DIY Dummy Load

A DIY Dummy Load that will handle about 100w RF from 1.8 Khz to 450 Mhz.

I made a dummy load before using a bunch of resistors, a paint can, and added some electronics so it could measure the radio’s power output. Unfortunately that dummy load is only good through 2 meters and a lot of my stuff is done on 70cm now.

This dummy load is a bit different and is based on a design from Nuts & Volts Magazine, 2019, Issue 3.

Completed 100w Dummy Load

Parts List

1100w 50ohm RF Resistor
1SO-239 Panel Mount Connector (or other suitable connector)
64-40 Bolts ½ inch Long
24-40 Bolts 1 ¼ inches Long
84-40 Nuts
4Washers that fit 4-40 bolts
11in long piece of heat shrink tube Size 1/4in Wide
112V PC Fan 80mm
1Aluminum Sheet 10in X 4in X 1/16in Thick
1PowerPole or DC Connector
1Solder Wick / Copper Braid / 16AWG Solid Copper Wire 1in Long
4Stick on Rubber Feet
1Good Quality Thermal Paste (Arctic Silver or similar)


  • Drill & Drill Bits (3/32 Inch & 5/8 Inch)
  • Soldering Iron & Solder
  • Wire Strippers
  • PowerPole Crimping Tool (If you use PowerPoles)
  • Screw Driver
  • Socket or Pliers for the 4-40 Nuts
  • Block of wood at least 5in long on one side
  • Permanent Marker
  • Ruler or Tape Measure


  1. Measure 4 inches in from the end of the aluminum sheet (this will make a 4in square on the sheet) and draw a line.
  2. Next measure ½ inch from the other end of the aluminum sheet and mark that line. These lines are where you will bend the aluminum, but don’t do that yet.
  3. Now mark the center of the 4in square you created with that first line, by measuring in 2 inches from each side or use a ruler to draw two diagonal lines from the corners of the aluminum sheet to the corners of the line you drew. Where these lines meet should be the center of your square.
  4. Next place the aluminum sheet on your block of wood and clamp it down.
  5. Drill a 5/8 inch hole through the aluminum sheet on that center mark. The wood will ensure you don’t drill through your good desk or table. 🙂
  6. Now ensure the SO-239 will fit in that hole. You may need to file the edges a bit with a round file.
  7. Once the SO-239 fits in the hole, mark the 4 holes around the connector and drill them out with the 3/32 inch drill bit. I would place the wood under the holes you’re drilling.
  8. Now place the aluminum sheet over the block of wood, lining up the line you drew with the wood and bend it over. It might be easier to do this in a tabletop vice.
  9. After that, use 4 of the ½ inch 4-40 bolts and 4 nuts to secure the SO-239 into the aluminum sheet.
  10. Now place the resistor on the inside of the sheet as close to the SO 239 connector as possible. Remember that your nuts and bolts must fit. Mark the two holes for the resistor.
  11. Drill two 3/32 inch holes for the resistor. DO NOT drill through the holes already in the resistor, remove it first.
  12. Apply thermal paste to the bottom of the resistor.
  13. Ensure that the center pin of the resistor (the little foil tab) is facing the SO-239.
  14. Put two ½ inch long 4-40 bolts through the resistor and the holes in the aluminum sheet. Add one washer to each bolt against the aluminum sheet. Then add one 4-40 nut to each bolt and hand tighten only. You just want it slightly snug, don’t break the resistor.
  15. Next, add flux to the foil tab of the resistor and solder a small piece of copper braid (or wire) to the foil tab, slip the heat shrink tubing over the wire and foil tab and then solder the other end of the braid to the center pin of the SO-239.
  16. Place the aluminum sheet over the wood again, lining up the other line with the edge of the wood or edge of the vice and bend it up.
  17. Place the fan against this lip of the sheet and mark the holes. Drill two 3/32 inch holes.
  18. Place the fan so that it blows air towards the resistor and put the fan on the inside of the lip.
  19. Next using two 1 ¼ inch long 4-40 bolts, 2 washers, and 2 nuts, bolt the fan to the sheet with a washer between the fan and the nut.
  20. Next add a PowerPole or DC connector to the fan’s wires.

Now you have a 100W dummy load that should work from 160m through about 70cm. Use the least amount of power possible when testing. I wouldn’t use this over 50w except for very short durations. The more power you use, the shorter the life of the resistor and you could burn it up.

SWR Graphs

So how does it stack up? Check out these SWR graphs. Under 1.2 to 1 SWR from 1.8Mhz to 148Mhz is good enough for me. On 70cm it has an SWR under 1.46 to 1, so it’s not the best, but it’s usable.

1.8Mhz to 50Mhz SWR
2m Band SWR
50Mhz to 148Mhz SWR
70cm Band SWR

More Images

Dummy Load – Top View
Dummy Load – Resistor/SO-239 Connection
Dummy Load – Bottom View

Live Streaming & DIY Microphone


During 6 and 1/2 of my 7 years of college (part time), I was President of and helped build a student organization for LGBTQ+ students and their allies. Every year we host an event comprised of speeches from students, faculty, and staff that I video tape. Eventually we began streaming the event live on Facebook. The event is always around the 2nd week of October.

Now I’m an alumni advisor to the organization and I’m still recording/live streaming the event. I wanted to post about how I do this, mainly for my future reference.

Recording/Live Streaming Setup

The event is held in a small, but beautiful chapel on campus. I normally setup the camera in the back corner of the room and the speaker is in the opposite corner in the front of the room. The recorded audio has always been decent, but it could definitely be better.

I connect the camera’s A/V output into the composite input cable on the TV Tuner, which is then plugged into the computer’s USB port. I use the software that came with the TV tuner or VirtualDub to set the channel on the TV tuner to the composite input. Then I can close VirtualDub or the TV tuner software and open OBS.

Computer Specs

OBS Setup

In OBS, I have a scene with just a photo source (the club’s logo). In another scene, I have the TV tuner’s video and audio sources. I run the photo a few minutes before we start and then switch over to the camera when the event begins. I do the actual recording in the camcorder at 1080p 30fps. Composite video has a standard resolution of 480i or 576i, so I set OBS to stream at a resolution of 640 x 480 to Facebook using a bit rate of 2000 kbps.

DIY Electret Condenser Microphones


I am NOT responsible for any loss or damage resulting from the use or building of this project. PLEASE make sure your camera/device accepts this type of powered microphone before connecting it. I am NOT going to be responsible for any damage you may cause by building or using this microphone.

Parts list

  • 6 Feet of 2 Conductor Audio Cable – I used thin coax cable, RG-174.
  • 2 Mono 3.5mm (1/8″) Phone Plugs (male)
  • 2 Capacitors (100nF – Code 104) – I used the ceramic disc type.
  • 2 resistors (680 Ohm) – Value should match the impedance of the microphone elements you purchase.
  • 2 Microphone Elements – I used PUI Audio AUM-5047L-3-R.
  • 2 short lengths of PVC Pipe – I used 1/2″ Sched. 40 PVC cut at 6.5″.
  • 2 PVC End Caps – Size needs to match the PVC pipe.
  • Spray Paint
  • 1 Length of wood or PVC pipe cut at 26″
  • 2 Bolts – 6-32 size. Use a length that will go through 1/2″ PVC (OD = 0.84″) and your piece of wood/pipe. (I used 2.5″ bolts and they were a 1/2″ too long).
  • 8 Flat Washers – #6 Size
  • 2 Hex Nuts – 6-32 Size
  • 2 Wing Nuts – 6-32 Size
  • Heat Shrink Tubing (various sizes)
  • Air Conditioner Foam Filter
  • 3 Panel Mount 3.5mm Phone Jacks (female)
  • 9V Battery
  • 9V Battery Holder


  • Soldering Tools
    • 15W Soldering Iron
    • Solder – I used 63% Lead / 37% Tin with a rosin core.
    • Desoldering Wick
    • Flux Pen
  • Pliers
  • Wire Cutters
  • Wire Strippers
  • Tweezers (optional)
  • Lighter
  • Audacity Recording Software

I’m not going to go into painting the PVC and putting the bolts through it. Nor will I cover cutting the slots in the PVC pipe or the sewing of the windscreen foam.

How to Build The electronics

wiring the microphones & connectors

  1. The first thing I did was solder the connectors onto the ends of the coax or audio cable. Make sure you put the plug’s cover on first. I did not care about interoperability so I soldered the center conductor of the coax to the center pin and the shield of the coax to the sleeve of the connector. Put one connector on each 3 foot piece of cable.
  2. Put heat shrink over the cable first. (we’ll shrink this later).
  3. On the other end of the cable we’ll solder the microphone element to the wire. The center pin goes to the positive and the shield of the coax connects to the negative on the mic element. Try to do this quick and precisely or you’ll ruin the mic like I did on my first try. I had to order another one after ripping the solder pad off the mic element. The mic’s solder pads are tiny, about 1mm wide by 3mm long. Don’t short the two solder pads either.
  4. Slide the heat shrink tubing up enough to cover the wires a bit, but do NOT cover the back of the microphone element. Shrink it over the wire. Don’t over heat the wire or mic element.
  5. Make sure you’re soldering the center conductor to the positive solder pad on the mic element. I messed this up on one microphone and could not figure out why neither microphone worked with the circuit.
two microphones
Two completed microphones.

create the circuit

Mono Directional Electret Microphones – Power Supply & Mono to Stereo Connection Schematic

Using the above schematic, we’ll create the circuit used to power the microphones.

Basically, the resistors provide bias to the microphone elements. The value of these 2 resistors should match the impedance of the microphone elements you bought. Mine were 680 Ohm. These resistors should be 1/2 Watt resistors.

The capacitors block the battery’s direct current (DC) from getting into the camera. While capacitors block DC current, they allow AC current, such as an audio signal, to pass through.

NOTE: Some capacitors are polarized and must be installed with the positive leg facing the microphone elements. If you install the capacitor backwards you could damage your camera and/or the microphone elements. The capacitors used in this schematic are NOT polarized.

Project box with power switch.
Project box with power switch

Test for DC on the Output before use

Just to be on the safe side, I tested for any DC voltage on the output before I plugged this into my camera. Plug in the battery, turn the switch on and test the output connector with a multimeter for DC voltage to make sure there will NOT be any DC getting into the camera. If your multimeter detects DC voltage on the output do NOT plug it into the camera. Go back and check your connections.

Project box output side
Output Side of Project Box

Creating the Stereo Effect (Extremely Basic)

This part is a little over my head, but according to what I’ve read, you can create a stereo effect in one of many ways. Since I couldn’t find microphone elements with the correct polar patterns to create a single stereo microphone, I made two mono microphones that get connected to the circuit which connects both of them to the stereo mic input on my camera. By separating the microphones at least 24 inches using the piece of wood or pipe, you will get a “stereo” sound effect.

completed microphones showing spaced stereo positioning
Showing Spaced Stereo Positioning of Completed Microphones

Matching the microphones

Typically, when a stereo microphone is built, the two elements are “matched” to be sure that the sounds they record are the same volume and gain on both the left and right channels. This is done so that the volume of one channel is the same as the other, else the audio would not sound right.

I personally did NOT match my microphone elements. I am not recording professional audio and I needed something cheap. I also wasn’t 100% sure how to do it at home (it can be done fairly well at home though).

That said, the two microphones are from the same manufacturing lot, the audio sounds the same on both channels to me and the gain of each channel is extremely close when looked at with Audacity.


The microphones sound MUCH better than the camera’s internal microphone. They’re fairly clear and provide decent audio for a “home” video. The volume of the recorded audio was almost exactly the same as the source audio I recorded.

The internal camera microphones sounded quieter than the source I was recording. While they were clear, the audio didn’t sound as clear or as full as the microphones I built. The audio from the camera sounded like it was too deep.


I used many sources and forums to find the information necessary to build my microphones. I originally was trying to build a shotgun microphone (a very directional microphone employing an interference tube design), but there was significantly more information on building a DIY Directional Microphone, so I went with that. I did research over the course of a couple of weeks trying to learn enough to build this project. The circuit is the basic manufacturer’s recommended design for a single microphone element that has been modified to connect two microphone elements to the same battery and stereo output connector.

I want to thank everyone who put this information out there for others to use.