4 ms·

The other way I've seen this phrased is that unlike a transmitter, it's OK for a receiving antenna to be incredibly thermodynamically inefficient, because a wea

The other way I've seen this phrased is that unlike a transmitter, it's OK for a receiving antenna to be incredibly thermodynamically inefficient, because a weak signal can always be amplified -- as long as it's not swamped by noise generated within the receiver itself. Let's say an AM radio station transmits a 1kW signal at 1MHz, with a bandwidth of 10kHz. If you have a receiver 100 miles away, and ideal isotropic antennas at both ends, then by plugging some numbers into a link budget calculator, it looks like the received power is about 12 orders of magnitude above the thermal noise floor. That means it's plausible that you could still get a clear signal even if your antenna only manages to capture a millionth or a billionth of the incoming energy.

hilbert42 5y ago
Ferrite antennas are remarkably efficient considering their size. If one were to examine the magnetic flux around a ferrite antenna one would find that the flux is channeled from the surrounding area into the ferrite rod (the rod effectively concentrating the field). In effect this makes the antenna's effective size (its aperture) much bigger than its actual physical size. Also, remember broadcast services are designed to put sufficient field (signal) strength into a 'notional' receiver that's operating within the rated service area of the broadcast. Both the transmitter field strength and the receiver sensitivity are designed in combination to provide a worthwhile listening signal. The designed signal-to-noise ratio is dependent on the type of service and for the AM B/C band it is in the order of 40dB or so for the primary service area (the secondary service area typically 10dB less).