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

LM35 Thermometer Project

LM35 is a precision IC temperature sensor with an output voltage linearly proportional to the Centigrade temperature. Thus for a 0 to 100 Degrees C, the sensor output will be 0 to 1V accordingly. The sensor works over a wide operating voltage from 4 to 30V with only less than 60 microAmperes current drain which makes it favorable on a battery operation.

My prototype is connected to a 9V battery and gives me a reasonable accuracy when compared to our calibrated ROTRONIC Thermometer using a type K thermocouple. A digital voltmeter is connected to the sensor to display the voltage output of the sensor. I also tried connecting the sensor into our Envitech Ultimate logger to record and monitor the room temperature in one of our AQMS  station.

Another possibility is to use the sensor to trigger some devices such as relays, blowers, fans, alarm, etc. I Had a great time building several automatic blower in some of my power supplies using this LM35 sensor, have fun with it.  ---73 de du1vss

Stripline SWR Meter

Another weekend project that I made is this SWR (Standing Wave Ratio) meter which rf pick-up is a directional coupler  fabricated from microstrip. FR4 double sided pcb was chosen  with thickness of about 1/16 of an inch on a 2mil copper thickness. Processing the microstrip starts from applying a masking tape on both sides of the PCB and manually drawn the three main important traces on the top side of the PCB. The masking tape on the bottom side was retained so that at the end after etching, it will serve as a ground plane of the microstrip. After etching the board, please verify that the dimension of L1, L2 and L3 are  closely followed so that our strip line impedance will be close at 50 ohm, the thickness, width and the material of the PCB has also a direct influence on  the impedance.

On my prototype I intentionally separate the directional coupler from the meter unit so that when time comes, I can easily swap between other types of directional coupler design. During the initial test, I found that the prototype respond well to VHF while it requires greater amount of rf energy to produce the same full scale deflection of the 100uA meter at HF. Germanium diode used in this project is not linear specially at different power levels therefore this project is best suited for QRP application only.

The face of the meter can be modified so that a calibrated lines can be drawn using a reference dummy load each with different impedance values.  73 de du1vss.

Two Stack Dipole Using Power Divider

My friend ask me If I can make a two stack dipole for his 92.9MHz FM station and I decided to construct this project using a home made coaxial power divider using a two piece quarter-wave long 75 ohm coaxial cable. The two 75 ohm coax was connected in parallel to obtain 37.5 ohm which is the required impedance for our power divider. I also use a 1 wavelength long 50 ohm coax feeding the two dipoles and along with it, a short piece of wire that act as a pawsey stub that makes the dipole feed point balance. The 1 wavelength long 50 ohm coax was also chosen so that each dipole can be vertically spaced at 1 wavelength for gain optimization.

A plastic element insulator hold the dipole segments on a 1"x1" square tube boom. The feed point connection was carefully protected with a layer of rubber tape and on top of it is an electrical tape for UV and water protection.

A  fabricated  bracket  will hold the antenna boom and this was made of a thick plastic material, the one used here was a chopping board 5mm in thick.

A close up view of the coaxial power divider during initial tuning and testing.

A quick frequency sweep was made to evaluate the antenna SWR after the trimming of dipole segments. The antenna bandwidth was quite narrow for this design and I was able to obtain an SWR value of 1:1.2 across 92MHz to 93MHz only.

During testing, the two dipoles were spaced 1 wavelength horizontally and was 1 meter above the ground  just only to evaluate the SWR and tune the antenna while at the ground. When installed permanently, this antenna is expected to yield  3dBd of gain theoretically.    73 de du1vss

Switch Mode Power Supply Unstable Output Cure

I got this switch mode power supply from a friend and my plan was to use this on my 80W linear amp. Things went the other way when I tested this supply with my 80W amp. The output voltage won't stay at 12V and every time i keyed my transceiver, the output voltage swings to 15V and sometimes the worst hitting a peak at 17V which by any moment will surely blow the PA transistor of my amp. It took me a quite a while to figure out what was causing the issue but then it was the stray RF reaching the PWM switching regulator IC was the main culprit. My verification includes bringing a 6W portable hand held transceiver close to the switch mode power supply and pressing the PTT while monitoring the output voltage of the power supply. Every time i pressed the PTT the output voltage jumps to 17V  and the proximity of the rubber ducky antenna is directly proportional to the increased in output voltage of the power supply.

I got the courage to disassemble the switch mode power supply to access the circuit board and found the only chip mounted on the board. It was a AZ7500E pulse width modulator regulator chip that runs this power supply and found some important pins that are responsible for its regulation. I tried to bypass to ground pin1 and pin16 (both were (+) inputs of its built-in error amplifier) using a 0.01uF ceramic capacitor. After the modification I quickly reassembled the power supply and test again for stability and surprisingly works now normally. --- 73 de du1vss

Home Brew 60W RF Dummy Load

Another great way of making your own 60 watts dummy load is by the use of Caddock thick film power resistor.  Purchased some of these at http://philippines.rs-online.com/web/ which cost  at 270 Pesos each. Two 100 ohm / 30W resistors were paralleled to make a 50 ohm / 60W dummy load. The good thing about this product is that it comes in TO-220 package and mounting this on a suitable aluminum heat sink is a big help in heat dissipation during actual use. Been able to test my prototype on VHF and provides a very good matched at 144MHz exhibiting an SWR value of 1.1:1 across the amateur band due to the fact that reactive components can be easily minimized because of the unique package of this resistor.

Thermal compound was added between the aluminum surface in contact with the power resistors. A pig tail coax to PL259 connection was also included but a slight increase in SWR was noted. 73 de du1vss

Jpole Antenna with a Twist

Here is my own version of my Jpole antenna used in my ham station. Only the half wave radiator was exposed while the remaining quarter wave stub was kept inside on pvc pipe. The pvc pipe prevents rain water from entering the feed point that might affect the vswr of the antenna when raining. It also provides the required structure for our Jpole since this type of antenna works best if kept insulated from the tower.

The half wave radiator is made of 3/8" anodized aluminum tubing while the pvc pipe diameter is 1 &1/4". Marine epoxy was used to secure the half wave radiator into the center of the pvc coupling.

Several patches of (4" x 1") cartoon or thick cardboard used as spacer in the stub. Keep in mind that the 1 inch spacing of the stub is critical since the stub is actually a ladder line but only shorted at the bottom. Finding the exact position of the feed point is done by trial and error and of course with the aid of your swr meter. After the actual tuning, I found my feed point to be at 0.05 wavelength from the bottom of the stub.The inclusion of the pawsey stub is only my personal preference to minimize feed line radiation but other methods can be used here.
Once completed, secure your Jpole antenna using a pair of scaffolding clamp on the top of your tower.   --- 73 de du1vss