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.


Homebrew RF Ammeter
Trials were made using a 50Ω, 1mA FSD moving coil meter to display 1 RF Amp maximum. The RF toroid, picked up from the junk box, 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 H 12.5mm. Both the primary and the secondary were just wires passed through the toroid without winding.

Homebrew 1:1CT RF Ammeter - Schematic
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.

Homebrew 1:1CT RF Ammeter - Inside view
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.

Homebrew 1:20 CT RF Ammeter - Schematic
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.

Homebrew 1:20 CT RF Ammeter - Inside view
Tests showed very good linearity at 7 and 14 MHz but drastic loss of sensitivity at 145 MHz!

Related post: Salvaged RF Ammeter
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Homebrew ¼ λ Magmount for 2m

Few parts are required to homebrew this magmount.

A 5¼” floppy disk drive rotor, with its boss removed, serves as the magnetic base. A thin plastic sticker, covering the exposed face of the magnet, prevents damage to the vehicle paint surface.

The enclosure is a suitably drilled Melamine or Bakelite cup on which the SO-239 is mounted.

2m ¼ λ Magmount details
RG-58/U or smaller coax is used. A ¼ λ counterpoise of stranded insulated hook-up wire is soldered to the braid of the coax. This is a must in case the rig is to be kept isolated from the body of the vehicle.

The enclosure is potted with epoxy to waterproof it and make it base-heavy. The same epoxy holds the assembly in position on the magnetic base.

The driven element is a ¼ λ length of 1.6mm brazing rod soldered to the PL-259 pin. The space between the PL-259 body and the driven element is filled with epoxy to prevent water ingress.

This magmount proved its usefulness on many occasions when access to the vehicle battery was denied and a separate battery had to be used.

Related post: Simple ¼ λ Ground Plane for 2m
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