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100 ohm dummy load
- Thread starter kaos513
- Start date
Did you run an SWR meter inline, yet; during testing?
Do you have two coax jumpers that are 3 feet in length?
If so, put one from the radio to the input of the meter.
Then another coax connected to the dummy load and the SWR meter's output/antenna connector.
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Never trust the radio's built-in meter, ESPECIALLY if you've modified carrier levels/lowered your dead key. Modifications and no alignment often equals a sloppy meter.
Always go with an external meter (this is part of your test bench), an entry level one is like 20 dollars and it will serve you for years.
This tool will give you a solid reference for power measurements and standing wave ratio check, accurate enough to start experimenting! (after all, what you need is a reference point, to measure {change}!)
Best part about this cheap $20 meter is that it will survive some heavy abuse, trust me..
Here is my personal recommendation for the beginner.
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just make sure you didn't use wirewound resistors, as their inherent inductance will throw your measurements off.
I saw that you said that your SWR meter on the radio shows a 1 when you measure it using your dummy load.
What i didnt see was if you ever used an ohmmeter to measure the resistance of your creation in its final form.
If you did this measurement, what was it?
Making a dummy load that is specifically for calibrating SWR meters in radios is a good thing IMO.
although you should really make a 150 ohm load so that you can calibrate the meter using somewhere near center scale in order to maximize your accuracy.
in radios that don't have an adjustment for this, you can usually change one resistor to a variable and make it adjustable.
when you put the resistors in oil, you can multiply its power handing capacity by about 10 or so. I'm not sure what effect salt water has on the power handling.
it does look like the beginnings of a way to heat pasta with your linear amp though!
LC
I saw that you said that your SWR meter on the radio shows a 1 when you measure it using your dummy load.
What i didnt see was if you ever used an ohmmeter to measure the resistance of your creation in its final form.
If you did this measurement, what was it?
Making a dummy load that is specifically for calibrating SWR meters in radios is a good thing IMO.
although you should really make a 150 ohm load so that you can calibrate the meter using somewhere near center scale in order to maximize your accuracy.
in radios that don't have an adjustment for this, you can usually change one resistor to a variable and make it adjustable.
when you put the resistors in oil, you can multiply its power handing capacity by about 10 or so. I'm not sure what effect salt water has on the power handling.
it does look like the beginnings of a way to heat pasta with your linear amp though!
LC
That device looks dangerous. Like a Molotov cocktail with electric trigger.
Use mineral oil because it won't turn rancid with time like olive oil does.
There are wire wound resistors that are non-inductive or if you have a bunch of no-chrome you can make your own non-inductive resistor. Magnet wire that is coated with HAPTZ works better.
It can made with almost 0 inductance and be completely resistive or you could just buy a cantenna.
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Try using some Italian vinaigrette salad dressing instead; just be sure to shake it up first before using it - lol.
All kidding aside, if you ran the two 100 ohm 1/4 watt carbon resistors in parallel in just oil, you might expect to use maybe four watts thru it for no more than five seconds before the resistors start changing their values due to heat buildup.
Don't use water, as it will create a dead short. I understand you are just trying to experiment, but you will just end up shorting out the finals if you keep at it like this . . .
All kidding aside, if you ran the two 100 ohm 1/4 watt carbon resistors in parallel in just oil, you might expect to use maybe four watts thru it for no more than five seconds before the resistors start changing their values due to heat buildup.
Don't use water, as it will create a dead short. I understand you are just trying to experiment, but you will just end up shorting out the finals if you keep at it like this . . .
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Well, I was "in a pinch" to fill my 1 quart d.i.y. cantenna, and I didn't have mineral oil laying around, with the can sealed (and obviously not allowing light through) I haven't had any issues with the performance or smell of the oil, yet that is.. I will keep my "fingers crossed" for now. I eventually bought some mineral oil from the local dollar general, I just haven't been bothered enough to change the oil out.
Thank you for the tip on making resistors, is the wire you are talking about "nochrome" called Nichrome or kanthal (like the vape coil builders use)?
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Nichrome wire is difficult to wind to avoid shorting between the turns, and yes it is the same as the Vapor elements. Just a thicker gauge. Plus the wire crawls when heated.
To make a non-inductive copper wire element it just takes a lot of calculations and a wire data book and a clear idea of how what you are going to do with it. Power levels will drive the selection of wire gauge. All wire has a rating of ohms per foot and you need Xft to make 50 Ohms. Measuring is not critical with heavy gauge because of the low resistance per foot and an error of six feet might make a 49.975 ohm load.
Winding is tricky but doable if you are handy and have worked in an manufacturing environment.
To make a non-inductive copper wire element it just takes a lot of calculations and a wire data book and a clear idea of how what you are going to do with it. Power levels will drive the selection of wire gauge. All wire has a rating of ohms per foot and you need Xft to make 50 Ohms. Measuring is not critical with heavy gauge because of the low resistance per foot and an error of six feet might make a 49.975 ohm load.
Winding is tricky but doable if you are handy and have worked in an manufacturing environment.
Any kind of wire including nichrome will make an inductive resistor unless it is wound such that there are two opposing fields. It is a poor choice. Use carbon composition if you can find them or maybe carbon film type or a higher power carborundom type as used in regular dummy loads.
Here is the easy way to 50 ohms non-inductive.
https://www.ebay.com/sch/i.html?_nkw=50+ohm+non+inductive+resistor
https://www.ebay.com/sch/i.html?_nkw=50+ohm+non+inductive+resistor
Those are way better http://www.ebay.com/itm/RF-Terminat...739484?hash=item4668d8a61c:g:6bMAAOSwjTlZhyk4
Mike
Mike
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