Salvaged RF Ammeter

Years ago, a 0.75A 'thermocouple heating element' was salvaged from a meterless BC-442 Antenna Relay Unit. Only recently was it taken out, to build an RF Ammeter for low-power rigs.

Salvaged RF Ammeter

A junk-box 50Ω, 1mA FSD  moving coil meter, with a bakelite housing, was also put to good use. It has graduations only upto 50% of its scale.

An aluminium baking tray was reworked to accept a couple of SO-239 sockets and the meter.

The unit was tested after completion of assembly and wiring.

Thermocouple RF Ammeter - Schematic

Salvaged RF Ammeter - Inside view

A homebrew CW rig was loaded to get a reading of 0.75A on a Weston 1.5A RF Ammeter. The Weston was then replaced by the unit to be tested.

The meter pointer stood at exactly 50% of full scale, indicating that the graduated portion of the scale covered the full range of the thermocouple. Notwithstanding the non-linear output of the thermocouple and the unmarked meter scale, staying within the graduated portion of the scale would avoid damage to the element!

Related post: Adjustable RF Ammeter
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Another ¼ λ Ground Plane for 70 cm

The elements of this antenna are cut from 3/8" aluminium tubing. The length of the driven element is 6½" and that of the the radials 6¾".

The connector is SO-239.

Another ¼ λ Ground Plane for 70 cm

The radials are fastened to the connector flange after their mounting ends are flattened and drilled. They are then bent, making an angle of 30 degrees to vertical, with the connector flange corners taking the bend.

The driven element is fixed to the SO-239 centre terminal using a brass bush. A plastic sleeve makes the joint water-tight. The open end of the driven element is then sealed with rubber compound.

SWR at 435MHz is 1.5:1.

Related post: Twin-Radial ¼ λ Ground Plane for 70 cm
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