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| Twin-Radial ¼ λ 70 cm Ground Plane |
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.
Related post: A ¼ λ Ground Plane for 70 cm
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Twin-Radial ¼ λ Ground Plane for 70 cm
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.
Related post: A ¼ λ Ground Plane for 70 cm
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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.
Related post: A ¼ λ Ground Plane for 70 cm
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Open-stub J-Pole for 70cm
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.
Related post: End-fed Sleeve Antenna for 70cm
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| 70cm open-stub J-Pole Antenna |
The dimensions are: Driven Element - 495mm, Stub - 165mm and Spacing - 16.5mm.
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| 70cm open-stub J-Pole Antenna - Schematic |
SWR at 435MHz is less than 1.5:1.
Related post: End-fed Sleeve Antenna for 70cm
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½ λ End-fed Sleeve Antenna for 70cm
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 '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 PVC door buffers 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.
Related post: A ¾ λ Ground Plane for 70cm
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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 '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 PVC door buffers 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.
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| 70 cm End-fed Sleeve Antenna |
SWR at 435MHz is around 1.5:1.
Related post: A ¾ λ Ground Plane for 70cm
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Sleeve Dipole Antenna for 2m
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.
Related post: Wire Slim Jim for 2m
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| 2m Sleeve Dipole Antenna |
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| 2m Sleeve Dipole Antenna details |
A good weekend project, awaiting tests for effectiveness.
Related post: Wire Slim Jim for 2m
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Moxon Antenna for 2m
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.
Related post: Simple 2-element Array for 2m
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| 2m Moxon Antenna |
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.
Related post: Simple 2-element Array for 2m
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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').
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.
Related post: J-Pole Collinear for 2m
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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').
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| 2m 2-element array |
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| 2m 2-element array - close-up (element folded) |
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.
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.
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.
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.
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!
Related post: Salvaged RF Ammeter
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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.
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| Homebrew RF Ammeter |
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| 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.
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| Homebrew 1:1CT RF Ammeter - Inside view |
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.
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| 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.
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| Homebrew 1:20 CT RF Ammeter - Inside view |
Related post: Salvaged RF Ammeter
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