**EMRFD Message Archive 3610**

MessageDateFromSubject3610 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?

HA5RXZ3611 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'm assuming that the load on the system output is another 50 Ohms in parallel

>

> 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?

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, K8IQY3616 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