Coil or Loop Antenna

dnyberg2

I'm wrestling with the notion that a small coil of wire in a planar flat spiral can be a real tuned antenna. The fact that the coil is only three turns and about the diameter of a quarter further irritates my ability to understand how such a simple thing can be viewed as a real antenna. The coil measures about 390 nH on an analyzer with about .7 ohms. On top of that, the operating frequency of the system is 49 MHz! How does one go about taking a simple coil, on FR4 and make it radiate at 49 MHz with respectable SWR's and an impedance of 50 ohms? Dont you have to add a capacitor in series with this coil inductor to make it resonate at 49 MHz? And, doesn't the value of the capacitor have to do with matching the input to 50 ohms? I'm assuming that the other end of the coil has to be grounded, so one side of the CAP is the input, which is connected to the other end of the coil... Any help in understanding this is greatly appreciated.

skeptic2

True, antenna like that will not be very efficient but most likely the manufacturer has to comply with the FCC regulations for unlicensed operation (in the US) and must keep the radiated power below specified limits. Using an inefficient antenna is a way of doing that. Adding a capacitor in series may make the antenna more efficient but it also may narrow the bandwidth of the antenna unacceptably.

There is no reason the antenna must be matched to 50 ohms if that is the only output. It is a good idea to do so anyway so that the output of the last stage can be measured by standard test equipment. How would you be able to check the output with a direct connection to a spectrum analyzer or power meter without matching the output to 50 ohms.