Homebrew your 4:1 Coaxial Balun

When the antenna has an input impedance of 300 ohms balanced line, the 4:1 coaxial balun easily transforms this to match with our 50 ohms unbalance transmission line. One example to this is the folded dipole whose input impedance is 300 ohms  and is balanced at its feed point.

To start with the construction, first we need to know the operating frequency of the balun. Assuming at 145MHz, so we may have the formula below.

Solve for the wavelength at 145MHz;

    wavelength (feet) = 984 / 145MHz
    wavelength (feet) = 6.78 ft.

We can express our unit of measurement by "inch" for ease in the calculation;
    
    wavelength (inches) = 81.43 inches

We need also to consider the velocity factor of the coaxial cable used which in this case for the RG-58 is 0.66 (dielectric is made of PE).

Solving for the actual physical wavelength;

    wavelength (inches) = 81.43 (0.66)
    wavelength (inches) = 53.75 inches

Solving for the required 1/2 wavelength;

    1/2 wavelength (inches) = 53.75 / 2
    1/2 wavelength (inches) = 26.87 inches

Given above formula, you can easily compute for the required length of the 1/2 wavelength at any operating frequency.   --- 73 de du1vss

The VHF J-pole antenna

The J-pole antenna earned its name as one of the most popular VHF antenna because of its simplicity and its considerable gain when compared to any dipole or ground plane. Aside from its low angle radiation, the J-pole can be DC grounded which provides excellent protection from lightning overload that pose potential damage to the receiver.

The J-pole antenna evolved in may ways but its basic parts remained the same. The bottom portion is the quarterwave matching section wherein balance feeding can be connected. Connected above the quarterwave matching section is its radiator. The radiator can be cut 1/2 wavelength but if you want greater gain it can be cut to 5/8 wavelength without degrading the SWR at the feed point.

In my project, the bottom portion which is the quarterwave matching section and the lower 5/8 wave radiator is made of 1/2" OD anodized aluminum for added rigidity to sustain strong wind loads from storm. The upper 5/8 wave radiator is made of 3/8" OD anodized aluminum. The two radiator is connected via 3/8 wave phasing stub and is made of # 10 AWG solid electrical wire localy available at hardwares. The transmission line is coupled by 1:1 balun at the feed point of the antenna wherein minimum SWR is found. Once fully constructed, the j-pole exibits a theoritical gain of approximately 6 dB in reference to a standard dipole. --- 73 de hevir

Regulated 15 Amps. Power Supply

This project is very simple to construct using cheap few components and easy circuit lay-out. Regulation of this project uses the fixed 3 terminal regulator 7815 as we know. You may noticed that 7815 will give an output of 15volts but because of the 3 amperes rectifier diode connected right after the output of the regulator, it drops the voltage down to 14.3volts furthermore the pass transistor used on this circuit are wired in series pass configuration, it adds another voltage drop down to 13.6volts which is pretty close to 13.8volts, anyway to beef-up the required current, two sets of 2N3771 NPN transistors capable of 30amps each connected in parallel and in series pass configuration. 

In the construction make sure that the heat sink is large enough to provide sufficient dissipation of the heat generated by the two power transistor.  Large diameter wire must be also used such #12 WAG stranded wire for the wiring of the circuit. If desired you can also add a small air blower to control the temperature of the power supply. ...73 de hevir

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3 Elements 2-Meter Yagi Antenna

This antenna project uses another approach in feeding the driven element. Instead of using gamma-feed dipole as its driven element, it uses a center feed dipole in combination of a 1:1 coaxial balun. This method of feeding a dipole eliminated the difficulty in tuning the driven element. The details of the coaxial balun was earlier discussed in my previous project. I build mine in less than 3 hours so this should be easy for most beginners. ---73 de hevir

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Simple Over-Voltage Protection For Regulated Power Supply

This little protection circuit can be very useful in protecting your transceiver unit. I was able to design this circuit out from my horrible experience wherein my 30 Amps Zebra power supply unit suddenly went wild exploding my 90 watts Daiwa RF amplifier. Its too late but never the less, this project will help you a lot. The original circuit uses an SCR in conjunction with a fuse but later modified it using a much simplier approach using a 12V relay. Although the author is aware by the delay of closure brought by the relay contacts but actual test showed no problem with the radio transceiver during a simulated over voltage test at the power supply.---73 de hevir

VOX Type PC to Radio Interface for Internet Link

The usual two meter radio communication is very limited when talking about coverage distance. This is due to the fact that at VHF frequencies and beyond, radio propagation is likely on line of sight. The higher your antenna is, the greater the distance it will cover. Nowadays technology has gone far; with the aid of internet communication thru VOIP, voice communication brings the world closer together. To cope with the advancement of technology, amateurs able to communicate worldwide thru their radios but not with the use of a huge and bulky HF transceivers, instead they were able to use the internet technology to hook up their radios to the internet. Available commercial interface unit cost so much that putting up a server is difficult.

 

This simple PC to radio interface project functions as a voice activated switch that will enable the audio from the radio to be retransmitted to the internet and vice versa with the use of a PC. The connecting wires must be a shielded type to avoid stray rf pick-up that might affect interface functionality. Internal power supply unit is needed especially when a portable transceiver is used while this can be eliminated if a mobile base transceiver is available since the 8 volts supply can be taken directly from the microphone input connector. The software for this project is the SKYPE because it can be downloaded for free on the internet. Implementations of this project can be a server, station identifier or to link several repeaters all over the world.  ---73 de hevir

Attachment: vox_type_server_interface.jpg

Switching Power Supply Modification

Most industrial equipments today uses switching type power supply in replace for the conventional linear power supply because of its efficiency and savings on electricity. Aside from its efficiency, switching power supply are lighter in terms of its current per pound ratio, its no wonder why all computer’s CPU are powered by a single switching power supply capable of delivering high current at the rated voltages.
In this project, it uses a switching power supply salvaged from old CPU. Before finding a second hand switching power supply, ensure that the 12 volts rating has a total current above 15 amps so that it will handle most mobile transceiver with a power output range of 50 watts.
Modification starts by removing the unwanted wires from the switching power supply. Unsolder all the colored wires except for the yellow (+12V), red (+5V), green (power enable), and the black (ground). You can attach binding posts at the external casing for the 12 volts DC output (yellow and black). Another binding post for the 5 volts DC output (red and black). A single SPST switch can be included for the POWER_ON (green and black). Overall modifications can be finished within an hour so there should be problem in building this project. ---73 de hevir