EMRFD Message Archive 3610

Message	Date	From	Subject
3610	2009-10-13 13:04:59	ha5rxz	Power Taps
In QST June 2001 there is an article by W7ZOI and W7PUA titled "Simple RF Power Measurement and as part of the article they describe a 40 dB power tap. This uses a series resistor of 2460 ohms and a resistor to chassis of 50 ohms. My calculations show this as a 50:1 voltage divider. I come unstuck when considering RF power. If I inject 47 dBm into the power tap (50v RMS) then I should get 13 dBm out (1v RMS) This represents 34dB of attenuation, not 40dB. Where am I going wrong? HA5RXZ
3611	2009-10-13 13:16:25	Nick Kennedy	Re: Power Taps
Did you consider the load being another 50 ohms in parallel with the 50 ohms of the divider? Will that give you the extra -6 db? 73--Nick, WA5BDU
3612	2009-10-13 14:03:01	Jim Kortge	Re: Power Taps
ha5rxz wrote: > In QST June 2001 there is an article by W7ZOI and W7PUA titled "Simple RF Power Measurement and as part of the article they describe a 40 dB power tap. This uses a series resistor of 2460 ohms and a resistor to chassis of 50 ohms. My calculations show this as a 50:1 voltage divider. > > I come unstuck when considering RF power. If I inject 47 dBm into the power tap (50v RMS) then I should get 13 dBm out (1v RMS) This represents 34dB of attenuation, not 40dB. > > Where am I going wrong? I'm assuming that the load on the system output is another 50 Ohms in parallel with the internal 50 Ohms to ground in the divider. That makes the divider nearly 100:1. One should also add in the 50 Ohms from the source that adds to the resistor string, but has less effect on overall accuracy. That's the way I see it. 72, Jim, K8IQY
3616	2009-10-14 07:55:57	ha5rxz	Re: Power Taps
Thank you for the two replies. Both of you are correct in that the power meter meant for use with the power tap has a 50 ohm resistor on the input, this then gives 40 dB of attenuati