I'm interested in omnidirectional antennas or maybe I should say antennas that don't care too much if you move/rotate them. Now how would you build a good antenna for a specific frequency range. (e.g 1920-2170MHz or 2401-2473MHz or 87.5-108MHz)
And what's the magic behind getting the best dbi?
I heard that the antenna height is very important and that the ideal antenna height is the wavelength divided by two, is that correct?
Also: Could you use two antennas and wire them together for better results?
And does it matter if you roll up or bend an antenna? (Is there a limit, maximum angle or so?)
Edit: One more thing, I found an antenna that claims to be optimized for 890 - 960 MHz / 1710 - 2150 MHz. How is that possible? Does it really not pick up 960-1710MHz?
Best Answer
So to throw out some basics:
Everything acts as an antenna if it is conductive. Some shapes and materials work better for some frequency but you can not say that an antenna will only accept 1 specific frequency unless it is designed with very special parameters and for high frequency. The receiver will always have some filtering, for ~900MHz for example a SAW filter is typically employed, this is basically a pass band filter to help reduce all the other frequencies.
A Wavelength / 4 antenna is typically referred to as a quarter wavelength monopole antenna. Whenever you see something like a pointy antenna on a radio or similar, this is exactly what it is. In your example, the reason the same antenna can work for 920MHz and 1900Mhz is that they are integer multiples of each-other. One frequency is received as a 1/4th wavelength and the other is most likely received as a 1/8th wavelength or similar.
Multiple antennas really get into complex theory that is frequency dependent. For example if you have 4 antennas in a square shape, in the FM frequency range only the antenna closest to the transmitter will receive. You can actually use this to some extent for determining what direction the signal is coming from. Typically you only use 1 antenna, because to use more then one gets complex and is far beyond what can be explained here.
As if you didn't get the hint yet, antenna design is greatly dependent on the frequency and transition style (PCM, AM, FM, PM, FHSS etc.). The lower frequency FM you mentioned above is typically designed with a stick antenna (1/4 wavelength is ideal but any piece of metal that is linear will work OK). Now the high frequency 2100MHZ that is more of an art, Google some images of them, often they are weird shapes with multiple bends etc. that are found with complex simulation software. Although you could use a 1/4 length antenna again and still get some signal it would just not be ideal...
The quality and type of the antenna needed depends on the link strength, how much power you will receive vs the noise floor and the RX minimum receive strength etc.
Your best bet is to read a few books, take a class, then if you are still interested, get access to a network analyzer and spectrum analyzer and try out some designs in a Faraday cage.