SWR meter
| SWR meter | |
|---|---|
| Also called | SWR bridge, reflectometer |
| What it measures | Forward and reflected power on a feed line, and the SWR worked out from them |
| Key part | A directional coupler |
| Built into | Most HF transceivers and many antenna tuners and amplifiers |
| Types | |
| Single-meter | Switches between forward and reflected; needs calibrating |
| Cross-needle | Two needles; SWR is read where they cross |
| Digital | Calculates SWR automatically; often reads peak and average power |
| Wattmeter | Reads forward and reflected power directly, such as the Bird 43 |
An SWR meter is an instrument that measures the standing wave ratio on a feed line, showing how well the antenna system is matched to the transmitter. It is connected in the coax and reads only while the transmitter is on, usually while sending a steady carrier at low power. Most HF transceivers have one built in, but separate meters remain common for checking an amplifier's output, for VHF and UHF work, and for watching the match at more than one point in the station.
How it works
The heart of an SWR meter is a directional coupler, a circuit that samples the wave traveling one way along the line while largely ignoring the wave traveling the other way. Some designs use a short pickup wire running beside the center conductor inside the meter; others use a small transformer wound on a ferrite ring around it. Either way, the meter produces two samples, one of the forward power headed toward the antenna and one of the reflected power coming back. Diodes turn the samples into DC voltages that drive the meter.
SWR depends only on the ratio of the two, not on how much power is flowing. With forward power Pf and reflected power Pr:
- SWR = (1 + √(Pr/Pf)) / (1 − √(Pr/Pf))
For example, 100 watts forward and 11 watts reflected gives an SWR of about 2:1. The power actually delivered into the line is the forward reading minus the reflected one.
Types of meter
- Single-meter types have a switch marked forward and reflected and a calibration or sensitivity knob. The operator transmits, sets the needle to full scale in the forward position, then switches to reflected and reads SWR from the scale. The calibration must be repeated whenever the frequency or power changes.
- Cross-needle meters put two meter movements in one case, with needles that cross. One shows forward power, the other reflected, and curves printed on the face give the SWR at the point where they meet. All three values appear at once, with no calibration knob, though the operator still picks a power range.
- Digital meters compute SWR automatically, and many show both average and peak envelope power (PEP). Some can switch an amplifier off or sound an alarm when SWR climbs.
- Wattmeters such as the Bird Model 43, long standard in commercial two-way radio shops and a familiar sight at hamfests, read forward and reflected power in watts. The Bird uses plug-in elements, each covering a frequency range and a full-scale power; turning the element around reads reflected power, and SWR is found from the formula or a chart.
An average-reading meter shows much less than the peak power of an SSB voice signal. To measure SSB output, use a meter with a PEP mode.
Frequency range and calibration
A meter is accurate only over the frequencies it was designed for. The coupling of a simple pickup-wire design rises with frequency, so the same power reads differently on different bands, and the coupler's directivity, its ability to reject the wave going the other way, falls off outside its range, producing false SWR readings. Use an HF meter on HF and a meter rated for VHF or UHF on those bands. Meters built for the 27 MHz Citizens Band may be inaccurate on other bands.
Power matters too. The diodes need a certain voltage before they respond, so many meters read poorly at a few watts, and a QRP station needs a meter designed for low power. At the other end, the meter must be rated for the full output of the station.
Where to put it
- Between the radio and the antenna tuner the meter shows what the radio sees. This is the place to watch while adjusting a tuner for 1:1.
- Between the tuner and the feed line it shows the SWR on the coax, which reveals how well the antenna itself is matched.
- After an amplifier it shows the amplifier's output and the load it sees. The meter must be rated for the power.
- Near the antenna it gives the truest reading. Loss in the coax makes SWR look better at the radio end than it really is at the antenna, as explained under SWR.
An SWR meter needs a 50-ohm line on both sides; it cannot be used on ladder line or other balanced line. An antenna analyzer measures SWR without transmitting and shows how it changes across a band.