Budget Dummy Load for HAM Radio

In this tutorial, we'll learn how to create a cost-effective 50-ohm dummy load suitable for radio frequency testing and calibration. This can be a great project for amateur radio enthusiasts who want a homemade solution that's both functional and economical.

The 50-ohm dummy load will be built using the resistors as the main load element. Start by securely connecting the two 100-ohm resistors in parallel, which will yield the desired 50-ohm resistance. Use the zip ties to hold the resistors in place and maintain a good electrical connection.

The SO-239 connector is mounted onto the lid of the Tostitos jar, which will require drilling an appropriate-sized hole. Connect the resistors to the SO-239 using the 20 gauge wire, ensuring that the connections are solid and well insulated.

Once the resistors are wired to the SO-239 connector, submerge them in the jar filled with your chosen oil, which acts as a coolant to allow for extended periods of operation without overheating the resistors. The oil type doesn't affect the electrical properties significantly, but it's important to ensure that it has good thermal conductivity and is non-conductive.

After sealing the jar, you'll have a simple yet effective 50-ohm dummy load that can handle up to 100 watts of power. This can be a handy piece of equipment for any ham radio operator's bench, ideal for testing, tuning, and calibration purposes.

Tools Needed:

1. Drill
2. Step Bit or 16 mm (5/8 in) drill bit
3. Soldering Iron
4. Multi-Meter
5. Micro-VNA (Optional)

Materials Needed:

1. Two non-inductive 100-ohm, 50-watt resistors: These will be connected in parallel to create a 50-ohm load.
2. Two small zip ties: These will be used to secure the resistors together.
3. One Tostitos jar or an equivalent glass jar: This will serve as the enclosure for the dummy load, providing a neat package and some dielectric properties.
4. One bulkhead SO-239 connector: This UHF female connector will serve as the point where the coaxial cable attaches to the dummy load.
5. Some 20 gauge solid wire: This will be used to make connections between the resistors and the SO-239 connector.
6. Baby oil, mineral oil, or transformer oil: Any of these will be used as a coolant to dissipate the heat produced by the resistors.

Materials:

• Metal lid from a Tostitos jar or equivalent
• Bulkhead SO-239 connector
• Power drill
• Center punch (optional, but recommended)
• Drill bit appropriate for the SO-239 connector (usually around 5/8 inch or 15.875 mm, but measure your connector to be sure)
• Hammer (if using a center punch)
• Safety glasses

Instructions:

1. Prepare the Lid: Begin by cleaning the lid thoroughly, ensuring it's free of any residue or debris.
2. Mark the Drill Point: Identify the center point of the lid, or the location where you want the connector to be mounted. Use a marker to make a visible dot at this point.
3. Center Punching: (Optional) Place the center punch on the mark and gently tap it with a hammer. This creates an indentation that will help to guide the drill bit and prevent it from wandering.
4. Drill Pilot Hole: If you have a smaller drill bit, drill a pilot hole at the indentation. A pilot hole will make it easier to drill the larger hole accurately.
5. Safety First: Put on your safety glasses. Metal shavings can fly during drilling, so it's important to protect your eyes.
6. Drill the Main Hole: Secure the lid, preferably in a vise or with clamps onto a stable surface. Using the power drill with the correct size bit, drill a hole where you've marked. Apply steady pressure and drill at a moderate speed to avoid overheating the bit or tearing the metal.
7. Deburr the Hole: Once the hole is drilled, remove any sharp edges or burrs with a file or deburring tool. This ensures a clean fit for the connector and reduces the risk of injury when handling the lid.
8. Test Fit: Place the SO-239 connector in the hole to ensure a proper fit. It should be snug and not move around. If the fit is too tight, gently enlarge the hole with the file. If it's too loose, you might need to use washers or a gasket to ensure a secure mount.

With these steps, you’ve successfully prepared the jar lid for the next phase of construction – mounting the SO-239 connector and attaching the resistors. Remember, precision and safety are key. A well-drilled hole will make the rest of the project go smoothly.

In this step, we will prepare the two non-inductive 100-ohm, 50-watt resistors to be used in our 50-ohm dummy load.

Materials:

• Two non-inductive 100-ohm, 50-watt resistors
• Two small zip ties
• Soldering iron and solder
• Wire cutters
• Heat shrink tubing or electrical tape (optional for insulation)

Instructions:

1. Inspect Resistors: Start by inspecting the resistors for any physical damage. They should be in perfect condition to ensure reliable operation.
2. Parallel Connection: We need to connect the two resistors in parallel to achieve the desired 50-ohm resistance. This means connecting the leads of the same sides of each resistor together: one end of the first resistor to the same end of the second, and likewise for the other ends.
3. Secure Leads: Before soldering, twist the leads that will be connected together to ensure a good mechanical connection. Ensure the leads are tightly twisted and aligned.
4. Soldering: Heat up your soldering iron and solder the twisted leads together. Make sure to apply enough solder to create a solid joint, but avoid using so much that it creates a large blob, which might cause issues when fitting everything into the jar later.
5. Insulation: After the solder has cooled, insulate the exposed connections. You can slide heat shrink tubing over the joint and apply heat to shrink it into place, or you can wrap the joint securely with electrical tape.
6. Securing Resistors: Use the zip ties to secure the resistors together side by side. This ensures they stay in place and makes the whole assembly easier to handle. Trim off the excess length of the zip ties for a neat finish.
7. Final Check: After assembly, use a multimeter to check the resistance across the combined resistors. You should see a reading close to 50 ohms. This confirms that the resistors are connected properly.
8. Preparation for Mounting: Prepare leads that will connect to the SO-239 connector. If necessary, use wire cutters to trim and solder on additional lengths of wire or to create clean, fresh ends for a solid connection to the SO-239.

With the resistors now correctly configured, soldered, and tested, we're ready to proceed to the next step, which will involve attaching the resistor assembly to the SO-239 connector.

Now that we have the resistors prepared and the SO-239 connector mounted to the jar lid, it’s time to attach them together to create the load for the dummy load project.

Materials:

• Prepared resistor assembly (from Step Two)
• SO-239 connector (mounted to the jar lid from Step One)
• Soldering iron and solder
• Wire cutters
• Heat shrink tubing (if not already applied)

Instructions:

1. Positioning: Position the resistor assembly so that the leads align with the contacts on the SO-239 connector. One lead should connect to the center pin of the SO-239, and the other to the ground.
2. Trimming Leads: If the leads on the resistor assembly are too long, trim them to the appropriate length with wire cutters. Leave enough length to make soldering to the SO-239 easy but not so much that they are unwieldy.
3. Tinning Leads: If not already tinned, use your soldering iron to coat the ends of the resistor leads with solder (tinning). This will make it easier to solder them to the SO-239.
4. Soldering to Center Pin: Heat the center pin of the SO-239 connector with your soldering iron and then bring the tinned lead of one resistor to the pin. Apply solder until a solid joint is formed.
5. Soldering to Ground Connection: The other resistor lead should be soldered to the ground part of the SO-239, which is typically the outer casing. Make sure to heat the connection sufficiently so the solder flows and makes a good electrical connection.
6. Securing the Assembly: Once the leads are soldered, ensure the resistor assembly is securely positioned and not stressing the soldered joints.
7. Inspection and Testing: After the soldering is complete, inspect the joints for any cold solder or loose connections. Use a multimeter to test for continuity and ensure there are no shorts between the center pin and the ground.
8. Final Assembly: Carefully place the resistor assembly inside the jar, and screw the lid on. Ensure that none of the resistor leads or joints are touching the jar, as this could cause a short circuit.
9. Filling with Oil: With the resistor assembly and the SO-239 connector securely attached, you can proceed to fill the jar with your chosen oil, submerging the resistors completely for cooling purposes. Be sure to leave some air space to allow for the expansion of the oil when it heats up.

With the resistor assembly now attached to the SO-239 connector, your dummy load is almost complete. The next steps will involve sealing the jar, filling it with oil, and testing it before use.

The purpose of filling the jar with oil is to dissipate the heat generated by the resistors when the dummy load is in use. The oil acts as a coolant, and the type of oil used (baby oil, mineral oil, or transformer oil) should be non-conductive to avoid any risk of electrical shorts.

Materials:

• Filled and sealed Tostitos jar with resistor assembly and SO-239 connector attached
• Baby oil, mineral oil, or transformer oil
• Funnel
• Measuring cup
• Paper towels or clean rags for any spills

Instructions:

1. Clean Workspace: Start with a clean workspace to avoid contamination of the oil.
2. Estimate Oil Volume: Before filling, estimate the volume of oil needed by considering the size of the Tostitos jar and the volume displaced by the resistor assembly. A standard Tostitos queso jar holds approximately 15 ounces when completely full.
3. Preparing the Jar: Ensure the resistor assembly is securely in place and that the SO-239 connector is tightly mounted on the jar lid.
4. Filling with Oil: Using a funnel and measuring cup, slowly pour the oil into the jar. Pouring slowly helps prevent air bubbles from being trapped. Fill the jar until the resistors are completely submerged in oil, leaving about an inch of space at the top to allow for expansion when the oil heats up. Based on the jar size, you may need approximately 12-14 ounces of oil, taking into account the displacement of the resistors and the air space required.
5. Checking for Bubbles: After filling, gently tap the jar to encourage any trapped air bubbles to rise to the surface and escape.
6. Cleaning Spills: If any oil spills occurred during filling, wipe down the jar and lid with paper towels or a clean rag.
7. Sealing the Jar: Carefully screw the lid onto the jar to ensure a tight seal. It's important to ensure that the lid is as tight as possible to prevent any oil leakage.
8. Wipe Down: Give the outside of the jar one final wipe down to remove any oil residue that could cause slippage during handling.
9. Safety Check: Ensure the lid is secured and there are no leaks. Double-check that the assembly is correctly put together, and no parts are loose.

With the jar now filled with oil, your dummy load is ready for the final steps of sealing and testing. Remember, while in use, the dummy load should be monitored to ensure it is functioning properly and not overheating.

The final step in creating your dummy load is to test its functionality to ensure that it performs correctly when in use. Testing will involve measuring both resistance (DC) and impedance (AC) to confirm that the load presents the correct 50-ohm impedance across the frequency range of interest.

Materials:

• Completed dummy load filled with oil
• Digital multimeter
• Vector Network Analyzer (VNA)

Instructions:

1. Resistance Check: Using the digital multimeter, measure the resistance across the dummy load. Connect the multimeter leads to the SO-239 connector—one to the center pin and one to the ground. The multimeter should be set to the resistance (Ω) setting. You should observe a reading of approximately 50 ohms, which indicates that the resistors are correctly configured in parallel.
2. Impedance Check: Now, use a Vector Network Analyzer to measure the impedance. Connect the dummy load to the VNA and set the frequency range over which you want to test the load. For ham radio applications, this might cover the bands you intend to use. The VNA will provide an SWR (Standing Wave Ratio) and impedance measurement. The closer the SWR is to 1:1 and the impedance to 50 ohms across the frequency range, the better the dummy load is performing.

Explanation of Resistance vs. Impedance:

• Resistance (DC): Resistance is the opposition to the flow of direct current (DC). It's a static measure that doesn't change with frequency. It's measured in ohms and is relevant when dealing with direct currents or steady-state signals.
• Impedance (AC): Impedance, on the other hand, is the opposition to the flow of alternating current (AC). Unlike resistance, impedance varies with frequency. It includes both the resistive element (which remains constant with frequency) and the reactive element (which changes with frequency). The reactive component consists of inductance and capacitance, which can cause the impedance to be different from the resistance at frequencies other than DC.

When testing the dummy load, it’s important to remember that the resistance measurement with a multimeter will likely only be accurate at DC or very low frequencies, while the impedance measured with a VNA will show how the dummy load behaves across a range of frequencies.

With the successful completion of these tests, your dummy load is ready to be used in various applications, such as adjusting antennas, testing RF transceivers, or any other application where a stable 50-ohm load is required. Always ensure to monitor the temperature of the load during extended testing to prevent overheating and damage to the load or your equipment.

Summary:

Congratulations on completing your DIY 50-ohm dummy load! This project has taken you through the essential steps of constructing a budget-friendly yet effective device for your ham radio operations, from drilling and preparing the jar to assembling the resistors and testing your creation.

Here's a quick recap of what we've accomplished:

1. Drilled the Jar Lid: Created a precise hole in the lid of a Tostitos jar to mount the SO-239 connector.
2. Prepared the Resistors: Connected two non-inductive 100-ohm resistors in parallel and secured them together.
3. Assembled the Load: Soldered the resistor assembly to the SO-239 connector, ensuring solid electrical connections.
4. Filled with Oil: Submerged the resistor assembly in an appropriate non-conductive oil for heat dissipation, filling the jar to the correct level.
5. Tested the Load: Conducted thorough testing to ensure the dummy load presents the correct resistance and impedance.

Now that you've got a fully functional dummy load, you can use it to test your RF equipment, tune antennas, or safely dissipate power without transmitting.

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