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Message |
    
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 118
Registered: 9-2004
| | Posted on Sunday, January 09, 2005 - 2:34 am: | 
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maybe the reason i havent found this already in this forum is because its very simple but, just for those doing searches, here's how to build your own dummy load.
A x B = C
A=50 ohms, the value you are trying to acheive.
B=number of resistors you wish to use.
C=value of each resistor
the one i made is like this: 50 x 3 = 150 ohms.
so i bought three 150 ohm resistors. each resistor will handle 25 watts so my dummy load will handle 75 watts.
buy a metal project box, size doesnt matter.(this time)
buy an SO-239 connector and mount it to your project box.
twist together the ends of all resistors.
solder one end to the center pin of the so-239 and screw or solder the other end to the box.
thats it. mine works great and cost me a total of 12.00 to build.
here are some other combos.
20 1K ohm resistors @1 watt= 50 ohms and handles 20 watts.
6 300 ohm resistors @5 watts= 50 ohms and handles 30 watts.
hope this helps.
matt |
Paper765
Junior Member
Username: Paper765
Post Number: 10
Registered: 12-2004
| | Posted on Sunday, January 09, 2005 - 8:15 pm: |
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Kid, that's pretty cool. If I go to Rat shack, or someplace similar, I could use this formula to build a dummy load and test out my radios?
For those newbies out here, (and those who just are rusty) tell us how the dummy load helps us out for testing.
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Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 120
Registered: 9-2004
| | Posted on Sunday, January 09, 2005 - 9:30 pm: |
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no problem, yes, thats correct, you can go to rat shack and build this. the only problem is that they dont carry much of a variety of resistors anymore. so you might have a hard time finding what you want. a dedicated electronics parts store is your best bet. i found 25watt 150 ohm resistors for $1.50 each. you would need three of these.
the advantage of using this is that you can tune your radio without putting out a signal on the air. have you ever heard someone on 19 continually saying "aaahhhhhh" into their mic? its pretty annoying. dont be that guy. the other advantage is that it is as close to a perfect swr match as is humanly possible. so the results of your tuning are accurate.
here's how i just used mine. i was installing a 102" whip on my truck and the swr was really high, i suspected the cable was the culprit so i hooked the dummy load to the antenna end of the coax and keyed the radio. a perfect match. now i knew that the antenna was the culprit. buffed off some paint and voila! the antenna works great and i didnt spend extra money buying a new piece of cable that i didnt need. this is just one example. another would be if you werent sure if your antenna was bad or that your swr meter is off. use the dummy load and if the swr is 1.1 to 1 or 1.2 to 1 then you would know that the antenna was the problem. build one, you'll like it.
matt |
Chad
Member
Username: Chad
Post Number: 75
Registered: 11-2004
| | Posted on Monday, January 10, 2005 - 1:33 pm: |
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One thing to remember is that standard wirewound resistors are very inductive. One needs to obtain carbon film resistors or resistors intended for RF applications.
Chad |
Hatchet
Junior Member
Username: Hatchet
Post Number: 11
Registered: 7-2004
| | Posted on Monday, January 10, 2005 - 6:53 pm: |
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Great info Chad....Wirewounds wont work for a dummyload |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 127
Registered: 9-2004
| | Posted on Monday, January 10, 2005 - 11:49 pm: |
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they wont? i kinda thought that they would work but would radiate a bit more of a signal than their carbon cousins. ive been known to be wrong a lot though.
matt |
Chad
Member
Username: Chad
Post Number: 76
Registered: 11-2004
| | Posted on Tuesday, January 11, 2005 - 8:56 am: |
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Kid Vicious
The most popular auction site often has electronics wholesalers offloading components. One can find RF resistors there very cheap. Heck I bought a 250 Watter there for 9 bucks 
Chad |
Stepchild
Intermediate Member
Username: Stepchild
Post Number: 146
Registered: 11-2002
| | Posted on Saturday, February 05, 2005 - 7:13 pm: |
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Hey Kid,Is key down time a factor with this dummy load?
Stepchild |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 210
Registered: 9-2004
| | Posted on Saturday, February 05, 2005 - 9:44 pm: |
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stepchild, yes, keydown time is a factor with any dummy load. as far as to how long you can key up without overheating it, that is a complicated answer. i built my dummy load to handle 75 watts. this means that the resistors insite are rated to handle that much power. so if my transmitter put out 75 watts dead key, my dummy load would heat up pretty quickly. im guessing less than 30 seconds. all of my radios put out between 2 and 35 watts so im not really worried about over heating. when im tuning a radio, i never key the mic for longer than 10 seconds anyway, to prevent overheating the radio. if extended key down time is an issue for you; build one with double the capacity you need, build it inside of a paint can, and fill the can with transformer oil.
matt |
Tech291
Moderator
Username: Tech291
Post Number: 64
Registered: 11-2004
| | Posted on Saturday, February 05, 2005 - 11:30 pm: |
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another thing to consider is the heat generated could get high enough to remelt your solder connections depending on your construction methods.(dont ask how I know  ) |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 211
Registered: 9-2004
| | Posted on Sunday, February 06, 2005 - 12:22 am: |
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wow! i hadnt thought about that! maybe thats why my motto is "always overbuild everything!"
works fer me!
matt |
