Saturday, February 6, 2016

Resistance IS NOT FUTILE

The Beautiful Genius Load

In this age of hyper-political-correctness it seems that I should not call my homebrew contraption an "Ugly Dummy Load".  

Ugly is so negative. It probably has a great personality and an inner beauty all its own.
And calling it a dummy is sooo degrading.  Why that's akin to calling it a Dolt, a Dullard, a Dunce, a Fool, a Blockhead, an Ignoramus, a Numbskull or a Simpleton.

So in the spirit of the age where Everyone Gets An Award

Behold the "Beautiful Genius Load"
Beautiful Genius Load

Construction

Up to now I only had one dummy load and it was built into my MFJ Deluxe Versa Tuner II so I thought it was high time to have something a bit more portable.  I already had the UHF connector in my junk box and lots of spare 12g solid copper wire.  So I ordered some 2w/1000ohm 1% resistors for 40 cents each and built the contraption above for about $10.  The bare copper wire is soldered to the bulkhead connector; one wire to the center conductor and the other to a screw terminal.  I simply wrapped the resistors leads around the bare wire and soldered them in place.  In the near side of the photo it looks as though they are not soldered but that is because it was flipped over when I soldered that side.

Concerning 1% resistors... I could have used 5% resistors rather than 1%.  The number of resistors would have averaged out the differences but I like the blue color and everyone wants to be a 1%'er.

Exhaustive parts list:
  • UHF bulkhead connector
  • 20 x 1000ohm 2watt resistors
  • Two stout gauge copper wires

Not an especially good design

This is not a very good design for a dummy load due to its length.  The two parallel wires will generate some capacitance especially at higher frequencies but for ease of building it can't be beat.  In the following section I show the results on the analyzer and you'll see that there is some capacitive reactance that causes the SWR to be 1.1 starting at 14 Mhz.  This certainly isn't ideal but it is sufficient for most amateur uses.  If you think you need totally pure resistance all the way up to 28 Mhz you'll want a different design.

I may place it in a 2 inch PVC pipe with some holes drilled for ventilation to give it some mechanical stability but the heavy gauge wire seems plenty sturdy for now.  Of course, while it's in use you want to keep it clear of metallic objects and not touch it during transmission.  Also it will get HOT after extended use so don't go and grab after you've transmitted into it without determining if it will burn you.

Performance

Well it works mostly like you'd expect a Dummy... errr Beautiful Genius load should.  The twenty 2watt resistors give me plenty of heat dissipation for QRP levels.  I transmitted at 5w into it for a full minute and it was just starting to warm. It should handle QRO loads up to 100w for a few seconds at a time as long as it's given time to cool down between uses.

The SWR is 1.0 on the lower bands and 1.1 - 1.2 on the higher frequencies...

80m SWR 1.0
40m SWR 1.0 
30m SWR 1.0 
10m SWR 1.2
15m SWR 1.1 
17m SWR 1.1
20m SWR 1.1

Make your own Dummy Load... errr Beautiful Genius Load

It's always handy to have a small dummy load handy. You can use more or fewer resistors depending on your needs.  Just remember the formula for calculating resistance is:

Calculating Parallel Resistance
You want to end up with 50 ohm as your total.  Also be sure to use resistors with decent power handling.  Soldering 1/8 watt resistors in parallel isn't going to give you much power handling capacity.

That's all for now

So lower your power and raise your expectations

72/73

Richard, N4PBQ

2 comments:

  1. Hiya Richard! Great write up on brewing your own dummy! Just a quick note for simplification on this for anyone else wanting to build one of these. Just remember that, the total resistance of a number of EQUAL resistors in parallel is equal to the resistance of one resistor, divided by the number of resistors. So in Richards case it is 1000ohms divided by 20, giving you your 50 ohms.

    ReplyDelete
    Replies
    1. Good point. Yes you'd want to use the same value for all your resistors otherwise the lower value resistors would have more current flowing through them and likely burn up or at least get hot quicker and skew the value of resistance for the load.

      Delete