Directional Wattmeter: Forward and Reflected Power Explained

The meter on the front of a transceiver tells you roughly what the radio believes it is doing. A directional wattmeter tells you what is actually travelling along the cable. It is a small section of transmission line with two sensing circuits, one that responds to power going towards the antenna and one that responds to power coming back. The two readings together show how much power leaves the transmitter, how much returns because of a mismatch and, with a little arithmetic, what the SWR is. This guide explains how the instrument works, how to read it, how to convert reflected power into SWR, why a peak and an average reading differ, how the classic slug-type wattmeter and the modern cross-needle meter compare and how to check that your meter tells the truth.

What a Directional Wattmeter Does

A directional wattmeter is built around a directional coupler. A short length of line carries the transmitter power to the antenna. Near it, a sensing line couples a small, known fraction of the energy, and a clever arrangement of the coupling makes the sensing line respond to a wave travelling in one direction much more than to a wave travelling the other way. A second sensing line, or the same one reversed, responds to the reflected wave. Each signal is rectified by a diode, and the DC voltage is shown on a meter or processed in a digital circuit and converted to watts, as the general article on the power meter describes.

The instrument therefore reports two numbers. The forward power is the power that the transmitter delivers into the line. The reflected power is the part that comes back from a mismatch at the antenna or elsewhere in the line. The net power that actually reaches the load is the difference. If the meter shows 100 watts forward and 4 watts reflected, then 96 watts is delivered, and 4 percent is wasted in the transmitter or the line. For the loss that comes with that figure, see the article on return loss and mismatch loss.

From Reflected Power to SWR

The two numbers are related to the voltage standing wave ratio. The square root of the ratio of reflected to forward power is the magnitude of the reflection coefficient, written rho. The SWR is then (1 + rho) divided by (1 − rho). It is a small calculation, and the table gives the usual points.

The table shows why a high SWR is less dramatic than it sounds. At 2:1 only 11 percent of the power is reflected, and since most of that is re-reflected at the transmitter end, the real loss is a fraction of a decibel. But the table also shows why a directional wattmeter is a better tool than a simple SWR indicator when the power is high: the reflected power in watts matters to the transmitter, and 25 watts coming back from a 100-watt transmitter is a heavy load on the final stage, which is why transceivers fold back their power at high SWR. The article on what an SWR meter tells you and what it hides explains the limits of the reading in more detail.

Two Families of Instrument

The wattmeters in the shack fall into two groups, and they behave differently.

Slug-type meters. The classic example is the Bird Model 43 and its many imitations. It has a fixed line section with a socket, and a plug-in element, called a slug, which holds the coupler and the diode. Each slug is rated for a range of frequencies and a full-scale power, such as 5, 25, 100, 250 or 1,000 watts, and the arrow on the slug sets the direction, forward or reflected. To measure reflected power you turn the slug round. The meter scale is then right for that slug. The advantage is accuracy: a good slug is accurate to within a few percent in its frequency range, and you choose the power range that puts the needle in the upper half of the scale. The drawback is that you need the right slug for the band and the power, and that each slug is another item to buy and keep.

Cross-needle meters. Instruments such as those in the SWR and power meters for HF and VHF have two needles on one scale, one for forward and one for reflected power. The point at which they cross on a printed curve gives the SWR. They are convenient, cheap and fast, since you do not turn anything, and are very popular in the shack. The accuracy is typically lower, often within 10 percent, and the range switches select the full-scale power.

Digital meters have joined both. They sample the forward and reflected voltages, calculate watts and SWR, and display a peak or an average value, and some measure peak envelope power directly, which is the quantity that regulations refer to. The article on peak envelope power explains why that matters for single sideband.

Peak or Average: What the Needle Shows

A transmitter carrying a steady carrier, as in FM, CW with the key down or a digital mode with constant amplitude, gives a clear reading on any wattmeter. An SSB signal does not. The envelope of speech varies, and a normal analog meter with a slow needle follows the average of the rectified voltage, which for voice is far below the peak. A 100-watt PEP transmitter may read 20 to 40 watts on an average-reading meter during speech. A true peak-reading instrument, with a fast detector and a hold circuit, shows the real peaks. When you compare transmitters, or check a legal limit, use a peak-reading meter or a two-tone test with a steady tone to read a stable value.

A related point is the effect of the detector on low power. Diode detectors are non-linear at low voltage, so a meter that is accurate at 100 watts may read poorly at 1 or 2 watts. For QRP work, choose a meter with a low-power range or a digital instrument that is made for it.

Where to Put the Meter

The usual place is in the coax run between the transmitter and the antenna tuner or antenna, as close to the transmitter as practical. At this point, the wattmeter reads the forward power the transmitter delivers and the reflected power that the transmitter sees. If you use an external tuner, as explained in the guide to the external antenna tuner, put the meter between the radio and the tuner to see what the transmitter sees, and a second meter after the tuner to see what the antenna gets. With only one meter, the position between the transmitter and the tuner is the most useful for protecting the final stage.

Keep in mind that the feedline loss affects the reflected reading. The reflected wave loses power on its way back to the transmitter, so with a long, lossy coax the reflected power at the shack is lower than at the antenna, and the SWR at the shack looks better than at the antenna. For example, 3 dB of line loss one way reduces reflected power by 6 dB for the round trip, and an antenna SWR of 3:1 appears as roughly 1.7:1 at the transmitter. The reading is correct for the place where it is taken, but it does not describe the antenna itself.

How to Check Your Wattmeter

A wattmeter you cannot trust is worse than none, so a few simple checks are worth doing, and they use items that every shack has, such as a dummy load. The guide to the dummy load shows how to make a safe test.

• Dummy load test. With a good 50-ohm load on the output, the reflected reading should be near zero on every band within the range of the meter. If it is not, the meter or the load is off, or a connector is faulty.
• Reverse test. Turn the meter round, or swap the input and output sockets. The forward and reflected readings should swap. If they do not, the coupler is unbalanced.
• Open and short test. At low power, put an open circuit or a short circuit on the output, and the reflected reading should rise to nearly the forward power. Use no more than a few watts, because the transmitter may not like this.
• Cross-check. Compare readings with a second meter, a friend’s meter or the built-in meter of the transceiver, at several power levels. Differences of 10 percent are normal for inexpensive meters; much larger differences show a problem.
• Band check. Measure on the highest and the lowest band that you use. Cheap meters can be accurate on 14 MHz and wrong on 50 MHz.

The wider list of instruments that a station needs is in the article on ham radio test equipment, where the wattmeter is one of the basic items next to the multimeter and the antenna analyzer.

Power Levels, Heat and Safety

A wattmeter is a passive device, but it is not immune to abuse. A slug rated for 100 watts can be damaged by a kilowatt, and the diode in a cheap meter can fail from a brief overload, so always start with a higher range and work down. The line section of a meter is rated for a certain power and for a certain SWR, so do not use a small meter with a linear amplifier unless the maker approves it. Remember also that the meter is part of the RF path: a loose connector in it can arc and heat, and a damaged case lets RF out. During a long, high-power transmission, feel the connectors afterwards, because warm connectors point to a poor contact. A meter that stays cold and reads consistently is a sign of a healthy line.

Common Mistakes

• Using the wrong slug. A slug rated for 2 to 30 MHz does not read right on 144 MHz, and a 1,000-watt slug barely moves at 5 watts. Check the frequency and the power range.
• Trusting a single reading. Read at several frequencies across the band, because SWR changes with frequency.
• Measuring with tuning pulses. Tune with a carrier, not with speech, to get a stable reading.
• Ignoring the connector. A bad connector on the meter gives a false mismatch. Use short, good-quality jumpers.
• Reading a peak scale with an average meter. A scale labelled in PEP does not make a slow meter into a peak meter.

The Bottom Line

A directional wattmeter shows forward and reflected power in the line, and from them you can calculate the SWR as (1 + rho) divided by (1 − rho), where rho is the square root of reflected over forward power. Slug-type meters are accurate within their frequency and power range, cross-needle meters are quick and convenient, and digital meters add peak reading. Use a peak-reading instrument for SSB, keep the meter near the transmitter, remember that line loss hides reflected power, and check the meter on a dummy load and by reversing it. With these habits the numbers on the dial can be trusted, and you will know what really leaves your radio.

Frequently Asked Questions

What does a directional wattmeter measure?

The power travelling towards the antenna, called forward power, and the power returning from a mismatch, called reflected power. It can show SWR from the two.

How do I calculate SWR from forward and reflected power?

Take the square root of reflected divided by forward power, call it rho, and compute (1 + rho) divided by (1 − rho). At 4 W reflected and 100 W forward, rho is 0.2 and the SWR is 1.5:1.

Why does my meter read low on SSB?

An average-reading meter cannot follow the fast peaks of speech, so it shows a fraction of the peak envelope power. Use a peak-reading meter or a steady carrier for tests.

What is a slug in a Bird-type wattmeter?

A plug-in element that holds the coupler and detector. Each slug covers a certain frequency range and full-scale power, and its arrow sets forward or reflected.

Where should I connect the wattmeter?

In the coax between the transmitter and the tuner or antenna, close to the transmitter. With a tuner, a position before it shows what the transmitter sees.

How accurate are cheap cross-needle meters?

Often within about 10 percent, and less accurate at low power or on the highest bands. They are good for tuning and trends, and a dummy load check shows whether yours is reasonable.