I²R heating. This additional loss depends on both the SWR and the matched-line loss. For a low-loss feedline, the additional loss is small even at high SWR. For a high-loss feedline, additional SWR-induced loss can be severe.
2. Stress on the transmitter's output stage: The transmitter sees an impedance that is not 50 ohms, which means its output transistors or tubes are not loaded in their designed operating region. Solid-state transceivers typically respond by reducing output power (fold-back) to protect the output transistors. Older valve transmitters simply operate less efficiently and may overheat.
3. Possible connector damage: At current antinodes and voltage antinodes, the peak current and voltage on the feedline are higher than their matched-load values. High-quality connectors handle this well; corroded or poorly assembled connectors may arc, heat up, or fail at elevated peak voltages.

Effects on the Transmitter

Modern solid-state transceivers contain automatic level control (ALC) and SWR protection circuits that monitor the reflected power and reduce the transmitter's drive (and therefore output power) when SWR exceeds a threshold — typically around SWR 2:1 to 3:1. This fold-back protects the output transistors from damage but means the transmitter delivers less power at high SWR.

The exact fold-back behavior varies by manufacturer and model. Some radios are more tolerant than others. The protection is necessary because at high SWR, the peak voltage on the coax near the transmitter can be well above the normal matched-load value (2× the normal value at SWR 3:1 as discussed in lesson M13G), which exceeds the voltage ratings of the output transistors.

The solution to transmitter fold-back is always to match the antenna system — either at the antenna feedpoint (ideal) or at the transmitter using an antenna tuner. After matching, the transmitter sees 50 ohms and delivers full rated power.

Effects on Feedline Loss

You studied this in lesson M13E, but it bears repeating in the context of real system analysis. The additional feedline loss caused by SWR is only significant when the feedline already has appreciable matched-load loss. The following worked example illustrates this clearly:

How an Antenna Tuner Works

An antenna tuner — more accurately called a transmatch or antenna matching network — is an adjustable impedance transformer placed between the transmitter and the feedline. Its purpose is to present a 50-ohm resistive impedance to the transmitter, regardless of what impedance the feedline and antenna system present at the tuner's input terminals.

A typical L-network, T-network, or Pi-network tuner contains adjustable capacitors and a switched inductor (or continuously variable inductor). By selecting the right combination of inductance and capacitance, the tuner creates a reactive matching network that cancels the reactive component of the feedline's input impedance and transforms the resistive component to 50 ohms.

When the tuner is adjusted for a 1:1 SWR reading on the meter between the transmitter and the tuner, it means: the transmitter sees 50 ohms. The tuner is doing its job. The transmitter delivers its full rated power. The SWR between the tuner and the antenna (on the feedline) has not changed — it is still whatever it was before the tuner was connected.

What a Tuner Cannot Do

This is one of the most important and most commonly misunderstood points in amateur radio:

Consider a 100-foot run of RG-213 to a poorly-matched antenna presenting SWR 5:1. Without a tuner, the transmitter sees about 12 ohms at its output and reduces power due to fold-back. With a tuner in the shack, the transmitter sees 50 ohms and delivers full power. But that full power is now being fed into a feedline with SWR 5:1 — the additional feedline loss from the SWR (which you calculated above: about 0.65 dB extra loss) still exists. The 100-foot run of RG-213 at SWR 5:1 at 14 MHz wastes about 34% of the transmitter's power as heat in the cable. The tuner has not reduced this; it has actually slightly increased the absolute power wasted, because the transmitter is now delivering full power (100 W) rather than fold-back power (perhaps 30 W).

This does not mean tuners are bad — they are extremely useful, particularly for multiband operation. But their benefit is that they allow the transmitter to deliver full power without fold-back, not that they improve antenna efficiency or reduce feedline loss. The true solution to high-SWR feedline loss is either to match the antenna at its feedpoint, or to use a low-loss feedline (ladder line) on which high SWR has negligible effect.

Matching at the Antenna vs. the Shack

There are two fundamentally different approaches to impedance matching:

Matching at the antenna feedpoint

Place the matching network directly at the antenna feedpoint. This is the ideal approach: the SWR on the feedline is always 1:1 (or very close to it), so feedline loss is at its minimum and no power is wasted in heating the cable due to SWR. An automatic remote antenna tuner mounted at the base of the antenna mast accomplishes this. Many modern transceivers with built-in antenna tuners can use a remote head for exactly this purpose.

The disadvantage is cost and weatherproofing. A remote ATU must be waterproofed, must handle operating temperatures, and must be accessible for maintenance. For a permanent installation on a single band, a fixed matching network (a matching transformer or impedance transformer) is often preferred over a remote tuner.

Matching at the shack (transmatch)

Place a transmatch or antenna tuner in the shack, between the transceiver and the feedline. This allows multiband operation with a single antenna without going to the antenna. The tradeoff is that the feedline operates with whatever SWR the antenna presents — which may be high on some bands — and the feedline loss increases accordingly. For low-loss feedlines (LMR-400 or ladder line) this tradeoff is entirely acceptable.

The worst case for a shack tuner is a long run of standard coaxial cable (RG-213 or worse) on a band where the antenna is far from resonance. The combination of high SWR and high matched-loss creates large additional losses. In this scenario, a remote antenna tuner or a better feedline is the correct solution.

Practical Matching Guidelines