The Result of the Regular Examination Held at NTC Batangas

The exam was held last October 3, 2009 at NTC Batangas. I took the exam for my upgrade (Class B license). Thanks to 4F1DY(Jyrus) of DX1CW who shared me the reviewer that made possible for me to pass the exam. ---73 de hevir

Microphone Preamplifier

Another weekend project is this microphone preamplifier which came from my new book Hints and Kinks for the Amateur Radio. The amplifier is a single transistor powered by the 8 volts from the radio transceiver. I can't find any vintage desk microphone casing so I ended up with this pink jewelry box which I obtained from a novelty store, anyway the output audio was very nice and a little treblish. All the needed parts are available at my favorite electronic store at very minimal cost. ---73 de hevir

My Home made Morse Code Keyer and Oscillator

Learning morse code is fun and exciting. At first, I don't see the importance of it thinking that this is just an obsolete form of communication. With the encouragement and help of my friends on the air, I was hooked with it and began to search different circuits about the morse code oscillator. I started with the 555 timer IC taken from my ARRL 1989 edition handbook but the audio quality of the oscillator was very irritating to my ears. This was due to the inherent square wave output of the 555 IC. Several search in the internet brought me the idea of constructing an audio oscillator that will generates a pure sine wave at 800Hz. In my project the circuit was the popular Bridge-T oscillator and then feeds to an audio amplifier to increase the output audio. The resulting output tone was nice and pleasant to hear as what the other station gives compliment to my oscillator.

The PCB board and the speaker were enclosed on an empty CPU power supply casing. On the side of the casing, my homemade straight key is mounted using a contact cement. The straight key was constructed using a hard disk motor. Kindly check the attachment for the oscillator circuit.  ---73 de hevir

My 24Amps 13.8V Modified Regulated Power Supply

I just brought my new Steady-Volt 24Amps, 13.8Volts power supply
that cost several thousand bucks at RadioCom somewhere there in
Pasay. I was excited and at the same time curious on what is inside
of the unit. When I came home, I did not bother to check
the output voltage if it was correct but I did open the
unit to check the inside. To my surprise, there was not even one IC
present in the regulator board of the supply. Further checks revealed
that regulation was accomplished using an ordinary zener diode,
resistors,capacitors and few transistors. To my contentment, I tried
to trace their original circuit to find out how it works and the
results are:
1. Zener diode was used to provide regulation. It is known that
zener diode is not capable of sustaining good regulation
specially on higher current.
2. The power transistors were wired in series
pass configuration and is installed after the regulator. This design
flaw is the main reason why the supply could not maintain
regulation when the load draws high current.
3. The OVP circuit (over voltage protect) is continuously on (relay)
while the power supply unit is idle. With this condition, the relay
coil might easily burned and will not sustain long hours of continuous
operation.
Several research on the internet leads me to the data sheet of LM317
from National Semiconductor which have the example applications of
their product. One example of LM317 circuit looks promising and can be easily
be replaced with a 7812 regulator to eliminate the use of biasing resistors.
The regulator circuit employs a PNP transistor that drive the bases of the
current hungry pass transistors. The 7812 regulator IC was wired as
wrap-around thus,it can able to maintain good regulation even if the load
draws more current. Prototype followed after I have secured all the
needed parts on the circuit. Final testing of the modified unit showed a
very stable output voltage at a varying load currents. My modified power
supply was able to power my IC-2200H at 65 watts and to the IC-720 HF
transceiver 100 watts on SSB.    ---73 de hevir

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