Power Supply Dummy Load

A 13.8V 25A DC Linear Regulated Power Supply was in need of testing after its defective main rectifier was replaced. It had protective circuits for under-voltage, over-voltage, short-circuit and over-current.

The filaments of a couple of 12V 55/60W automobile headlight bulbs were connected in parallel and used as a dummy load, but their very low cold resistance caused instant tripping.

A length of galvanised curtain wire (~ 1.5m long), connected in series, saved the day. The power supply did not trip and the bulbs emitted a feeble glow.

Power Supply Dummy Load - Schematic
The connecting wire was then slid along the curtain wire. The illumination progressively increased and so did the load current. When the curtain wire went out of circuit, the bulbs were fully illuminated and drawing 23A from the power supply.
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Simple ¼ λ Ground Plane for 2m

This simple ¼ λ ground plane antenna is based on G3OGR - OM F.G. Rayer's design for a 2m vertical antenna.

 ¼ λ Ground Plane for 2m - details
It's built-in at the top end of a 25 foot long, 2-piece telescopic aluminium pipe mast with the elements made of 3/8" aluminium tube.

A view of the ¼ λ Ground Plane for 2m
The elements are flattened at the ends to facilitate sealing/mounting/coax connection and the joints are made water-tight/corrosion-resistant with epoxy sealant.

This antenna has been up for more than 10 years and still going strong.

Related post: Moxon Antenna for 2m
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Coax Collinear for 70cm

This 70cm Coax Collinear Antenna was homebrewed one evening in the year 2008.

The job was made simple, thanks to N1HFX - OM Mike Martell and his detailed article 'Build A 9 dB, 70cm, Collinear Antenna From Coax'.

A view of the Coax Collinear for 70cm
All instructions were strictly followed.

A brass sleeve was soldered to the braid at the feedpoint and a SO-239 connector was used.

Both the ends of the antenna were waterproofed with epoxy.

SWR was close to 1.5:1 when checked at a power level of 50mW.

Signal strength with this antenna was 2-3 'S' points higher compared to that using a ¼ λ Ground Plane Antenna at the same height.

Related post: Direction-finding Yagi Beam for 70 cm
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J-Pole Collinear for 2m

A makeshift, phasing-coil variant of a 2m J-Pole Collinear was recently homebrewed.

J-Pole Collinear for 2m - details
It was fabricated with a single length of 20SWG copper wire and taped to a bamboo pole for rigidity.

The coil section was 64 turns close-wound on a 10mm mandrel and then stretched to a length of 300mm.

SWR measured close to 1.5:1 across the 2m band.

Antenna performance exceeded that of a normal J-Pole at the same height.

A 16SWG rugged version, encased in a PVC pipe, is in the works.

Related post: PVC-encased Vertical Antennas for 2m
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Noise cancellation on 2m using 2 antennas

A while ago, static-type QRM was encountered right across the 2m band.

It proved to be a nuisance while receiving weak signals using either of the shack antennas - a 1/4λ Ground Plane at a height of 15m and a Slim Jim at a height of 10m, located 5m apart.

The noise was presumed to be of commercial origin as it was not present at nights and on weekends.

Then, on an impulse, both the antennas were connected to the rig through a BNC-T connector. Surprisingly, the QRM disappeared and the audio level went up considerably.


The offending noise had been phased out just like that!

The antenna feeder lengths were then changed one at a time. The QRM promptly returned, confirming that it was indeed phase cancellation at work.

SWR was 3:1 with both the antennas connected. Inspite of that, it was now possible to comfortably work through a distant repeater.
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PVC-encased Vertical Antennas for 2m

These PVC-encased 2m vertical antennas were built years ago.

PVC-encased TV Ribbon J-Pole & Slim Jim Antennas
One is a J-Pole, housed in a thick-walled  ¾” PVC pipe, with a SO-239 connector. The other is a Slim Jim, in a thin-walled PVC pipe, with a BNC.

Both were homebrewed, using 300 Ω TV Ribbon, with good results (SWR < 1.5:1). The ends were sealed against water ingress using 'M-Seal' (epoxy sealant in dual component lump form).  
     

TV Ribbon
Thanks are due to W0IPL - OM Pat & N4UWJ - OM Don, for construction details of the J-Pole Slim Jim respectively.

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Audio Signal Injector


This decades-old audio signal injector was wired on a piece of perfboard.  All components were sourced from the junk box.
Audio Signal Injector
The enclosure was fashioned from an empty tapered spool of industrial sewing thread, a cosmetic tube cap and a film canister cap on which the push button switch was fixed.

A machine screw, filed to a point and fixed to the cosmetic tube cap, served as the prod.

Audio Signal Injector - Schematic
Over the years, this unit has helped service the audio stages of several ham rigs.
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Quartz Crystal Tester

This unit was built years ago when there was a need to test surplus crystals. It also came in handy as a signal generator for receiver alignment.

It's basically a Pierce Oscillator Circuit which works even with low-activity crystals.

Quartz Crystal Tester
'Octal' and '807' valve bases were wired for vintage crystals in addition to crystal holders suitable for HC-6/U, HC-18/U & HC-25/U crystals.

Quartz Crystal Tester - Schematic
The indicator is a 6V 0.06A dial lamp. Tuning / Loading is done using the PVC-2J 500+500 pf variable. The 2000 pf is switched in parallel for very low frequencies.

Quartz Crystal Tester - Inside view
This tester works in the range 455kHz to 20Mhz, with overtone crystals oscillating at their fundamental frequency.
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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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