All powers SWR bridge and power meter

1st edition: may 2004

Revision 12: December 2014

by Dominique - F1FRV

Aide technique (Français)       Droits d'auteur (Copyrights)

EXCEL sheet with toroid true measured AL input.

Peak power optional board (link at bottom of this page).


This design is applicable for small or high powers, from 100 kHz (or less) to 50 Mhz, from miliwatts to kilowatts, depending on the selected components. For best results, for WIDEBAND use the ferrite "43" material. Materials "4C6 or 4C65" generaly painted in purple, are identical to material "61" and can also be used (with much more turns than "43" for same inductance).

Material "2" iron powder (painted in red), are not in tables, as i strongly do not recommand their use for wideband. They works well for monoband designs, but definelitely not for wideband designs.

In theory, "61" or "4C6/4C65" could be used for 144 MHz, but i did not experimented by myselves...

Good suppliers for toroids are: PartsAndKits , RF microwave.it and AMIDON ...

PartsAndKits and RF microwave.it can also supply 1N5711, 2N7000, and trimmer capacitors .

With good electrical and mechanical design, using parts described here, and PCB designed for, 1.8 to 30 (50) MHz directivity can be better than 35 dB, and accuracy in power measurements in the range 1 to 2 %. Not so bad, isn't it ...

With different electrical or mechanical design, or misunderstanding of the excel sheet, please, do not complain if you obtain bad results, no or poor directivity, or transform resistor and wires into smoke and flammes (1 case in eastern Europe). Hundreds of people all around the world were happy with this design... If it is not your case,  please, re-read carefuly what is written, and review what you made.

Generaly, if it dont works, it is with people more clever, and better technicians than me, who dont want to follow EXACTLY what is written, and do what they feel instead.

EXCEL sheet design example

Nota: It is better to select the schematic with one resistor R2 and mid tap at coil, than the other scheme with 2 half values resistors. As it is easier to have EXACTLY the same nb ot turns in coil, than having EXACTLY the same values of the 2 resistors. Any difference in the 2 resistors values would give errors in SWR measurements results. Dont forget that turns number is the nb of turns counted INSIDE toroid hole.

Coaxial line shield must be grounded ONLY 1 side of toroid, 1 time at input connector, and 1 time just near toroid. Place the WIDE ground return strip(s), not a single wire, between coaxial line, circuit board and main ground frame, as close as possible to the toroid, to have the best accuracy in POWER measurements over a wide range of frequencies. For SWR measurements ONLY, this is not very important, as the ratio FWD/REF is independant of absolute values of detected voltages.To avoid parasitic capacitances, place the PCB at least 10 mm above metallic enclosure,

To have perfect balance of the bridge, with the calculated values given by EXCEL sheet, it is important to know the " TRUE " values of the coil(s), resistor R2, and capacitor C1. To know what is the exact Al value of your toroid, coil 5 turns of wire into it, measure with a LC meter, and divide value by 25. With an Ohmmeter, adjust VR1 to the value given by excel sheet.The variable C2 capacitor, as shown, is MANDATORY, to compensate for all tolerances in components values, parasitics. During tests, dont hesitate to modify calculated value of C2 (fixed + adjustable value) to obtain the best balance.Tolerances and parasitic capacitances management are unfortunately not exact sciences. Adjustment of phase compensation C2 must be made, ONLY at bridge maximum frequency of use. If null is not OK with C2, readjust slightly VR1 and C2, at max frequency of use, to obtain perfect null. Reverse bridge in /out to check null symetry.

Check of coil(s) and capacitor C1 exact values can be made with a LC meter, like the one described in pages " Technical " and proposed by DF1SR (ex DD0SB).

Adapt cross sectional area of wires in toroid to maximum current in R2, do not undersize, 3 Amps by square milimeter is a maximum.

Load on detector diodes is a first order parametre for rectified voltages output. Do not hesitate to use operational amplifiers LM358 or LM324 as buffers (they include GND in common mode range). In this case, with very high impedance detectors load, to avoid too high time constant in SWR readings, reduce, if necessary, the values of decoupling capacitors (ceramic) in FWD & REF outputs. 2.2 to 10 nF is OK with operational amplifiers, 100 nF or more without operational amplifiers.

PCBs availables. See HERE

Note the wide ground strap (12 x 0.5 mm) behind toroid


Simulations of the bridge phase & balance


Bridge old revision 3 PCB & components picture

Detail of the 0.5 pF @ 4000 V capacitor ( 2 x 10 mm of copper of RG402 )

Measurement of a 0.7 pF @ 4000 V capacitor ( 2 x 14 mm of copper of RG402 )

Front plate with meter

Meter scales can be made with F5BU’s POSTCARDWARE “GALVA” available HERE.

Download EXCEL sheet, components datasheets, example files for GALVA (SWR and power meters scales), and files for PCB: SWR bridge F1FRV.ZIP


Internal link to Automatic SWR meter

Internal link to DD0SB low cost LC meter


Thatís All Fox ! ! ! !

Dominique - f1frv@sfr.fr OR

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