Construction and Testing of the 40M Band VPA
A small tutorial on how the 40 M band VPA was constructed follows:
A used Hammond die cast chassis, a piece of single-sided copper clad board, an SO-239 RF connector and an old switch were obtained. A layout was planned on paper. Then 3 chassis holes were drilled and reamed to accommodate the switch, connector and an LED holder, which was temporarily lost when these photos were taken.
The inner chassis width and length was measured to ensure the copper clad board would fit without any rubbing or warp. I draw on copper boards using a felt "Sharpie" marker and a ruler or small T-square. After a few cut and tries, it fit within the Hammond chassis.
I cut my copper boards with a pair of straight aviation sheet metal sheers. As seen in the picture below, a rectangle section was removed for the SO-239 connector. The 90 degree cuts were made with the aviation sheers, while the sections at right angles between these cuts were removed with a hammer and chisel.
Placing the copper board on the floor and giving 2 or 3 hits with the chisel gave a clean looking cutout. Finally, 4 holes were drilled into the copper board and chassis to allow the copper board to be secured with 4-40 hardware. See below.
Boring holes in chassis to mount hardware, pots, switches and other stuff is much easier with a ream tool. To the right is a picture of my 2 reaming tools.
The top unit goes into an electric drill and is very quick and efficient. The hand version also works well, but is a little slower to use. Both require a pilot hole be drilled prior to reaming. Another quintessential tool is the hand pick, which along with my needle nose pliers and soldering iron, is in constant use.
Below. After placing the switch and SO-239 on the chassis, plans were made to add an antenna holder and a battery compartment made from copper clad board.
The battery compartment was made from a single, small one-sided piece of copper which will be close to the switch.
The antenna mount consisted of a small piece of double-sided copper clad board. When soldered on both sides, two-sided board will be able to stand the "windage" generated by the 6 foot whip antenna.
Below. Looking from the opposite angle, the antenna mount can be seen in detail. It has the top and bottom copper halves separated by a groove cut by a hobbyist tool on both sides. This prevents the antenna from being shorted to ground.
The antenna is factory tapped. A 4-40 bolt is used to anchor it to the mounting board. A 4-40 nut will also be added for extra support when the project is ready for final assembly.
Where to solder the antenna mount board is learned experimentally. The antenna must pass through the chassis lid. Therefore, before soldering the antenna mount board, the lid is drilled at the place where you want the antenna to be.
The hole must be lined with a rubber grommet to prevent the antenna from touching/shorting the chassis lid. Ensure the hole and grommet will fit your antenna snugly to prevent antenna sway. Once drilled, the lid is fitted with the grommet and passed over the collapsed antenna, which is placed inside the chassis in its approximate permanent position.
Once the antenna is sitting relatively level, the lid is carefully raised with one hand while the other hand holds the whip trying not to move the antenna. Once the lid is high enough, the lid and antenna are held by one hand while the other reaches into the chassis to hold the antenna mount board while the lid is completely removed over the antenna.
A mark may be made on the copper main board to indicate the correct position for the antenna board. If you did not move the antenna and feel satisfied it is in good position, solder a very small bead to one point between the main and antenna mount boards. Use just enough to support the antenna free hand.
Next take the lid, putting the grommeted hole over the antenna, and see if the lid fits properly with no undue pressure on the antenna. If it moved, and the antenna is not in proper position, note how far to the left or right or up and down it has to go. Remelt the solder bead and move the board accordingly. Ensure the whip is standing straight up and down.
When you are 100% satisfied with the antenna board location, solder it on both sides for maximum strength. Copper clad board soldering goes well if you use a 100 watt watt gun (Weller or equivalent).
Below. A ruler and marker was used to draw 3 island pads for each of the tank circuits and the B+ battery connection. The lines are cutout using a motorized hobby tool.
Manhattan construction would also work. For the Manhattan style, copper side up strips are glued to the main board to provide the 3 pads. Test all of your pads with an ohm meter or continuity tester to ensure there are no shorts to ground.
Note that the antenna was removed after the antenna mount board was soldered in place to keep the board from tipping over.
The parts are soldered onto the copper board in a good "ugly" fashion. A -6dB 50 ohm pad was permanently soldered onto the project for testing purposes. The VPA is easy to build. The die cast chassis costs much more than the project parts.
Once all the parts are soldered, it is time for testing your DC voltages. If you have an RF oscillator and scope you can also measure AC voltage and peak your tank circuits.
In some of the VPA schematics, voltage readings labeled VDD, VD and VS were listed in addition to drain current. It is essential to measure the voltages and calculate the drain current value to ensure that the FET is working properly and/or there are no mistakes.
Evaluating each FET is easy. Set your voltmeter on the 20 volt scale. While grounding your black lead to the main copper surface, first measure the VDD (battery voltage) on Q1. If you are using a 9 volt battery, it should be some value around 9 volts.
Next measure the drain voltage (VD), which is the voltage drop across the 100 ohm resistor. The value should be somewhere between 0.2 to 0.3 volts less than the VDD value.
From this, calculate the drain current using ohm's law. ID = VDD - VD divided by the 100 ohm resistor. For example, if VDD is 9.0 volts and VD is 8.75 volts, the drain current is 2.5 milliamperes.
The VS value is measured to confirm the FET is working properly. It should range from about 0.5 to 0.6 volts or so. Next perform the same measurements/calculation for Q2. If you do not measure values close to these, something is wrong.
On one VPA experiment, the MPF102 VDD was 9.3 volts and the VD was 7 volts. This FET was drawing 23 mA! The VS was 2.6 volts. The part was thrown away and replaced with a functioning version. Watch out for blurry-eyed late night mistakes!
If you have a scope and an RF generator, the tanks can be peaked for maximum output voltage by adjusting each trimmer capacitor.
Above. A crystal oscillator was attached to the Q1 gate by a small capacitor. The scope probe is connected to a 50 ohm -6dB pad and the trimmer caps were turned to give the largest AC voltage reading on the scope display. The tanks tune very sharply. Also above, is a photograph of the sinusoidal output in the scope.
Above. More photos of the project after the switch, battery clip, and SO-239 were connected up.
Above. The near final product. All that remains is installation of the LED, the chassis screws and the rubber feet. The grommeted antenna hole helps supports the six foot whip.
