RF propagation - RF Cafe Forums

 Post subject: RF propagation

Posted: Mon Apr 10, 2006 9:51 am 

Hello,

Does this seem logical?

1. The signal strength (example in dBm or mW) from a transmitting source is measured.

2. Assuming most of the attenuation is due to free space loss, the free space loss factor is "added" back to the measured signal to get the ERP of that specific radiation lobe (it could be main, side, or back lobe).

3. The transmitter output is a known value (example 2 Watts).

4. Using the calculated ERP for a specific lobe, I can use the following to determine the loss / gain due to the antenna & cable: loss/gain = 10 log (transmitter output / ERP specific lobe).

5. If I rotate the transmitting antenna to 360 degrees, then I should be able to have a rough picture of its azimuthal radiation pattern at a certain elevation angle.

I'm probably using the terms the wrong way because I think ERP is synonymous to the main lobe of the antenna (right?). But I'm just treating the ERP as specified only to a certain lobe without knowing if its the main, side, or back lobe.

I'm really not sure if this is right. I just want a confirmation whether my logic is wrong, and if so, why? Thanks!

 Post subject: Antenna pattern measurement

Posted: Mon Apr 10, 2006 8:02 pm 

Hi!

Don't forget about the gain of your receive antenna, and about possible reflections (the bane of antenna measurements!). Reflections from the ground are particularly problematic with certain antenna designs. Reflections from nearby structures are often also a big problem. Remember, it doesn't have to be shiny or even reflective at light frequencies to be a jim-dandy reflector at RF.

Remember that the effective area of the antennas is intimately connected to the gain. The transmitted power is spread out over an area, and your receive antenna gathers power over an area. That's the origin of the inverse-square law.

Rotating a transmitting antenna through 360 degrees is a standard way of making antenna pattern measurements.

Good Luck!