Simple 2-element array for 2m

This array was homebrewed using four 18" long telescopic whips and a length of ¾" diameter thick-walled CPVC water pipe.

Data for the 2-element array was obtained from an old ARRL Handbook.

Dimensions in inches: Driven Element - 5540/f MHz, Director - 5263/f MHz, Spacing - 2750/f MHz.

The spacing close to 0.25λ would result in a good match for 50Ω coaxial cable.

Maximum gain would be possible with a director at 0.1λ or a reflector at 0.15λ. However matching issues related to the very low radiation resistance would then have to be tackled (F.C.Judd G2BCX in '2 meter Antenna Handbook').

2m 2-element array

The shorter length of the telescopics was made up with spacers which also served as mounts for the elements. The spacers were made using scrap bakelite strips. Self-tapping screws were used to fasten the elements to the CPVC pipe. A BNC socket was mounted at the feed point and wired to the elements.

2m 2-element array - close-up (element folded)

The array is easy to carry as the elements can be 'telescoped in' and folded. Hence it lends itself to portable operation or direction finding.

Theoretical gain for this array is 3dB.

Preliminary on-the-air checks for gain, front-to-back ratio and null, were encouraging.

Related post: J-Pole Collinear for 2m
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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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