| Author |
Message |
    
Climber
| | Posted on Saturday, March 01, 2003 - 7:08 pm: | 
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I thought I'd build a Dummy load to test with.
Trouble is when I tested it, my SWR's were high 1:3.5+ and I stopped the test after 3 or 4 seconds.
Here is what I have,
I used a metal timer box about 9 inches by 6 inches and 4 inches deep.
Using a S0-239 for the input and a single ceramic 100 watt 50 ohn resister.
The center contuctor of the SO-239 goes to one side of the resister, the outer side of the resister goes to ground on the metal box floor with the paint and primer epoxy removed with a grinder. the resister is ceramic glass and is suspended in air by two L shaped spring clips included.
The resister looks like a 7 inch long hollow cigar with a metal conector at each end.
It seeems my SWR's should be much lower, and I can't use this Dummy Load as is.
Help, |
Tech833
| | Posted on Sunday, March 02, 2003 - 11:11 am: |
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It sounds like you used a wire-wound resistor. You need to use a non-inductive resistor, like a carbon type.
The wire-wound resistors do not present a 'perfect' 50 ohm load. The load will be inductive. Also, to RF, a coil to ground will lower the resistance- even though it is 50 ohms with your DC ohmmeter, it will not be 50 ohms at 27 MHz. |
Buck
| | Posted on Sunday, March 02, 2003 - 3:59 pm: |
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Tech 833 hit the nail on the head...I built one also but I used a quart paint can and filled it with oil. I soldered one side of my(5- 5 watt,250 ohm) resistors to the inside of the lid and the other side to the center of a so-239 mounted in the lid. My swr was very high also and with the help of Bruce found out the wire wound resistors were not what I should have used....I havent done anything more yet because I havent been able to locate any 5 watt 250 ohm carbon resistors
Buck |
Climber
| | Posted on Sunday, March 02, 2003 - 7:58 pm: |
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Thanks,
climber |
Crafter
| | Posted on Sunday, March 02, 2003 - 11:25 pm: |
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I make alot of those little 5 watt dummy loads for friends out of 47 ohm resistor's and pl259's just for CB testing. |
Climber
| | Posted on Monday, March 03, 2003 - 11:45 pm: |
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Thanks for the information Tech833, Buck & Crafter.
I just built a smaller Dummy Load tonight.
Used twenty 1/2 watt, 1k ohm resistors between two boards. (1000 or 1k)/Quanity of 20 = 50ohm hope that's correct.
Its a handy 10 watt unit I sealed in a larger tuna can with mineral oil. Test of my ability to seal that can up with liquid in it, harder than it sounded.
Tomorrow I will pick up twenty (3) watt "Zener Diods" sp? & two more copper boards to make the big RF sponge. Not sure what this will be capapble of handling?
Not store bought pretty, no fancy powder coating.
Looks decent, works fine so far. |
Crafter
| | Posted on Tuesday, March 04, 2003 - 4:18 pm: |
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You know I still have one a friend gave me before he passed away it aint pretty either but it was good enough to work means alot to me too. I make them for friends around here to tune-up with just to keep them off the air while whistling. |
Bigbob
| | Posted on Tuesday, March 11, 2003 - 8:40 pm: |
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If you really want to get ugly,get a spool of spark-plug cable,old style carbon core"rag wire"high tension wire,7000 ohms per foot.Cut off several lengths for 50 ohms pin the ends with copper stubs solder accordingly,voila a really"OOGLY" dummy load.you'll have to experiment because the rf may travel through the jacket as well. |
Applejack
| | Posted on Monday, January 12, 2004 - 12:49 pm: |
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dummy load,dummy question. better asked here than to find out the hard way. can i use a dead mobile with a jumper as a dummy load. almost tried it last nite but decided to wait and ask instead. aj |
409
| | Posted on Tuesday, January 13, 2004 - 4:32 am: |
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I don't think a "dead mobile" will work as a dummy load. Might just fry something. |
Crafter
| | Posted on Tuesday, January 13, 2004 - 7:40 pm: |
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Who told you that Apple Jack, you need to slap them! A dead mobile ranks up there with swr grease and a box of radio checks, oh dont forget a can of squelch. I've used them all too. That dead radio dummy load is good though, hey tell them your doing it and it works great I bet they try it!!! LOL. |
Bigbob
| | Posted on Tuesday, January 13, 2004 - 10:12 pm: |
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A 500' spool of old lossy coax might work to,make a teeny tiny dummy load,1000watts in 1/4 watt out,put that dummy on the end first. |
Applejack
| | Posted on Wednesday, January 14, 2004 - 11:45 pm: |
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crafter, jeesh, i hate to say who told me to use a dead mobile for a dummy load. i also hate to have to slap myself. thats why my post said dummy load dummy question. i was in my garage looking at a dead cobra's guts, when the idea came to me. but i wanted to ask here before frying a good radio. remember i am the guy who has a 4 element beam mounted on his truck. |
Damyankee
| | Posted on Thursday, January 15, 2004 - 2:33 am: |
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Hey Applejack,
At least you asked first. I about busted a gut...LOL. Not about your question but the fact that you took "the heat" for it. Hat's off to you.It would make an interesting rumor to start though! Keep thinking outside the box...
73's - Ron |
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