VU2NAN (Nanda): A homebrew RF Ammeter

This project is the result of recently reading WB8EVI - OM Mike Herman's article 'DIY RF Ammeter'.

An available aluminium baking tray, though a bit oversize, came in handy as an enclosure. A piece of aluminium sheet was used to coarsely fabricate a recessed cover.

Trials were made using a 50Ω, 1mA FSD moving coil meter to display 1 RF Amp maximum. The RF toroid, picked up from the junkbox, had no markings but its relative permeability was quite good for a 1:1 transformer to work. It measured OD 20mm, ID 12.5mm and width 12.5mm. Both the primary and the secondary were just wires passed through the toroid without winding.

In the final assembly, the primary was a piece of 18SWG solid bare copper wire soldered to the BNC sockets and the secondary a length of flexible insulated copper wire. A rubber grommet ensured positioning of the toroid. Wiring was on a piece of perfboard, supported directly on the meter terminals.

Calibration was done using a homebrew CW rig and a Weston 1.5A RF Ammeter, after which the variable resistor was replaced by 3 series-wired 10KΩ resistors.

Measurements with this RF Ammeter proved acceptable at 7 MHz, 14 MHz and also at 145 MHz!

However, the 1:1 transformer could cause the secondary load to be directly reflected as a series load in the feeder.  Also, the higher secondary current could result in overheating of the toroid and the 82 Ω resistor.

Hence it was decided to have 20 turns on the secondary side, thereby dropping the load ratio to 400 :1.

The series load imposed on the feeder would now be in the region of only 0.1Ω.

The secondary was wound using solid hookup wire and the RF Ammeter rewired.

Tests showed very good linearity at 7 and 14 MHz but drastic loss of sensitivity at 145 MHz!
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