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Amateur Radio

Antennas -- SWR Meters

This section includes information about SWR (Standing Wave Ratio) meters.
Ideally, all of the RF energy that is fed into an antenna feeder cable should be delivered into the antenna. In practice, mismatches in impedance along the feeder cable or between feeder and antenna will cause some of this energy to be reflected back down the feeder and into the transmitter where it will be dissipated as heat, possibly harming the PA stage of the transmitter. Reflected energy will generate standing waves along the feeder cable which can be detected and measured using a Standing Wave Ratio meter (SWR meter).

An SWR meter is an ESSENCIAL item when tuning/matching an antenna system to a transmitter. Adjustments are made so as to obtain the minimum possible SWR -- ideally 1:1. SWR ratios above 1.3:1 are considered to be excessive for modern solid state transmitters.

Most modern transmitters include built-in SWR meters. SWR meters can also be found as stand-alone items, or as part of an ATU.






SWR meter

The essence of an SWR meter is shown in the top image on the left. Two signals are generated which are indications of:
a) the forward signal going towards the antenna --identified as FWD, or sometimes as DIR
b) the signal that is reflected back to the transmitter -- identified as REF





This form of SWR meter is typically produced on double sided PCB material so that the central conductor can be implemented as a 50 Ohm microstrip line, as shown in the second image. This form of construction can be made to be suitable for use at the higher HF, VHF, and even UHF frequencies.





Sensitivity of microstrip based SWR meters might not be good enough for low power use at HF frequencies. The inclusion of a toroid transformer, as shown in the bottom image can improve sensitivity at lower frequencies.

In some SWR meters, a single moving coil meter is switched between the FWD and REF points. More sophisticated SWR meters might use two separate moving coil meters, or even a single twin movement meter. In the latter case, the meter scale can be calibrated to indicate SWR at the intersection of the two meter needles.

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18/07/13