How to measure RF power from antenna


#1

Calling the RF engineers of the group:

I’m getting frustrated by my Google-fu not returning results that I particularly understand. I’d like to know the RF power (wattage) of my antenna at the point where it connects to my RTL-SDR dongle.

My current setup (cue ASCII art):

1/4 wave ground-plane antenna (spider)–>DC Blocker–>Perfect Vision PVLA-30 Amplifier–>50’ 50Ohm Coax–>Power Inserter (with 24VDC power supply)–>DC Blocker–>RTL-SDR

Specs on PVLA-30 (from Amazon):
950-2150 MHZ/ GAIN 30DB, NOISE FIGURE 6DB
LINE VOLTAGE 8 TO 29V DC IN/OUT, WITH POSITIVE SLOPE
CURRENT CONSUMPTION 125 mA
OUTPUT LEVEL -10DB

I’d like to reinsert my filter (ZX75BP-1100+) between the DC Blocker and the RTL-SDR, but I noticed that the RF power input is 3.2W Max. (as compared to the 7W Max on the VBFZ-1065+.) I wasn’t worried about the 3W max when I didn’t have an amplifier inline, but I suspect I should be a bit more careful about that rating with the amplifier inline.

What is the best way to determine the RF power wattage with the amplifier inline?

I do not have access to the expensive RF power meters that were being returned in my Google results, though I do have access to a Fluke 199C Scopemeter and likely a portable oscilloscope.


#2

Your power levels are well below a milliwatt - there’s a very useful PDF chart on the Mini-Circuits website for converting among volts, watts, and dBm - 0 dBm = 1 milliwatt; -10 dB (m) is way below that.

Nothing to worry about!

Bob k6rtm


#3

Are you TRANSMITTING into this antenna and filter setup? If not, and I don’t think you are, the wattage makes NO difference to you. There are instruments that measure SWR (standing wave ratio) that transmit very low power (watts) INTO the antenna system to make this measurement. You probably don’t use any of these as they are quite expensive and usually only RF techs use them.
As far as RF power coming IN from the antenna, an S-mode transmitter and antenna like they have on an aircraft would have to be sitting in your back yard to generate enough RF voltage to damage the filter.
So my advice is don’t worry about it.
I’m not an RF engineer but I am a ham radio operator and know a thing or two about such things.

Ed N2CTI


#4

Thank you both! I will not worry about it and instead worry about why my ISP decided to drop service for about an hour and a half this morning. :slight_smile: