Homebrew ¼ λ Magmount for 2m

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.

2m ¼ λ Magmount details
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.

Related post: Simple ¼ λ Ground Plane for 2m
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HT-powered CW Interface

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.

HT-powered CW Interface - 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.

HT-powered CW Interface Board
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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Direction-finding Yagi Beam for 70cm

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.

70cm Direction-finding Yagi Antenna
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.

70cm Direction-finding Yagi - detail of driven-element
A length of 18SWG copper wire was used for the conversion.

Related post: Wire Slim Jim for 70cm
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'Wire Slim Jim' for 70cm

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.

70cm Wire Slim Jim Antenna
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.

Related post: Open Stub J-Pole for 70cm
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A ¾ λ Ground Plane for 70cm

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 ¾ 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.

Related post: Another ¼ λ Ground Plane for 70 cm
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A homebrew 2m FM rig

This rig was homebrewed in the year 1987.

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

http://www.americanradiohistory.com/Archive-Practical/Wireless/70s/PW-1978-07.pdf
http://www.americanradiohistory.com/Archive-Practical/Wireless/70s/PW-1978-08.pdf
http://www.americanradiohistory.com/Archive-Practical/Wireless/70s/PW-1978-09.pdf

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 are 8 MHz crystal oscillator, reactance modulator and multipliers for 24, 72 and 144 MHz. All stages, from 8MHz crystal oscillator to 144MHz multiplier, use BC109B transistors. Three 2N3866 stages amplify the output to 0.5W. A vintage antenna relay does the T/R switching.

Homebrew 2m FM rig - inside view
The receiver section is a converter, wired on perfboard in 1983.

It is 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).

These articles are also covered in '50 (FET) Field Effect Transistor Projects' by the same author.

http://www.talkingelectronics.com/projects/TheTransistorAmplifier/50FETProjects.pdf

3N200 insulated-gate, dual-gate mosfets are used for the preamplifier, amplifier and mixer stages. A couple of 2N918 transistors for local oscillator and first doubler are 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.

Crystals ground for the 2m FM rig
The 12 MHz output of the converter was fed to my RCA BC-312-D 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' BC-312-D as reference.

The pads for FT-6U crystals and trimmers became redundant on account of non availability of the crystals. An 807 tube 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.

Related post: Simple ¼ λ Ground Plane for 2m
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Tests for power on a 2m antenna

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.

The ability of a neon lamp to glow in the presence of an electromagnetic field made it a handy RF indicator of yesteryear. Many a handie talkie (HT) was tested by just holding a NE-2 near its whip antenna.

Curiosity led to a search of the junk box for a NE-2 but a different type, having plate electrodes, was found (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.
Neon Lamp Test on J-Pole Antenna
At high voltage points the neon continued to glow even when moved away from the antenna.

Neon Lamp Test on Rubber Ducky
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.

Dipole-lamp pick-up - folded
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.

Dipole-lamp test on J-Pole antenna 
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