FM Broadcast 55W Push-Pull RF Amplifier

This prototype rf amplifier uses two 2SC2640 VHF power transistor wired in push-pull configuration. The 50 ohm impedance of the input is transformed to lower impedance by the 4:1 broadband coaxial cable transformer  providing a close match to the base of the two transistor.  To operate in class C, the base of the transistor must be at ground potential so the secondary winding of the 4:1 coaxial transformer is grounded at the center position( null).  The impedance of both collectors are transformed by the combination of  inductors L1 , L2 and the 120pF capacitor  and further transformed by the 1:1 coaxial balun.

Both the 4:1 coaxial transformer and the 1:1 balun are made from RG316 teflon coaxial cable and these are cut to a certain length in order to operate at the highest cut-off point which in this case at 110MHz. Actual test of the prototype amplifier, I was able to measure 55 watts output from a 1.5 watts input.   ---73 de du1vss

Quarter Wave Stub Demo

One of the trick that you can perform to your friend (not a Ham) is this quarter wave stub. Looking at the DC (direct current) point of view, the bulb should not light since there is a shorting wire (stub) that will ground any voltage present across the 12V bulb. But strange things happen when a strong rf source is brought near the dipole antenna. The bulb should light brightly! This phenomenon can be easily explained when we understand how the quarter wave stub behaves when excited at its resonant frequency.

In my prototype, I use a 435MHz  3w transceiver to excite the dipole antenna and the 12V pilot bulb was used as an indicator for rf current present across the feed point of the antenna. The antenna itself is approximately 13 inches in total length while the stub length is 6.5 inches. Spacing of the stub was merely approximated to around 1/16 of an inch. Part of the trick is that you can also cut open the far end of the stub and show to your friend how the bulb immediately off at this time.   ---73 de du1vss

Telephone Audio Coupler

This circuit is a simple one, audio signal from the telephone line is sampled by the 600:600 ohm isolation transformer. The combination of 0.1uF/200V capacitor and 500 ohm resistor suppresses the high voltage present in the line during the ring signal.

Signals appearing at the secondary side of the transformer is clamped at 1V peak to peak by the two diodes connected back to back. Audio level at this point can be easily processed by an audio mixer before going to the transmitter. ---73 de du1vss.

4:1 Unbalance to Unbalance Coaxial Transformer

This is my first time to implement the 4:1 unbalance to unbalance coaxial transformer to match the input and output impedance of an rf power transistor. My prototype rf amplifier originally had a lumped LC network in both input and output sections and only have +/- 3MHz of bandwidth but after the addition of 4:1 coaxial transformer, the bandwidth has now increased to +/- 6MHz from the center tuning frequency of the amplifier.
The coaxial transformer is about 1/16th wavelength long from the highest frequency in which the amplifier will work and should have 25 ohm of characteristic impedance. In my prototype amp it uses a pair of RG-178 cable paralleled to arrive at the 25 ohm requirement.

This is the simplified diagram when looking at the transformer however, the characteristic impedance of the coaxial cable also plays an important role in the impedance ratio. For a 50 ohm to 12.5 ohm transformation, it calls for a 25 ohm cable but since I don't have this kind, I simply paralleled two RG178, a 50 ohm cable.

The schematic diagram of my prototype amp tells that aside from the 4:1 coaxial transformer, LC network was still employed to further matched the impedance of the base and collector of the power rf transistor. The only limiting factor that prevents the amplifier for a broadband operation is the LC network!

My prototype amp after the inclusion of the 4:1 coaxial transformer. I might replace the old PCB to facilitate the addition of the two coaxial transformer. ---73 de du1vss

Experiment With Different Wind Turbines

Two months ago I was tasked to build a wind generator that can be use to charge batteries and power LED lights here in our main office. The first thing that I was thinking is the vertical axis wind turbine (VAWT) suitable enough for our other projects that will provide back-up power in some of our remote  installation of met sensors. After a month long planning and fabrication, I came up with a Savonius turbine that drives our 350W motor hub from an electric bicycle. During the testing, I was a bit disappointed with the result, the wind generator starts to spin at 2.5m/s wind speed and only reaches at 15 to 25 rpm from a wind velocity of 5m/s, not enough speed to generate few volts of electricity!

This prompted me to think with other turbine designs that are more efficient and have less wind drag as compared to my existing Savonius turbine. Anyway, earlier this morning I was busy setting my small test fixture that will test 2 other types of vertical axis wind turbine. We have an old anemometer that we no longer use and found that I can use the optical sensor to measure the relative wind speed of the turbine under test.

The first is the small scale model of my Savonius turbine, I'm aware that this is the least efficient from the rest of turbines but i would like to see how well it does when seated on the test fixture.

Not bad at all, the Savonius turbine able to spin at 0.2m/s., rotation sometimes varies over time but is still provides me a consistent slow rotational speed.

The next is a Darrieus turbine installed in the fixture. It has a hard time getting to start initially even if the wind is moving at the blades at substantial speeds. It sways back and forth until it catches the right angle and start to spin rapidly. I was able to get an average of 0.5m/s wind speed as displayed in the anemometer unit but it is not self-starting, not a good candidate for my project.

The last model of turbine that I'm going to test is the Lenz turbine. From the appearance of the model, it looks like a combination of Savonius and Darrieus turbine.

The performance is very promising, it automatically start to spin rapidly and I was able to obtain an average of 0.8m/s of wind speed reading from the anemometer unit.

 Comparing all the three models that were evaluated, the Lenz turbine is very well suited in my project and just a matter of time, an actual turbine will be made and soon running in my wind generator project. 73 de du1vss

Callsign Plate

Beautiful callsign plate which I got from Philippine Amateur Radio Association (PARA) last year. The price is much cheaper than the one from the NTC (National Communications Commission) and that's why I bought two of these. One of the plate was hung at DU1VSS station and the other one is kept for future use. 73 de du1vss.

100W Dummy Load

A 100W quick and easy rf dummy load using a 100W 50ohm Caddock thick film power resistor. The large aluminum heatsink was bought at Rox Electronics. The dummy load was tested at 144MHz exhibiting an swr of 1:1.0 at 150W of rf power. ---73 de du1vss