Amateur Radio

A salvaged RF Ammeter

2012-05-13T21:44:00.000+05:30

Years ago, I had salvaged a 0.75A 'thermocouple heating element' from a meterless BC-442 Antenna Relay Unit. Only recently did I take it out of cold storage, to build an RF Ammeter for use with low power rigs.

A junk-box 1mA FSD 50Ω moving coil meter, with a bakelite housing, could also be put to good use. It had graduations only to 50% of full scale.

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

On completion of assembly and wiring, the unit had to be tested.

My homebrew CW rig's antenna current was set to 0.75A, using a Weston 1.5A RF Ammeter for measurement. The Weston was then replaced by the unit to be tested.

As luck would have it, the meter pointer stood at exactly 50% of full scale. Inspite of the non-linear output of the thermocouple and the uncalibrated meter scale, at least staying within the graduated portion of the scale would avoid damage to the element!
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Yet another ¼ λ Ground Plane for 70 cm

2011-09-14T11:35:00.028+05:30

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.

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.
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A Twin-Radial ¼ λ Ground Plane for 70 cm

2011-09-07T16:34:00.033+05:30

The driven element of this antenna is a telescopic whip. It is mounted in a tubular aluminium pill box using polythene end-caps as insulators. A BNC flange-type socket is mounted on the bottom end-cap, with its centre pin soldered to the whip.

Two aluminium-strip radials are bolted on to an aluminium base plate, beneath which the driven element assembly is attached. The whip clears the base plate through a central hole.

Mounting screws ensure that the base plate and the BNC flange make contact with the pill box.

SWR at 435MHz is better than 1.5:1.
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An open-stub J-Pole for 70cm

2011-09-04T21:51:00.029+05:30

Telescopic whips and an old terminal block insulator make this open-stub J-pole for 70cm.

The stub portion of the driven element is of 3/16" copper tubing (if only to make up for the shortage in length!).

The dimensions are: Driven Element - 495mm, Stub - 165mm and Spacing - 16.5mm.

The elements are wired to a BNC socket mounted on the block.

SWR at 435MHz is less than 1.5:1.
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A ½ λ End-fed Sleeve Antenna for 70cm

2011-08-30T08:34:00.076+05:30

One of the well-known end-fed ½ λ vertical antennas is the J-Pole. It's ¼ λ stub may be closed and fed at the point of lowest SWR or be kept open and fed at the ends (ARRL Handbook). A closed stub is first choice as it ensures DC ground and static-free operation. The sleeve variant of this is the Sperrtopf Antenna.

Notwithstanding the above, ease of construction prompted homebrewing of an end-fed sleeve antenna with an open-stub.

OE7OPJ OM Peter's plans for a 'Bierdosenantenne' or 'Beer Can Antenna' fitted the bill.

The 19¼" long driven element is cut from a length of 3/16" scrap copper tubing. A 2" diameter aluminium can, 6½" long, is used for the sleeve. Two 1" diameter plastic bushes with 3/16" bores are used to hold the driven element centrally insulated from the can. One is press-fitted into the mouth and the other fastened with screws to the base of the can. A BNC socket is similarly fixed after it's centre pin is soldered to the driven element pushed out through a hole in the base of the can.

A tapered bakelite tube and a plastic end-cap complete the picture.

SWR at 435MHz is around 1.5:1.
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A Sleeve Dipole Antenna for 2m

2011-08-21T17:55:00.084+05:30

The elements for this Sleeve Dipole Antenna are cut from copper tube scrap (3/16" diameter for the driven element and 5/8" for the sleeve).

The driven element is pushed into a 2" long rubber hose and fitted 1" deep into the sleeve. A tapered bakelite tube gives rigidity to the joint. For an overall length of ½ λ (39" at 145MHz), the sleeve is 19½" long and the driven element 20½".

The ends are closed with plastic caps. A BNC socket is mounted on the sleeve end-cap and wired with RG-59/U coaxial cable. The centre conductor is soldered to the driven element and the braid to the sleeve. Care is taken to ensure that the BNC socket does not touch the sleeve.

A good weekend project, awaiting tests for effectiveness.
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A Moxon Antenna for 2m

2011-08-13T12:58:00.075+05:30

3/8 " aluminium tubular elements, left over from a cannibalised VHF TV Yagi beam, were used to fabricate this 2m Moxon Antenna.

Screw joints became a necessity since the elements were not long enough. The ends were flattened and drilled for the purpose.

A scrap plastic handle came in handy for the spacers. It was easy to cut it, drill the fixing holes and make a recess for the BNC socket.

Dimensions were obtained using the 'MoxGen - MOXON Rectangle Generator'.

For vertical polarisation, this antenna is directly mounted on a PVC pipe mast with a screw through each spacer.

Preliminary checks showed that this antenna is as good as, if not better than, my 'simple 2 element array'. Both, being 50Ω antennas, are capable of outperforming even a 3 element array with its associated matching problems.
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A simple 2-element array for 2m

2011-08-07T12:24:00.082+05:30

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').

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.

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.
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A homebrew RF Ammeter

2011-07-04T15:40:00.058+05:30

I recently came across WB8EVI - OM Mike Herman's article, 'DIY RF Ammeter', and was eager to homebrew my own.

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.

Trials were made using a 50Ω, 1mA FSD moving coil meter to display 1.5 RF Amps 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 width 12.5mm. Both the primary and the secondary were just wires passed through the toroid without winding.

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.

The RF Ammeter was calibrated using my homebrew CW rig and a Weston 1.5A RF Ammeter, after which the variable resistor was replaced by 3 series-wired 10KΩ resistors.

Measurements with this RF Ammeter proved acceptable at 7 MHz, 14 MHz and also at 145 MHz!

However, the 1:1 transformer would cause the secondary load to be directly reflected as a series load in the feeder. Also, the higher secondary current would 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.

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.

Tests showed very good linearity at 7 and 14 MHz but drastic loss of sensitivity at 145 MHz!
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A homebrew ¼ λ Magmount for 2m

2011-05-06T14:08:00.007+05:30

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.

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.
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A HT-powered CW Interface

2011-04-19T22:43:00.002+05:30

A keyed piezo beeper, connected to the microphone jack of a HT, appeared feasible as a ready-made MCW generator.

It failed on 3 counts (high pitch, low volume and interrupted carrier) resulting in a very poor-quality signal.

Hence a bit of design effort was called for. The result is the following schematic.

It's a keyed audio oscillator, with a low part count, working off 4.5V - 1.5mA available at the microphone jack.

Oscillation is obtained using an AC128 (Germanium PNP AF transistor) and an audio transformer with a turns ratio of 1:5.

The 220K and 4.7K trimpots enable adjustment for the desired tone without motorboating.

In the event of the circuit failing to oscillate, connections to one of the transformer windings is reversed.

Connection to the microphone jack is through a shielded cable.

The prototype was wired on a piece of perforated board with a microswitch serving as the PTT.

The unit was tested/adjusted while monitoring the signal with another rig. 100% modulation, with a clean note, was obtained.

Use of a homebrew electronic keyer precluded the need for a sidetone monitor.

It was an interesting weekend project using parts from the junk box.
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A Direction-finding Yagi Beam for 70cm

2011-04-13T13:59:00.000+05:30

The PVC pipe/brazing rod construction, used in homebrewing this antenna, is the brainchild of OM Nathan Loucks WB0CMT (A Portable 3-Element 2m Beam - April 1993 QST).

All it requires is a length of 25mm PVC pipe, 2 end caps, 1.6mm brazing rod, a BNC socket, a piece of RG-58U coax and M-Seal epoxy sealant.

Element lengths used:

Driven element: 330mm end-to-end, Director: 305mm, Reflector: 355mm

Element spacing:

Driven element to director: 130mm, Driven element to reflector: 75/85mm.

Conversion of the driven element, from a straight dipole to a half-folded one, was an afterthought to bring the SWR down to less than 1.5:1.

A length of 18SWG copper wire was used for the conversion.
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A 'Wire Slim Jim' for 70cm

2011-04-13T13:28:00.000+05:30

This Slim Jim for 70cm was homebrewed using a length of 18mm diameter thin-walled PVC tube, 20 SWG copper wire, a BNC socket and ‘M-Seal’(epoxy sealant in dual component lump form).

The wire ends and the flange-type BNC socket are secured with self-tapping screws.

Sealing against water ingress is done with M-Seal.

A low SWR (less than 1.5:1) was obtained with the feed point at 25mm from the lower end.

This antenna weighs only 90 grams.
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A ¾ λ Ground Plane for 70cm

2011-04-13T12:13:00.000+05:30

This antenna is my mast-mounted version of G2BCX OM F.C.Judd's design of a mobile
¾ λ collinear in his 'Two-metre Antenna Handbook'.

It is similar in construction to my ¼ λ Ground Plane for 70cm.

A SO-239 connector is used.

The driven element, consisting of the ¼ λ lower portion, the ¼ λ hairpin stub and the ½ λ upper portion, is bent from a single brazing rod.

The blue plastic strut gives rigidity to the driven element.

SWR measured lower than 1.5:1 at 435 MHz.
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A 5/8 λ mobile whip for 2m

2011-03-17T13:27:00.000+05:30

Here's the schematic for a a 2m 5/8 λ mobile whip, courtesy WB4MYL (73 Magazine August 1974).

The shunt feed ensures DC ground and prevents static build-up.
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My homebrew 2m FM rig

2011-03-13T13:00:00.000+05:30

This rig was homebrewed in 1987.

It was inspired by G8FWM - OM Brian L.Phillips' article, 'PW AVON - a 10W 2m FM Transmitter', in Practical Wireless (July - September 1978).

OM Brian's dedication to detail and his 1:1 PCB layouts made it easy to homebrew the coils and the 'island pad' style PCBs.

The transmitter stages were 8 MHz crystal oscillator, reactance modulator and multipliers for 24, 72 and 144 MHz. All stages, from 8MHz crystal oscillator to 144MHz multiplier, used BC109B transistors. Three 2N3866 amplifier stages gave an output of 0.5W. A vintage antenna relay came in handy for T/R switching.

The receiver section was a converter, wired on perfboard in 1983.

It was based on G3OGR - OM F.G. Rayer's two articles 'Converter for 144-146MHZ' and '144-146 MHz Amplifier', in his book 'Projects in Amateur Radio and Short Wave Listening' (1981).

3N200 insulated-gate, dual-gate mosfets were used for the preamplifier, amplifier and mixer stages. A couple of 2N918 transistors for local oscillator and first doubler were followed by a 2N2369 second doubler for 133MHz.

The quick and dirty 12VDC power supply was put together with junk box parts.

An aluminium baking tray served as the enclosure.

The 12 MHz output of the converter was fed to my RCA BC312D Receiver, used as a tunable IF and slope detector.

Vintage 8 MHz crystals were ground by hand to the desired frequencies using QSOs, received on the converter and 'well-warmed' BC312D, as reference.

The pads for FT-6U crystals and trimmers became redundant on account of non availability of the crystals. An 807 socket was found suitable to plug in all configurations of vintage crystals used.

With a ¼ λ Ground Plane Antenna, this rig served me well for both local and DX contacts till I could lay my hands on an ICOM IC-02A in the year 1990.
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Tests for power on a 2m antenna

2011-02-13T16:42:00.004+05:30

Caution! To avoid dangerous RF exposure, these tests are to be carried out using a maximum power of 5W only. A 2m HT is quite safe for the purpose.

Tests for power on a 2m antenna may be carried out with a NE2 neon lamp. However a different type, with plates as electrodes, was at hand(see inset).

Power was applied and the antenna probed with the neon lamp. A static-charged plastic bag, placed in between , helped trigger the neon lamp.The ensuing red glow was sustained by the RF energy. The plastic bag may not be required when dry weather and synthetic clothes make it more conducive for the test.

At high voltage points the neon continued to glow even when moved away from the antenna.

A more reliable method is to use a dipole as a pickup and a 6.3V-150mA dial lamp as the indicator.

My version of the pickup has two 21" telescopic whips, fixed on a wooden strip, with the dial lamp soldered in between. It's quite convenient to push the telescopics in and fold them for storage.

With only 3.5W from my HT, and the dipole length adjusted to ½λ,the lamp glowed to more than full brilliance at a good distance from the antenna.

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A 'Wire Slim Jim' for 2m

2011-02-13T09:41:00.000+05:30

This is a 'Wire Outside PVC' Slim Jim for 2m. It was made using enamelled copper wire and a vacuum cleaner nozzzle!

The wire ends and the BNC connector were fixed with self-tapping screws. The feed points were

connected to the BNC using the same wire. Water ingress points were sealed off with 'Araldite'.

An SWR of 1.5:1 was realized.
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2m J-Poles and a Slim Jim

2011-02-13T09:40:00.001+05:30

Last summer an attempt was made to homebrew a couple of 2m antennas, using parts salvaged from a disused VHF TV Yagi Beam.

The radiating element (folded dipole) provided the stub portion for two J-Poles. Bakelite pieces were used to ensure rigidity and also locate the BNC connector.

The moulded plastic box was used for the second version of the J-Pole. For the stub portion, the 2 elements were shorted inside the box. A BNC connector was located at the lower end of the box. A length of PVC pipe was fastened to the box to facilitate mounting of the antenna,

'M-Seal’(epoxy sealant in dual component lump form) was used at the joints and to waterproof the connections.

The dimensions were arrived at using K4ABT's 'J-Pole Calculator'. Absolutely no trials / adjustments were required to get the SWR very close to 1:1!

With the remaining material a Slim Jim came into being.

Construction was on similar lines with good results.
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UHF on a VHF Rig

2011-02-13T09:33:00.002+05:30

This circuit enables fair copy of strong 444 MHz signals, off a local repeater, using a 2m rig and an indoor 70cm ¼λ ground plane antenna.

The 300 MHz 5th harmonic of the local oscillator, mixing with the incoming signal, delivers output on 144 Mhz.
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A ¼ λ Ground Plane for 70 cm

2011-02-13T09:32:00.001+05:30

It is easy to homebrew this ¼ λ ground plane antenna for 70cm. All one needs is a TNC connector (see inset), 1.6mm brass brazing rod and a pack of ‘M-Seal’(epoxy sealant in dual component lump form).

Radiating element length in inches is 2808/f MHz minus the projecting length of the TNC hot end.

Radial length in inches is 2950/f MHz plus a little for soldering on to the TNC connector flange.

The soldered points are sealed from water ingress using M-Seal.
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Fox Hunt Attenuator

2011-02-13T09:13:00.001+05:30

Here's the circuit of my homebrew fox hunt attenuator. It was dictated, on-the-air, by my friend OM Guru VU2GUR.

It's a simple offset attenuator with a low part count.

The signal strength of the attenuated signal, available at +1MHz, +2MHz, -1MHz, -2MHz of the fox frequency, can be varied with the potentiometer.

It's to the credit of the original designer that it proved successful in a couple of fox hunts in Bangalore.
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Rubber Ducky Gimmick

2011-02-13T09:11:00.001+05:30

How does one couple a 2m HT (with fixed rubber ducky) to an external antenna? Can another rubber ducky be used for coupling? I checked this out, on my homebrew SWR meter, using a dial lamp as a load.

The glow of the dial lamp indicated an efficiency of about 20% (2.5W to 0.5W). I then connected my 2m external antenna instead of the dial lamp. On-the-air tests were successful. I had comfortable QSOs with stations I could never work with the rubber ducky.
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Handy 1 kW HF Dummy Load

2011-02-13T09:09:00.008+05:30

Years ago, after replacing the tubes of a Heathkit SB-200 Linear Amplifier, I was on the lookout for a dummy load to test it.

It then struck me that a 230V ~ 1kW electric hot plate, with its heating element resistance close to 50Ω, would do.

The mains plug of the hot plate was replaced with a PL-259 before connecting it to the output of the linear amplifier.

Mismatch, if any, was taken care of by the pye tank and the amplifier was successfully tested.

At a later date, the same exercise was repeated with a Dentron GLA-1000.
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SWR Meter - Easymatch

2011-02-13T09:07:00.023+05:30

The 'Easymatch' has a length of solid copper wire as its transmission line. The diode leads,

running parallel to the transmission line, double as sensing lines. There is no indication till
a load is connected.
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