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Forum Index : Other Stuff : Building RF attenuators...
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Hi folks. 
I am trying to build a very basic T-type RF attenuator, but no matter how much tinkering I do with the values, I never get the wanted attenuation. I am using this site to calculate the values: http://www.everythingrf.com/rf-calculators/attenuator-calcul ator I have 5W of juice, and I want to attenuate this by 3.5W as the transmitter is an older unit, and there is no power output control on it. If I use another calculator here on the same site: http://www.everythingrf.com/rf-calculators/watt-to-dbm ...to calculate the dB value of 3.5W, I end up with 35.4dB. If I then put that figure into the attenuator calculator, and also specify 50-ohm impedance, I end up with values for R1 of 41.2R and R2 of 1.7R. In the real world, I changed these such that R1=4x220R/1W carbon resistors in parallel(equaling 55R @ 4W rating) and R2=4x8R2/1W carbon resistors in parallel(equaling 2R @ 4W rating). All this is inside a die-cast aluminum case, with BNC sockets. Problem is, that with these values, and 5W in, I get 100mW output. I tried increasing the value of R2 to 5R(4x22R/1W in parallel), but the output is still 100mW - I would have expected some kind of change in that - expecting an increase in output power. RF meter says that the match is perfect(Reflected power from the attenuator is zero, and SWR is 1:1) Does anyone here know more about RF attenuators and can help me work out what I am doing wrong? I figured you should just be able to follow the on-line calculators, and it would be easy as pie - but it is not...  Smoke makes things work. When the smoke gets out, it stops! |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
ADDITIONAL: I have made some kind of mistake somewhere, as a quote from another website says that: [quote] A one-watt transmitter is +30 dBm. A 4-watt transmitter is +36 dBm. Use the logarithmic scale for you power calculations. A 5-watt transmitter is 36.9 dBm. With a 20 dB pad the power level is 16.9 dBm or 50 mW. [/quote] So, if I am putting a 30dB attenuator on a 5W tx, this basically bleeds the entire power off the transmitter, which is exactly what is happening. ...the investigation continues... Smoke makes things work. When the smoke gets out, it stops! |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
Ahhh, yes - my maths was off just a bit. 3dB represents twice the power, so with a 6dB attenuator made as above, I am getting about 1W of output. Seeing as I am aiming for about 1.5W out, I will increase the values a little and build a 5dB attenuator, and I should be pretty much in the ball park give or take a hundred mW or so... Smoke makes things work. When the smoke gets out, it stops! |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9306 |
4dB attenuation was the magic figure. 5W in, 1.5W out now - exactly what I need. Smoke makes things work. When the smoke gets out, it stops! |
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