POCKET SIZE DUMMY LOAD FOR
Instructions for building a 10 watt QRP dummy load using a
Editors Note: This simple project
enables you to build a very small QRP 50 ohm dummy load that fits inside
a suitable sized pill bottle and can take up to 10 watts of rf
On the opposite end, strip about 3" of the jacket & separate the center & shield so you can join them to your "resistor bank".
(Try crawling under & behind your workbench & you might find an assortment that yields 50 ohms when paralleled).
Anyway; use any combination of at least 5 - 1 watt resistors that will give you 50 ohms.
I used 9 - 470 @ 1W in parallel, they = 52.22 ohms, (95.75% accurate) and if you add a 1200 ohm to the group, you now have
50.04 ohms (99.92% accuracy).
(For the "Accuracy Police" use 1 % or closer tolerance), or grab your $10,000 VOM & start measuring.
(Hey folks; been a ham over 50
yrs. its a hobby, not a profession!)
Now the hard part; find any pocket-sized pill bottle that will hold your assembly of resistors.
(Hey its your pocket, your pills, find something.)
Bore a hole in the center of the bottle lid, & run the coax thru it - with the connector on the outside!
On the inside of the lid, put a wire tie snugly around the coax to keep it from pulling out.
Leave yourself room to work with on the coax so you can attach it to the resistor bank.
Solder the shield to one end, & the center conductor to the other, put it together, and Voila!
Your QRP dummy load is done!
DON'T LET THE SMOKE
Use only short transmissions and don't cook the resistors or
melt the plastic container. Give them some time to cool. If pill container
starts to get warm, fine, but..........
For use with higher power ratings, use larger wattage resistors of the same values and a larger pill bottle accordingly! Or if you prefer, mount it all in a small aluminum box suspended and insulated from the metal! Your choice. You might also want to drill some small ventilation holes in the plastic bottle. There are many ways to get it all done to suit your tastes, materials on hand and your expertise.
Resistor color code tips.....
If you don't know the resistor color code, use the handy visual resistance calculator in the link below (off site) to compare what you have in the junk box with what is needed for this project or use the chart below:
Visual Resistance Calculator (Off site)http://www.csgnetwork.com/resistorvalimagecalc.html
Hold the resistor with the gold or silver band to the right and read the resistor from the left to the right.
In the QRP dummy load project above you
need 9 ea 470 ohm resistors, 1 watt rating and......
So using the chart above... break 470 into 3 numbers, 4 7 0 .....now going down the second column from the left you see that 4 is yellow.
Then for 7 the color is violet and for 0 the color is black.
So now all you have to do is match those colors with the bands on your resistors in that order from left to right found in your parts bins, under your desk, in the corners of the drawers, etc. Make sure you are reading from left to right and then look at the 4th band which should be either no color (20%), Silver (10%) or Gold (5%). The tolerance of the resitsors are plus of minus the tolerance color in value. The article suggest using 5% tolerance resistors.
For the 1.2k ohm (1200 ohms) resistor you should come up with brown, red, red = 1200 ohms and whatever tolerance you want for the 4th band.
A + or - 5% tolerance of 50 ohms is 2.5 ohms so as an example, it could be anywhere from 47.5 to 52.5 ohms in actual resistance value. That is it, now put it all together and get testing...have fun! 73---KF9F
Hamuniverse.com uses Green Geeks Web Hosting!