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