antenna length RX and TX question

[kaushalpkk]

Hi, I don't know much about radio electronics. But I wanted to know why the antenna length matters when a transceiver transmits RF and not while receiving it.

as I see in this product **broken link removed** the tx freq is 144-146 & 430-440 MHz and RX freq 25-2000 MHz...

thankss

 

[E Kafeman]

Antenna length is important also for receiving signals. A good Bluetooth antenna is just a few cm long. Try that antenna length for an 100 MHz FM radio and not much will be heard. Antenna efficiency is related to what wavelength it needs to receive or transmit and wavelength is related to frequency. Similar as for a loudspeaker. Low frequency tones needs big speakers to be effective but high frequency sound emits better from a small speaker with a lightweight membrane.
Regarding any super duper antenna with extreme wide bandwidth, it will actually work on even wider band then this, but most super wide antennas have that in common that they have rather poor performance for most frequencies. Actual antenna gives exactly same performance as any 40 cm long iron-wire.

 

[enjunear]

Antennas are resonant (or near-resonant) devices. Like Kafeman described with the speaker analogy, the same effects apply to antennas. The closer the electromagnetic field's wavelength is to the resonant frequency(s) of your antenna, the more signal it will either transfer into the air (transmit mode), or collect from the air (receive). The vast majority of antennas seen in the commercial marketplace are, at their core, bi-directional and reciprocal devices. Thus, they operate under the same rules in transmit, as they do in receive.

Picking a proper antenna for receive is just as critical as picking a good antenna for the transmitter. Think about trying to pick up broadcast TV stations using a **broken link removed**.... it doesn't work at all, because the loop's resonant frequency is typically around 1 MHz, while broadcast TV is on the order of 100 MHz. The loop antenna has very, very poor gain at 100 MHz, thus making picking up a wide range of OTA TV stations virtually impossible with that antenna on your receiver.