RF current

Hello.

I'm using a CC2430 micro-controller, which has a transmitting-receiving unit.
Its output power is programmed to +0.6dBm.

I was wondering what is the amplitude of the RF current that flows through the Antenna.

This is a single-ended Antenna - a piece of wire, 3.3cm length, made of copper.
When I came to order such copper wire, I was asked to specify the current and voltage rating of the desired wire.

The Uc's VDD is 3.3V, therefore the voltage rating of the Antenna is 3.3V, right? (or at least, not more than that).
I also need to know what is the max current that the Antenna should be able to handle, how can i tell the RF current's amplitude?

Thank you.

Call it a milliWatt (actually 0.6db higher than 1mW). If the antenna where a perfect center-fed dipole, the feed point impedance would be 70Ω. Because it is actually an end-fed non-resonant noodle, the impedance will be much higher. P=(i^2)*R or i = √(P/R) = √(0.001/70) ≤ 4mA, hardly anything...

Thank you.
How can you tell that 0.6dBm is around 1mW?

How does this transformation get done?

alphacat said:

Thank you.
How can you tell that 0.6dBm is around 1mW?

How does this transformation get done?

Click to expand...

0 dBm is a reference of 1 milliwatt into a load. The load is usually 50 ohm, but 75 ohm is also common in RF. The conversion formula is simple.
mW =10^(dBm/10) or dBm = log10 (mW)*10


Or you can refer to a conversion chart. **broken link removed**

Also be aware that audio people, telephone people etc, usually use a load of 600 ohm for their dBm calculations.

As an aside, I have a piece of test equipment here which came from a troposcatter communications system. It is a combined oscillator and level meter which was used for calibrating the baseband circuits (20hz to 120khz) and has switch selectable load impedances of 75, 140 and 600 ohms.
The attenuators and meters are all calibrated in dBm.

JimB

Minor nit: the OP asked how much current would flow into his antenna wire. His transmitter is rated at +0.6dbm, which is a POWER level, relative to 1 mW. The specification of Power Level doesn't , by itself specify the impedance.

Now somewhere else on the data sheet for the transmitter it will say something like RF Power was measured into an XXΩ load (usually 50 or 70Ω). The simplified calculation I did above was based on the bogus assumption that his antenna would be resonant, and that it presents something like a free-space resonant 1.2λ dipole feed-point impedance (72+j0)Ω to the transmitter, and that the transmitter will actually put +0.6dbm into that impedance.

If his antenna is actually a 1/4λ monopole operated against a non-existent or tiny ground plane (the PC board), then the feed-point impedance that the transmitter sees is 16 - j800, a very highZ, very capacitive.
Now the question becomes, how much power does the transmitter put into such a crappy load, (I'll be surprised if it is anywhere close to its rated power), and what is the actual current that flows into the antenna (it will be much, much less than the 4mA I calculated above).

The moral of this story is that a 1/4λ monopole without a proper 1/2λ by 1/2λ ground plane under it is a really crappy antenna!!!